# Molecules in the carbon-rich protoplanetary nebula CRL 2688

Jian-Jie Qiu(邱建杰) School of Physics and Astronomy, Sun Yat-sen University, 2
Da Xue Road, Tangjia, Zhuhai 519000, Guangdong Province, China  Yong Zhang(张泳)
School of Physics and Astronomy, Sun Yat-sen University, 2 Da Xue Road,
Tangjia, Zhuhai 519000, Guangdong Province, China  Laboratory for Space
Research, The University of Hong Kong, Hong Kong, China  Jiang-Shui Zhang(张江水)
Center For Astrophysics, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou
Higher Education Mega Center, Guangzhou 510006, Guangdong Province, China
Jun-ichi Nakashima(中岛淳一)  School of Physics and Astronomy, Sun Yat-sen
University, 2 Da Xue Road, Tangjia, Zhuhai 519000, Guangdong Province, China

###### Abstract

We present observations of the carbon-rich protoplanetary nebula (PPN) CRL
2688 made with the Institut de Radioastronomie Millimétrique (IRAM) 30 m
telescope in the 3 mm and 2 mm bands. In total, 196 transition lines belonging
to 38 molecular species and isotopologues are detected, among which, to our
best knowledge, 153 transition lines and 13 species are the first report for
this object. Additionally, in order to contribute to future research, we have
collected observational data on the molecular lines of CRL 2688 from the
literature and compiled them into a single unified catalog. We find that the
molecular abundance of CRL 2688 cannot be explained by the standard model of a
circumstellar envelope. The implications of metal-bearing molecules on
circumstellar chemistry are discussed.

ISM: molecules — circumstellar matter — line: identification — planetary
nebula: individual (CRL 2688) — surveys

††journal: ApJS

##  1 Introduction

Mass loss of asymptotic giant branch (AGB) stars fuels the recycling of
materials in the galaxy (see, e.g., Herwig, 2005; Ziurys, 2006; Höfner &
Olofsson, 2018, for reviews). Suffering from complex dynamic and chemical
processes, the resultant circumstellar envelopes (CSEs) are active sites for
the formation of substantial amount of dust and molecular gas, making them
bright infrared (IR) and radio sources. Along the evolution beyond AGB, the
CSEs gradually expand, resulting in decreasing density, and the central stars
would become visible at a certain moment (Bloecker, 1995). With further
evolution, the gas in CSEs are partly or fully dissociated and ionized by
intense ultraviolet (UV) radiation from the central stars, and emits strong
ionic emission lines at optical wavelengths, marking the beginning of a
planetary nebula (PN) stage. Passing through the PN evolution, the
circumstellar matters finally disperse into and contaminate the interstellar
medium (ISM). Recently, there are increasing observational evidences showing
that some molecules are able to survive in the PN evolution and then seed the
chemistry in diffuse and molecular clouds (Tenenbaum et al., 2009; Schmidt &
Ziurys, 2016, 2018; Zhang, 2017).

The variations of molecular composition during the CSE evolution is a hot
topic in astrochemistry (Cernicharo et al., 2011). So far, more than 70 kinds
of molecular species have been detected in circumstellar environments
(McGuire, 2018, 2021, and references therein). Many of these detections
greatly benefited from molecular line surveys performed with advanced single-
dish telescopes. However, spectral line surveys have been overwhelmingly
concentrated on the brightest nearby carbon star IRC+10216 (Cernicharo et al.,
2000, 2010; He et al., 2008; Patel et al., 2011; Gong et al., 2015; Zhang et
al., 2017, etc.). In comparison, line surveys of other CSEs, although
existing, are far sparser, which would have the risk of leading to biased
views of circumstellar chemistry. It is evidently necessary to extend such
observations to CSEs at different evolutionary stages.

Protoplanetary nebulae (PPNs) represent a transient phase before the PN stage
when the AGB winds cease (Kwok, 1993), during which the physical conditions of
the CSEs change drastically on spatial and temporal scales. Given their
relative short evolutionary timescale ($\sim$103 yr), the PPNs that are
suitable for line-survey observations are rare, and thus the chemical
processes in this transition phase are far from fully understood. CRL 2688 and
CRL 618 are two ideal PPNs for line-survey observations (Pardo et al., 2007;
Park et al., 2008; Zhang et al., 2013). Both are carbon rich, but CRL 618
appears to be closer to the PN stage (Bujarrabal et al., 1988). The spectra of
the two targets exhibit dramatically different molecular species, indicating
that quick chemical reactions have taken place during the PPN stage. Detailed
chemical models of PPNs are practically limited to CRL 618. Howe et al. (1992)
theoretically investigated the chemistry of PPNs using a interacting stellar
winds model, and found that a few small molecules and cations can be formed in
the shocked regions, while the molecules originally forming in the AGB stage
are quickly destroyed unless clumpy structures exist. Based on a model of a
dense, warm gaseous slab, irradiated by an intense UV radiation, Woods et al.
(2003a) concluded that complex organic chemistry can be driven in the torus of
late PPNs, which can qualitatively account for the radio observations of CRL
618. Cernicharo (2004) modelled the chemistry in the neutral layers of CRL 618
and pointed out that large carbon-bearing molecules can be quickly produced by
photochemistry. A general chemical model of late-AGB/early-PPN stage objects
was constructed by Woods et al. (2005), who suggested that the molecule-rich
PPNs should be attributed to the existence of a dense torus. More observations
of PPNs at different evolutionary stages are necessary to constrain these
chemical models.

This paper is devoted to investigate the molecular species in CRL 2688, a
prototypical bipolar reflection nebula surrounding a post-AGB star. This
nebula exhibits a wealth of structures, including a pair of bipolar lobes, an
equatorially dense dusty cocoon, a pair of nonuniform annular holes coaxial
with the polar axis, and a large number of roughly round arcs (Sahai et al.,
1998b). The interferometric observations in the CO $J=2\rightarrow 1$ line has
revealed two distinct high-velocity outflows along the optical axis and close
to the equatorial plane, respectively (Cox et al., 2000). A high-velocity
outflow is revealed by bright H2 emission arising from shocked gas (Sahai et
al., 1998a). Table 1 summarizes the basic parameters of CRL 2688 and its
central star. The distances estimated by several authors have a large
uncertainty. CRL 2688 is a bright IR source, and most of IR emission
originates from the central region with a diameter of 1$\hbox
to0.0pt{.\hss}\arcsec$5 and a temperature of about 150 K (Ney et al., 1975).
The central dust-obscured post-AGB star shows an F5 Ia spectrum (Crampton et
al., 1975). Assuming a distance of 1 kpc, Ney et al. (1975) obtained a total
bolometric luminosity of $1.8\times 10^{4}\,L_{\odot}$. Based on a distance of
420 pc and an optical extinction of AV mag of 0.8 mag (Ueta et al., 2006),
Balick et al. (2012) derived the luminosity and effective temperature of the
central star to be $5500\pm 1100$ $L_{\odot}$ and 7000 K, respectively.
Measuring the overall motions of the lobes at 2 $\mu$m based on the $HST$
NICMOS images from 1997 to 2002, Ueta et al. (2006) determined the nebular
dynamical age of roughly 350 yr. Using the same approach, but based on the
$HST$ images at 0.6 $\micron$ observed from 2002 to 2009, Balick et al. (2012)
obtained a dynamical age of about 250 yr.

One of the key motivations of this work is to investigate the gas-phase
molecules in sources exhibiting the so-called 21 $\mu$m and 30 $\mu$m
unidentified IR features (see Volk et al., 2020, for a recent review). The
rare 21 $\mu$m feature is observed exclusively in carbon-rich PPNs, indicating
its transient nature. It is always accompanied with the 30 $\mu$m feature.
Buss et al. (1993) found that the IRAS/LRS spectra of CRL 2688 revealed a weak
feature near 20 $\mu$m, which was later confirmed by Omont et al. (1995).
However, the ISO/SWS spectrum does not show any feature around this wavelength
(Volk et al., 2020), casting a doubt whether CRL 2688 is a 21 $\mu$m source.
The 21 $\mu$m sources also exhibit unusually strong 3.4–3.5 $\mu$m emission,
and this is the case for CRL 2688. If the 21 $\mu$m feature is indeed lack in
CRL 2688, Geballe et al. (1992) proposed that this PPN may be in a transition
between the ‘21 $\mu$m’ and normal phases. Strong 30 $\mu$m feature has been
discovered in the ISO/SWS spectrum of CRL 2688 (Hony et al., 2002). Zhang
(2020) found that the relative abundance between carbon-chain and Si-bearing
molecules are highly similar between CRL 2688 and a well-established strong 21
$\mu$m source, and then inferred that gas-phase molecules may hold key clues
about the carrier of the 21 $\mu$m feature.

The paper is organized as follows. In Section 2, we introduce the observations
and data reductions. In Section 3, we present the results of a abundance
analysis. In Section 4, we compare the molecules in CRL 2688 and other CSEs,
as well as observations and modelling results. A brief summary is given in
Section 5.

##  2 Observations and data reductions

The observations toward the central region of CRL 2688 (RA: 21:02:18.27, DEC:
36:41:37.0, J2000) were performed with the Institutde Radioastronomie
Millimétrique (IRAM) 30m-diameter telescope at the Pico Veleta Observatory
(Spain) in 2017 January 14. We used the Eight Mixer Receiver (EMIR) with dual-
polarization and the Fourier Transform Spectrometers (FTS) backend to cover
frequency ranges from 75.7–83.5 GHz and 91.4–99.2 GHz in the 3 mm window, and
160.8–168.6 GHz and 176.5–184.3 GHz in the 2 mm window. The instantaneous
frequency coverage per sideband and polarization is 4 GHz with a frequency
channel spacing of 195 kHz. The standard wobbler switching mode with a $\pm$
110′′ offset at a rate of 0.5 Hz beam throwing was applied. The on-source
integration times of each band are about two hours. Pointing was checked
frequently with strong millimeter emitting quasi-stellar objects. The typical
system temperatures are 84–112 K in the 3 mm band and 110–540 K in the 2 mm
band.

The antenna temperature $(T_{\rm A})$ is converted to the main beam
temperature $(T_{\rm mb})$ with the expression of $T_{\rm mb}=T_{\rm
A}\times\frac{F_{\rm eff}}{B_{\rm eff}}$, where the forward efficiency
($F_{\rm eff}$) and main-beam efficiency ($B_{\rm eff}$) of the telescope at
each band are listed in Table 2. The data are reduced using the CLASS software
of the GILDAS package111http://www.iram.fr/IRAMFR/GILDAS. The spectra at the
same frequency coverage were co-added to improve the signal-to-noise ratio
according to the weights proportional to $1/\sigma^{2}$, where $\sigma$ is the
root-mean-square uncertainty in the continuum. The identification of each
transition line was made by using the information from the NIST database
recommended rest frequencies for observed interstellar molecular microwave
transitions222http://pml.nist.gov/cgi-bin/micro/table5/start.pl (Lovas, 2004),
the Cologne Database for Molecular Spectroscopy
catalog333https://cdms.astro.uni-koeln.de (Müller et al., 2001, 2005), and
splatalogue database for astronomical
spectroscopy444https://splatalogue.online//advanced.php. To validate the
detection and identification of a given line, we checked the other transition
lines from the same species that are available in the current observations and
those in literature.

In order to get a more complete view of the chemistry in CRL 2688, this work
was complemented with observational data reported in the literature. We have
collected as many results as possible from previous molecular radio
observations of CRL 2688 using single-dish telescopes, except for a few old
observations where the line intensity was not given correctly. For the past
reports that did not explicitly give the half-power bandwidths (HPBW) of used
telescopes, we take the values derived by the expression of
$\rm{HPBW}=75531$.″$2\left[\frac{\nu}{{\rm
GHz}}\right]^{-1}\left[\frac{D}{{\rm m}}\right]^{-1}$, where $\nu$ is the rest
frequency of the transition lines in units of GHz and $D$ is the diameter of
the telescope in units of m. The results obtained from the present observation
and those collected from literature are summarized in Table Molecules in the
carbon-rich protoplanetary nebula CRL 2688.

##  3 Results

The spectra of the entire frequency range observed are shown in Fig. 1, in
which the identified strong lines are marked. We totally detect 196 emission
lines, among which four remain unidentified, and the others are attributed to
24 molecular species and 14 different isotopologues, as shown in Fig. 2 and
listed in Table Molecules in the carbon-rich protoplanetary nebula CRL 2688.
Most of the molecules, except SiS, NS, PN, and AlF, are carbon-bearing. This
is consistent with the rich carbon nature of CRL 2688. We note that the
detections of C33S, Si34S, 29SiS, NS, 26MgNC, Si13CC, 29SiC2, 13CCH, H13C15N,
C3S, $c$-C3H, HCC13CN, $c$-C3H2, and C13CCCH have not been previously reported
for this object in literature although some of them have been detected in
other CSEs of evolved stars. The features with a central intensity of larger
than 3$\sigma$ are regarded as real detections. The spectra also reveal a few
weak features with an intensity of less than 3$\sigma$, which have wavelengths
closed to those of lines from usual circumstellar molecules, and here we
classify them as tentative detection. The spectra show a line density of about
6 lines per GHz. The velocity-integrated intensities are obtained by fitting
the lines using the stellar-shell model in the CLASS package. For the lines
that are too faint to be fitted, the intensities are obtained by directly
integrating $T_{\rm mb}$ over a certain velocity interval (see Table Molecules
in the carbon-rich protoplanetary nebula CRL 2688). The fitting results are
shown in Fig. 3 (the complete figure set is available in the online version),
which are ordered according to the molecular sizes and the rest frequencies of
the transition lines. In the following, the results are presented species by
species, which are followed by a comparison with previous observations. Also
present are the molecular abundances derived from an analysis of rotation
diagrams and radiative transfer modelling.

###  3.1 Individual species

####  3.1.1 Carbon monosulfide – CS

The $J=2\rightarrow 1$ lines of CS and its 13C, 33S, and 34S isotopologues are
detected, as shown in Fig. 3. The CS $J=2\rightarrow 1$ line shows a broad
line wing. The blue wing is blended with the $l$-C3H
${}^{2}\Pi_{1/2}~{}J=9/2\rightarrow 7/2$ line. The 13CS $J=2\rightarrow 1$
line shows a double-topped line profile, and is partly blended with the C3S
$J=16\rightarrow 15$ line. All these lines are fitted by using the stellar-
shell model. High spatial-resolution observations with the Nobeyama Millimeter
Array (NMA) show that the CS $J=2\rightarrow 1$ line exhibits symmetrical
peaks on both sides of the optical dark lane, indicating a shock-induced
sulfur-bearing molecule chemistry (Kasuga et al., 1997). Compared to the NMA
observations (see Fig. 4 in Kasuga et al., 1997), our observations show great
improvement of revealing the line profile. The CS $J=2\rightarrow 1$ line
shows an asymmetric profile with a stronger red peak. Our observations suggest
that the local standard of rest velocities ($v_{\rm LSR}$) of the red peak is
$-33$ km s-1, slightly differing from that given by NMA ($-29$ km s-1),
although the ranges of this line are the same in $v_{\rm LSR}$ space. This
might be due to the higher spectral resolution and higher sensitivity in our
work. The $J=2\rightarrow 1$ lines of CS isotopologues show a similar profile
with the CS $J=2\rightarrow 1$ line. Both CS and C34S lines exhibit two
emission components at velocity ranges from $-60$ to $-50$ km s-1 and from
$-68$ to $-65$ km s-1, respectively. The NMA observations show that they are
associated with the regions located at about 2$\arcsec$ from the center to the
northwest.

####  3.1.2 Silicon monosulfide – SiS

SiS in CRL 2688 was firstly discovered with the Nobeyama Radio Observatory
(NRO) 45 m telescope by Fukasaku et al. (1994). So far, the SiS
$J=5\rightarrow 4$, $6\rightarrow 5$, $13\rightarrow 12$, and $14\rightarrow
13$ lines and the Si33S $J=8\rightarrow 7$ line have been detected in this
object (Fukasaku et al., 1994; Highberger et al., 2003; Zhang et al., 2013).
The four SiS and isotopic lines, SiS $J=9\rightarrow 8$ and $10\rightarrow 9$,
29SiS $J=10\rightarrow 9$, and Si34S $J=10\rightarrow 9$, lie within our
frequency coverage. All are clearly detected, as shown in Fig. 3. The double-
peaked profiles suggest extended ($>$14$\arcsec$) and optically thin natures.
There are intriguing similarities between the profiles of SiS and CS lines;
both exhibits a broad component distributed from $v_{\rm LSR}=-70$ to 0 km s-1
and the strengthened red peak, probably suggesting that sulfur-bearing
molecules are formed through a common chemical pathway.

####  3.1.3 Nitrogen sulfide – NS

We detected two NS lines with $>$3$\sigma$. Figure 3 shows the fitting result
to the NS ${}^{2}\Pi_{1/2}~{}J=7/2\rightarrow 5/2~{}l=e$ line. The other
tentatively detected lines of NS is blended with other lines and cannot be
fitted well. The NS line is too faint to accurately retrieve its profile, but
it seems to be asymmetric probably due to the blend of hyperfine components.
The NS ${}^{2}\Pi_{3/2}~{}J=7/2\rightarrow 5/2$ line at 162.67 GHz is also
within our frequency coverage, but is not clearly detected due presumably to
its intrinsic weakness.

####  3.1.4 Phosphorous mononitride – PN

The PN $J=2\rightarrow 1$ line is detected at a $4.5\sigma$ level, as shown in
Fig. 3. This is the only PN line lying within our frequency coverage. This
molecule has been previously discovered by Milam et al. (2008) in this object,
IRC+10216, and an oxygen-rich supergiant star VY Canis Majoris, using the
Arizona Radio Observatory (ARO) 12 m telescope.

####  3.1.5 Aluminum fluoride – AlF

AlF was firstly discovered in CRL 2688 with the IRMA 30 m telescope by
Highberger et al. (2001) through the $J=4\rightarrow 3$, $5\rightarrow 4$, and
$8\rightarrow 7$ lines. Two AlF lines lie within our frequency coverage, one
of which ($J=5\rightarrow 4$) is detected, as shown in Fig. 3. This line
exhibits an approximately flat-topped profile, probably suggesting that AlF is
located within a central compact region. The $J=3\rightarrow 2$ line at 98.9
GHz is unfortunately located at the edge of the spectrometer where the signal
to noise is relatively low.

####  3.1.6 Sodium cyanide – NaCN

NaCN in CRL 2688 was firstly detected with the ARO 12 m telescope by
Highberger et al. (2001) at the 2 mm band. Nine transition lines of NaCN are
clearly detected in our observations, as shown in Fig. 3. Eight of them are
newly detected. The NaCN $J_{K_{a},K_{c}}=10_{1,11}\rightarrow 10_{1,10}$ line
has been detected by Highberger et al. (2003), which shows a consistent
intensity and profile with ours. The NaCN $J_{K_{a},K_{c}}=5_{0,5}\rightarrow
4_{0,4}$ line is partly blended with the HC7N $J=69\rightarrow 68$ line (see
Fig. 3), and thus two stellar-shell components are used to decompose its
profile. Most of the transition lines show a double-peaked profile, indicating
that NaCN emission is optically thin and the emission region is resolved by
the telescope beam.

####  3.1.7 Magnesium isocyanide – MgNC

MgNC in CRL 2688 was firstly detected with the ARO 12 m telescope by
Highberger et al. (2001) through the $N=11\rightarrow 10$, $12\rightarrow 11$,
and $13\rightarrow 12$ lines. We detect three new transition lines,
$N=8\rightarrow 7$, $14\rightarrow 13$, and $15\rightarrow 14$ as shown in
Fig. 3. The double-peaked line profiles of the MgNC $J=8\rightarrow 7$ line
indicate an optically thin nature. We also marginally detect the weaker
$N=7\rightarrow 6$ and $8\rightarrow 7$ transition lines of 26MgNC (see Fig. 2
and Fig. 3). The 26MgNC $J=7\rightarrow 6$ line is blended with an
unidentified line (U-line) at 80.5 GHz, as shown in Fig. 2. We notice that
this U-line was also detected in IRC+10216 (see the NIST database).

####  3.1.8 Silacyclopropynylidene – SiC2

The detection of SiC2 in CRL 2688 was firstly reported with the IRMA 30 m
telescope by Bachiller et al. (1997) for the
$J_{K_{a},K_{C}}=6_{0,6}\rightarrow 5_{0,5}$ line. We detected nine transition
lines from SiC2 and one isotopic transition from Si13CC, as shown in Fig. 3.
Except for two transition lines at 165.5 GHz that are heavily blended with the
CH3CN $J=9\rightarrow 8$ line (see Fig. 3), all have a relatively high signal
to noise ratio and exhibit a symmetric double-peaked profile. Moreover, four
29SiC2 isotopic transition lines are marginally detected, as shown in Fig. 2,
but cannot be well fitted using the stellar-shell model assuming spherical
symmetry.

####  3.1.9 Dicarbon sulfide – C2S

Two weak C2S transition lines are detected, as shown in Fig. 3. The
$(N,J)=(7,8)\rightarrow(6,7)$ line is partly blended with one vibrationally
excited transition line of C4H. Park et al. (2008) reported a detection of the
C2S $(N,J)=(8,8)\rightarrow(7,7)$ line with a flux density of 3.5 Jy. However,
this line was not revealed in the more sensitive spectrum of Zhang et al.
(2013). Based on our measurements and the intrinsic strength ratio between the
$(N,J)=(7,8)\rightarrow(6,7)$ and $(8,8)\rightarrow(7,7)$ lines ($1:1.16$), we
estimated the flux density of the latter line to be less than 0.012 Jy,
confirming the speculation of Zhang et al. (2013) that this line was unlikely
detected by Park et al. (2008).

####  3.1.10 Ethynyl radical – 13CCH

Huggins et al. (1984) first reported the detection of C2H through the
$(N,J)=(1,3/2)\rightarrow(0,1/2)$ line in CRL 2688 using the NRAO 11 m
telescope. Although our spectral range does not cover any C2H lines, we
detected two 13CCH isotopic transition lines with hyperfine components, as
shown in Fig. 3. They are obviously optically thin.

####  3.1.11 Hydrogen cyanide – HCN, hydrogen isocyanide – HNC, and formyllum
cation – HCO+

The $J=2\rightarrow 1$ rotational transition lines of HCN, H13C15N, HNC, and
HCO+ and two vibrationally excited transition lines of HCN at the $\nu_{2}=1$
state are detected, as shown in Fig. 3. The HCN $J=2\rightarrow 1$ line is
blended with the much weaker HCN $\nu_{2}=1~{}J=2\rightarrow 1~{}l=1e$ line.
Zhang et al. (2013) reported a tentative detection of the HCO+ $J=1\rightarrow
0$ line. The existence of a small amount of HCO+ in CRL 2688 is confirmed by
our detection of the weak HCO+ $J=2\rightarrow 1$ line, although it is partly
blended with the HC5N $J=67\rightarrow 66$ line.

Similar to those of sulfur-bearing molecules, the HCN and HNC lines exhibit a
broad and asymmetric component with a stronger blue wing. A prominent
absorption feature, ranging in velocity from $-50$ to $-60$ km s-1, is
revealed in the broad wing of the HCN $J=2\rightarrow 1$ line. The absorption
ranging from $-50$ to $-55$ km s-1 has been detected in the CO $J=1\rightarrow
0$, $3\rightarrow 2$, and $4\rightarrow 3$ , HCN $J=4\rightarrow 3$, H13CN
$J=4\rightarrow 3$, and CS $J=7\rightarrow 6$ lines (Kawabe et al., 1987;
Jaminet et al., 1992; Young et al., 1992; Cox et al., 2000). Our observations
reveal that this feature is composed of two components, ranging from $-50$ to
$-55$ km s-1 and $-55$ to $-60$ km s-1, respectively. Based on a Voigt fitting
(Ginsburg & Mirocha, 2011), we obtained the column densities of the two HCN
absorption components to be 5.39 $\times$ 1013 and 1.51 $\times$ 1013 cm-2,
respectively.

####  3.1.12 Cyanoethynyl radical – C3N

The first detection of C3N in CRL 2688 was presented by Lucas et al. (1986).
We detected the $N=8\rightarrow 7$, $10\rightarrow 9$, $17\rightarrow 16$, and
$18\rightarrow 17$ lines, as shown in Fig. 3. The C3N $N=8\rightarrow 7$ and
$10\rightarrow 9$ lines show an optical thin double-peak profile, while the a
parabolic profile is revealed for the $N=17\rightarrow 16$ lines, suggesting
that the higher-$N$ lines might arise from the inner compact region that
cannot be resolved by the telescope beam. The two $N=18\rightarrow 17$ lines
are heavily blended with each other, and cannot be well fitted using the
stellar-shell model.

####  3.1.13 Propynylidyne radical – $l$-C3H and Cyclopropenylidene radical –
$c$-C3H

$l$-C3H has been discovered in CRL 2688 (Zhang et al., 2013) . In this work,
we detected eight vibrationally excited transition lines of $l$-C3H, as shown
in Fig. 3. Each line splits into two hyperfine structure. All show an optical
thin double-peak profile.

The $c$-C3H $N_{K_{-1},K_{+1}}=2_{1,2}\rightarrow 1_{1,1}$ line, including two
hyperfine structure components at 91.5 GHz is detected with a low signal to
noise ratio (see Fig. 2). $c$-C3H has no other lines within our frequency
coverage, except for the line at 91.7 GHz. However, the spectrum around 91.7
GHz is too noisy to allow us to confirm the secure detection. We thus infer
that the abundance of $c$-C3H is much lower than its linear counterpart.

####  3.1.14 Cyanoacetylene – HC3N

HC3N is detected through the $J=9\rightarrow 8$, $18\rightarrow 17$, and
$20\rightarrow 19$ lines, as shown in Fig. 3. These lines consistently exhibit
three peaks and a broad wing. We also detect six $\nu_{7}=1$ vibrationally
excited lines of HC3N and several isotopic lines from H13CCCN, HC13CCN, and
HCC13CN (see Fig. 3). The transition lines of HC13CCN and HCC13CN from the
same energy levels are partly blended with each other. We are not able to
adequately fit the profile of the $J=18\rightarrow 17$ and $20\rightarrow 19$
lines of HC13CCN and HCC13CN using the stellar-shell model.

####  3.1.15 Cyclopropenylidene – $c$-C3H2

The first detection of $c$-C3H2 in a PPN (CRL 618) was made by Bujarrabal et
al. (1988), who also observed CRL 2688. But they did not report any $c$-C3H2
line in CRL 2688 although some lines are obviously lying in their spectral
coverage. We detected three $c$-C3H2 lines, as shown in Fig. 3. The double-
peaked profile suggests that they are likely to be optically thin.

####  3.1.16  Butadiynyl radical – C4H

C4H in CRL 2688 was detected for the first time by Lucas et al. (1986) through
two $N=10\rightarrow 9$ transition lines. But it was only tentatively seen in
CRL 618 by Bujarrabal et al. (1988). All the eight rotational lines lying
within our spectral coverage are detected with a prominent double-peaked
profile, as shown in Fig. 3. In addition, we detected six and two
vibrationally excited transition lines within the ${}^{2}\Pi_{3/2}$ and
${}^{2}\Pi_{1/2}$ states, respectively, as shown in Fig. 3. The C4H
$\nu_{7}=1~{}^{2}\Pi_{3/2}~{}J=33/2\rightarrow 31/2$ line is significantly
blended with the C13CCCH $N=17\rightarrow 16~{}J=35/2\rightarrow 33/2$ line
(see Fig. 3), and cannot be well fitted with the stellar-shell model. Due to
the low signal to noise ratio, it is hard to tell whether the profile of these
vibrationally excited lines is double-peaked or flat-topped.

####  3.1.17 Pentynylidyne Radical – $l$-C5H

The first detection of $l$-C5H in CRL 2688 was made by Highberger et al.
(2001) through the ${}^{2}\Pi_{1/2}~{}J=65/2\rightarrow 63/2~{}l=e~{}\&~{}f$
line. Several vibrationally excited transition lines of $l$-C5H at
${}^{2}\Pi_{3/2}$ and ${}^{2}\Pi_{1/2}$ states were subsequently detected
(Highberger et al., 2003; Zhang et al., 2013). We detected six vibrationally
excited lines of $l$-C5H, as shown in Fig. 3. Each line is split into $l=e$
and $f$ hyperfine structure components. Other six vibrationally excited lines
of $l$-C5H are lying within our spectral coverage but not being detected due
presumably to their intrinsic weakness; their intensity upper limits are
estimated to be 10 mK.

####  3.1.18 Methyl cyanide – CH3CN

Matthews & Sears (1983) failed to detect CH3CN in CRL 2688, which was then
discovered by Bujarrabal et al. (1988) through the $J=8\rightarrow 7$ line.
More transition lines were subsequently detected by Zhang et al. (2013). We
detected the $J=5\rightarrow 4$, $9\rightarrow 8$, and $10\rightarrow 9$ lines
of CH3CN, as shown in Fig. 3. Each line includes several hyperfine structure
components. The CH3CN $J=9\rightarrow 8$ line is blended with the SiC2
$J_{K_{a},K_{c}}=7_{4,3}\rightarrow 6_{4,2}$ and $7_{4,4}\rightarrow 6_{4,3}$
lines (see Fig. 3).

####  3.1.19 Cyanodiacetylene – HC5N

The observations of Bujarrabal et al. (1988) using the IRAM 30 m telescope
resulted in an upper limit of the HC5N abundance in CRL 2688. This molecule
then detected by Truong-Bach et al. (1988) with the Effelsberg 100 m telescope
through the $J=8\rightarrow 7$ and $9\rightarrow 8$ lines at 21 and 24 GHz,
respectively. We detected nine transition lines of HC5N $J=29\rightarrow 18$,
$30\rightarrow 29$, $31\rightarrow 30$, $35\rightarrow 34$, $36\rightarrow
35$, $27\rightarrow 36$, $61\rightarrow 60$, $62\rightarrow 61$, and
$63\rightarrow 62$, as shown in Fig. 3. Generally, multiple peaks are found in
the line profiles, specially for the low-$J$ transition lines.

####  3.1.20 Hexatriynyl radical – C6H

C6H was marginally detected in CRL 2688 with the ARO 12 m telescope by
Highberger et al. (2001), and then the detection of the line at 102 GHz was
confirmed Highberger et al. (2003) using the IRMA 30 m telescope. Six and five
vibrationally excited transition lines of C6H, respectively in the
${}^{2}\Pi_{3/2}$ and ${}^{2}\Pi_{1/2}$ states, are detected in our
observations as shown in Fig. 3. The profiles of the eight strongest C6H lines
are well fitted by using the stellar-shell model, suggesting an optically thin
nature.

####  3.1.21 Cyanotriacetylene – HC7N

The first detection of HC7N in CRL 2688 was made by Nguyen-Q-Rieu et al.
(1984) using the Effelsberg 100 m telescope through the $J=21\rightarrow 20$
line. The present observation detected three new lines, $J=68\rightarrow 67$,
$71\rightarrow 70$, and $83\rightarrow 82$, as shown in Fig. 3. The
$J=69\rightarrow 68$ line is only marginally detected because of a blending
with the NaCN $J_{K_{a},K_{c}}=5_{0,5}\rightarrow 4_{0,4}$ line (see Fig. 3).

####  3.1.22 Unidentified lines

Four U-lines that cannot be assigned to known transition lines of any
molecular species are newly detected, increasing the number of U-lines in CRL
2688 to 13. It is clear that the frequency spans of these U-lines cannot be
accounted for by a single rotation constant, suggesting that they are unlikely
to arise from the same species. Probably they are associated with unknown
circumstellar molecules or internal rotations of polymer molecules, which call
for further experimental investigation. For comparison, we examine the U-lines
in the spectrum of IRC+10216 ranging from 129.0 to 172.5 GHz presented by
Cernicharo et al. (2000), but do not find any match. We cannot rule out the
possibility that some of them are artificial features.

###  3.2 A comparison with previous line survey observations of CRL 2688

Our spectral range has overlaps with those of Park et al. (2008) and Zhang et
al. (2013) in frequency coverages, allowing us to make a comparison between
these measurements. Within the overlapped frequency range from 91.38–99.16
GHz, the CS $J=2\rightarrow 1$ line is the only line discovered by Park et al.
(2008), while we unambiguously detect 35 lines. All the 23 lines observed by
Zhang et al. (2013) in the overlapped frequency range from 75.70–83.48 GHz are
visible in our spectra, in which we totally detect 102 lines. The HPBWs of the
IRAM 30 m and ARO 12 m telescopes are different (69$\hbox
to0.0pt{.\hss}\arcsec$4–64$\hbox to0.0pt{.\hss}\arcsec$0 and 26$\hbox
to0.0pt{.\hss}\arcsec$9–24$\hbox to0.0pt{.\hss}\arcsec$8, respectively, for
the overlapped frequency range), resulting in different main beam temperatures
for the the same line between ours and that measured by Zhang et al. (2013).
Through a comparison between these measurements, we can roughly estimate the
size of the region where the molecular lines originate. The same technique has
been used by Takano et al. (2019) and Qiu et al. (2020) to investigate the
molecular emission regions in extragalactic objects.

With the assumption of a Gaussian intensity distribution, the velocity-
integrated intensities measured by different telescopes should satisfy the
relationship $I_{\rm mb}(\theta_{\rm b}^{2}+\theta_{\rm
s}^{2})=I^{{}^{\prime}}_{\rm mb}(\theta_{\rm b}^{{}^{\prime}2}+\theta_{\rm
s}^{2})$, where ($I_{\rm mb},\theta_{\rm b}$) and ($I_{\rm
mb}^{{}^{\prime}},\theta_{\rm b}^{{}^{\prime}}$) represent the intensities and
beam sizes corresponding to the IRAM 30 m and ARO 12 m telescopes,
respectively, and $\theta_{\rm s}$ represents the Gaussian full width at half-
maximum (FWHM) of the source size. To estimate $\theta_{\rm s}$, we define a
parameter for the $i$th line

$f_{i}=\left|\frac{I_{{\rm mb},i}(\theta_{\rm b}^{2}+\theta_{\rm
s}^{2})}{I^{{}^{\prime}}_{{\rm mb},i}(\theta_{\rm
b}^{{}^{\prime}2}+\theta_{\rm s}^{2})}-1\right|\times\frac{1}{\sigma_{i}},$
(1)

where $\sigma_{i}$ is the uncertainty of $I_{{\rm mb},i}/I_{{\rm
mb},i}^{{}^{\prime}}$. Then an $F$ factor is introduced by $F=\sum_{i}f_{i}$.
Here, $f_{i}$ is the residual of the ratio between the velocity-integrated
intensities measured by IRAM 30 m and ARO 12 m telescopes considering the beam
dilution weighted by 1/$\sigma_{i}$. The source size $\theta_{\rm s}$ can be
obtained by minimizing the $F$ factor. We find that the $F$ factor reaches the
minimum value if the source size is 15$\hbox to0.0pt{.\hss}\arcsec$5, as shown
in the upper panel of Fig. 4. The CO mapping of CRL 2688 in the
$J=1\rightarrow 0$ line shows that the emission is dominantly from the central
core with a radius of 10$\arcsec$–13$\arcsec$ (Kawabe et al., 1987), while the
13CO image of the $J=1\rightarrow 0$ line reveals a high-brightness central
core, a high-velocity component near the center, and a less bright extended
envelope with a diameter of about 15$\arcsec$ (Yamamura et al., 1995). These
observations are in reasonable agreement with our size estimation based on
single-dish data. The $F$ factor cannot reach zero (see Fig. 4), probably
suggesting that the molecular gas has a stratified structure and/or the
brightness distributions deviate from the Gaussian assumption. To further
examine this, we plot in the lower panel of Fig. 4 the $[I_{\rm
mb}(\theta_{\rm b}^{2}+\theta_{\rm s}^{2})]/[I^{{}^{\prime}}_{\rm
mb}(\theta_{\rm b}^{{}^{\prime}2}+\theta_{\rm s}^{2})]$ ratios for all the
lines lying within the overlapped frequency range, where $\theta_{\rm
s}=15\hbox to0.0pt{.\hss}\arcsec$5\. The deviations from one in this plot is
an indication that different molecules may have different spatial
distributions, and thus the obtained source size only represent the average
value.

Alternatively, we could assume that the source has a disk geometry, where the
brightness distribution is constant within a diameter of $\theta_{\rm d}$.
Then we have $I_{\rm mb}/(1-2^{-\theta^{2}_{\rm d}/\theta^{2}_{\rm
b}})=I^{{}^{\prime}}_{\rm mb}/(1-2^{-\theta^{2}_{\rm
d}/\theta^{{}^{\prime}2}_{\rm b}})$. Following a similar way as described
above, but defining $f_{i}$ by

$f_{i}=\left|\frac{I_{{\rm mb},i}/(1-2^{-\theta^{2}_{\rm d}/\theta^{2}_{\rm
b}})}{I^{{}^{\prime}}_{{\rm mb},i}/(1-2^{-\theta^{2}_{\rm
d}/\theta^{{}^{\prime}2}_{\rm b}})}-1\right|\times\frac{1}{\sigma_{i}},$ (2)

we find $\theta_{\rm d}=24\hbox to0.0pt{.\hss}\arcsec$7 when $F$ approaches
the minimum (see the upper panel of Fig. 4). This means that we would obtain
the same beam-filling factors if assuming $\theta_{\rm d}=24\hbox
to0.0pt{.\hss}\arcsec$7 for a disk distribution or assuming $\theta_{\rm
s}=15\hbox to0.0pt{.\hss}\arcsec$5 for a Gaussian distribution. Therefore, the
$[I_{\rm mb}(1-2^{-\theta^{2}_{\rm d}/\theta^{{}^{\prime}2}_{\rm
b}})]/[I^{{}^{\prime}}_{\rm mb}(1-2^{-\theta^{2}_{\rm d}/\theta^{2}_{\rm
b}})]$ ratios for $\theta_{\rm d}=24\hbox to0.0pt{.\hss}\arcsec$7 are equal to
the $[I_{\rm mb}(\theta_{\rm b}^{2}+\theta_{\rm s}^{2})]/[I^{{}^{\prime}}_{\rm
mb}(\theta_{\rm b}^{{}^{\prime}2}+\theta_{\rm s}^{2})]$ ratios for
$\theta_{\rm s}=15\hbox to0.0pt{.\hss}\arcsec$5, as shown in the lower panel
of Fig. 4. It is natural that $\theta_{\rm s}$ is smaller than $\theta_{\rm
d}$ since the former is the FWHM of the surface brightness profile while the
latter represents the extent of the source.

###  3.3 Rotation diagram, column Densities, and fractional Abundances

The column densities ($N$) and excitation temperatures ($T_{\rm ex}$) of 22
molecules and isotopologues, including CS, 13CS, C33S, C34S, HCN, HNC, HCO+,
SiS, C2S, SiC2, $l-$C3H, C3N, HC3N, H13CCCN, HC13CCN, HCC13CN, C4H, $c$-C3H2,
C6H, CH3CN, HC5N, and HC7N, are estimated by means of the rotation diagram.
Under the assumption of local thermal equilibrium (LTE) and negligible optical
depth, the level populations can be expressed with

${\rm ln}\frac{N_{\rm u}}{g_{\rm u}}={\rm ln}\frac{8\pi k\,\nu^{2}\,\int
T_{\rm S}\,dv}{hc^{3}\,A_{\rm ul}\,g_{\rm u}}\,={\rm ln}\frac{N}{Q(T_{\rm
ex})}-\frac{E_{\rm u}}{kT_{\rm ex}},$ (3)

where $k$ and $h$ are the Boltzmann constant and Planck constant, $\nu$ is the
rest frequency in Hz, $Q(T_{\rm ex})$, $A_{\rm ul}$, $g_{\rm u}$ are the
partition function, spontaneous emission coefficient, and upper state
degeneracy, respectively. Under the assumption of a Gaussian distribution of
the surface brightness, the source brightness temperature $T_{\rm S}$ is
derived after correcting for beam dilution, $T_{\rm S}=T_{\rm mb}(\theta_{\rm
b}^{2}+\theta_{\rm s}^{2})/\theta_{\rm s}^{2}$, where $\theta_{\rm b}$ is the
antenna HPBW and $\theta_{\rm s}$ is the source size. $\int T_{\rm S}\,d\nu$
is the velocity-integrated intensity. $Q(T_{\rm ex})$, $A_{\rm ul}$, $g_{\rm
u}$, and $E_{\rm u}/k$ are taken from the CDMS catalog. Following the previous
works (Park et al., 2008; Zhang et al., 2013), we simply assume a source size
of 20′′ for the calculations of all the molecules. The resultant rotation
diagrams are presented in Fig. 5, and the results are listed in Table 4.

If the optical depth ($\tau$) poses an effect, the dots in the rotation
diagram might deviate from a linear distribution. If this is the case, an
optical depth correction factor of $C_{\tau}=\tau/(1-e^{-\tau})$ needs to be
introduced to construct the rotation diagrams (Goldsmith & Langer, 1999).
However, such a deviation cannot be clearly seen in Fig. 5. Using the
population diagram method developed by Goldsmith & Langer (1999), we estimate
$\ln C_{\tau}<0.03$, suggesting that the effect of optical depth is
negligible.

We use the expression given by Olofsson (1997) to derive the molecular
abundances with respect to molecular hydrogen ($f_{X}$),

$f_{\rm X}=1.7\times 10^{-28}\frac{v_{\rm e}\theta_{\rm b}D}{\dot{M}_{\rm
H_{2}}}\frac{Q(T_{\rm ex})\nu^{2}}{g_{\rm u}A_{\rm ul}}\frac{e^{E_{\rm
l}/kT_{\rm ex}}\int T_{\rm mb}dv}{\int^{x_{e}}_{x_{i}}e^{-4x^{2}{\rm
ln2}}dx},$ (4)

where $v_{\rm e}$ is the expansion velocity in km s-1, $D$ is the distance in
pc (assumed to be 1 kpc hereafter), $\dot{M}_{\rm H_{2}}$ is the mass-loss
rate in $M_{\odot}$ yr-1, $\nu$ is the rest frequency in GHz, $E_{\rm l}$ is
the energy of the lower level, $\int T_{\rm mb}dv$ is the velocity-integrated
intensity in K km s-1, $x_{e,i}=R_{e,i}/(\theta_{\rm b}D)$ with $R_{e}$ and
$R_{i}$ for the outer and inner radii of the shell, and $\theta_{\rm b}$,
$Q(T_{\rm ex})$, $g_{\rm u}$, $A_{\rm ul}$, and $k$ are the same as in
Equation 1. To simplify the calculations, we assume the values of $x_{e}$ and
$x_{i}$ to be 1 and 0 for all the species, respectively. For the molecules
that have more than one transition line detected, we adopt the average
abundances weighted by their velocity-integrated intensities. As mentioned in
Zhang et al. (2009), the abundances is essentially insensitive to $T_{\rm ex}$
in the case of $E_{\rm l}\ll kT_{\rm ex}$. The results are listed in Table 4.
It should be noted that if the transition lines are optically thick, the
derived $f_{\rm X}$ values should be considered as lower limits.

###  3.4 The fractional abundances derived from radiative transfer method

In order to determine the fractional abundances ($f_{X}$ in Table 4), we have
assumed that all the lines originating from a given molecule are characterized
by a single excitation temperature, which is generally valid if the molecule
is located within a narrow layer. However, CRL 2688 exhibits a moderate radial
temperature gradient (Truong-Bach et al., 1990). For the abundance computation
of molecules lying within a region with wide temperature variation, a more
sophisticated approach is to use a radiative transfer model. As such, we
redetermine the fractional abundances using the three-dimensional non-local-
thermodynamical (NLTE) radiative transfer code LIME (Brinch & Hogerheijde,
2010). We limited the calculations to a few strong lines, aiming to examine
the systematic uncertainties in the determination of $f_{X}$.

For the modelling, we assume a spherically symmetric structure with a velocity
law of $v(r)=v_{\rm exp}(1-{r_{0}}/{r})^{0.5}$, where $r_{0}$ is the inner
radius of the envelope and is taken to be $10^{15}$ cm (Truong-Bach et al.,
1990). Such a velocity law is the solution of the equations of the stellar
wind driven by a force decreasing with $r^{-2}$. The terminal velocity $v_{\rm
exp}$ is taken from Table Molecules in the carbon-rich protoplanetary nebula
CRL 2688. In most parts of the envelope ($r>10^{16}$ cm), $v(r)$ is very close
to $v_{\rm exp}$. We thus simply assume a H2 number density law of
$n(r)=\dot{M}/(4\pi m_{\rm H_{2}}r^{2}v_{\rm exp})$, where $m_{\rm H_{2}}$ is
the mass of a H2 molecule, and the mass-loss rate $\dot{M}$ is taken to be
$1.7\times 10^{-4}$ $M_{\odot}$ yr-1 (Truong-Bach et al., 1990) . Even though
the assumed mass-loss rate may be slightly larger than the current value of
CRL 2688, we consider that it would be acceptable as a first approximation.
Moreover, we apply a radial distribution of the kinetic temperature obtained
by Truong-Bach et al. (1990) through high-resolution mapping of the CO
$J=1\rightarrow 0$ and $2\rightarrow 1$ lines. The molecular data, including
the energy states, Einstein coefficients, and collisional rates, are taken
from the LAMDA database555http://www.strw.leidenuniv.nl/ moldata (Schöier et
al., 2005). The output spectral resolution is set to be 1 km s-1.

Fixing the distance between CRL 2688 and earth at 1 kpc and allowing the inner
radius ($R_{i}$) and outer radius ($R_{e}$) of individual molecules to vary,
we determine the fractional abundance by fitting the LIME model to the
observations. The fittings for the CS $J=2\rightarrow 1$, SiS $J=10\rightarrow
9$, PN $J=2\rightarrow 1$, SiC2 $J_{K_{a},K_{c}}=4_{0,4}\rightarrow 3_{0,3}$,
HCN $J=2\rightarrow 1$, HNC $J=2\rightarrow 1$, HCO+ $J=2\rightarrow 1$, HC3N
$J=9\rightarrow 8$, $c$-C3H2 $J_{K_{a},K_{c}}=2_{0,2}\rightarrow 1_{1,1}$, and
CH3CN $J_{K}=5_{3}\rightarrow 4_{3}$ lines are shown in Fig 3. The LIME models
appear to match the main components of the observed line profiles reasonably
well, but fail to reproduce the broad wings of CS, SiS, HCN, and HNC lines. An
interferometric observation has revealed that HCN and HC3N emission primarily
traces dense gas in polar and equatorial bipolar outflows (Dinh-V-Trung et
al., 2020). Therefore, the mismatches between the models and the observations
may suggest a deviation from a spherical geometry. In order to reproduce the
present observations more closely, it would be necessary to assume a more
realistic geometry, which is possibly determined by high-angular resolution
interferometry. However, such the observation is beyond the scope of the
present research. Table 5 summarizes our fitting results, showing that the
agreement between the molecular abundances derived from the LIME models and
$f_{X}$ is better than one order of magnitude.

###  3.5 Isotopic abundance ratios

Isotopic abundance ratios reflect the nucleosynthesis in the stellar interior
and mixing processes (Herwig, 2005). We determine the isotopic abundance
ratios of C, S, Si, and Mg elements in CRL 2688 based on the intensities of
the corresponding molecular transition lines. The results, along with those of
the Sun and IRC+10216 (Lodders, 2003; Kahane et al., 2000), are given in Table
6. The 12C/13C ratio is believed to be one of the most useful tracers of the
relative degree of primary to secondary processing in stars, which drops
during the first dredge-up and is enhanced by the third dredge-up. Our
measurements suggest a 12C/13C ratio of 15–40 in CRL 2688, slightly lower than
that in IRC+10216 and significantly lower than the solar value. This can be
attributed to the more evolved nature of CRL 2688. Strikingly, the 12C/13C
ratios derived from HC3N lines tend to increase with increasing $J$ (see Table
6). Chemical fractionation may alter the isotopic ratio. The reaction, 13C+ \+
CN $\rightarrow$ 13CN + 12C+, preferentially takes place under low temperature
(Watson et al., 1976; Langer, 1992; Milam et al., 2005). As CN is the key
parent molecule of cyanopolyynes, the enhanced 13CN would leads to a higher
abundance of the 13C isotopologues of HC3N in the cool region. If the gas
temperature of the CSE has a negative gradient form the inner side to the
outer side, the low-$J$ lines preferentially arise from the outer low-
temperature region, respective to the high-$J$ lines. This offers a seemingly
plausible explanation for the observations that the HC3N low-$J$ lines result
in a lower 12C/13C ratio. However, chemical fractionation is efficient only at
an extremely low kinetic temperature. Through theoretical calculations, Milam
et al. (2005) found that for a molecular cloud with a kinetic temperature
ranging from 10–100 K, the effect of fractionation causes negligible
alteration of the 12C/13C ratio during the typical life timescale ($10^{6}$
yr). Although PPNs are denser than molecular clouds, tending to decrease the
reaction timescale, their lifetime is much shorter ($\sim$103 yr). Moreover,
PPNs are energetic sources with a kinetic temperature of much higher than 10
K. Therefore, chemical fractionation is unlikely to affect the 12C/13C ratio
in CRL 2688. Isotope-selected photodissociation could also affect the 12C/13C
isotopologue ratio. For instance, 12CO is more abundant than 13CO, and thus is
more self-shielding from the interstellar UV field, resulting in an increasing
12C/13C ratio. HC3N is the product of photochemistry of HCN and CN, which thus
would result in a radial 12C/13C gradient for HC3N if their isotopologues have
different self-shielding factors. However, unlike CO, HCN and CN are
dissociated mainly in the continuum rather than bands, suggesting that their
isotopologues are equally affected by UV radiation (Saberi et al., 2017). As a
result, we can rule out the isotope-selected photodissociation as the cause of
the 12C/13C gradient observed for HC3N.

Giesen et al. (2020) found that the C13CC/13CCC isotopic ratio in the massive
star-forming region Sgr B2(M) is higher than the statistically expected value,
which is probably due to the lower zero energy of C13CC relative to 13CCC
making the position-exchange reaction of converting 13CCC to C13CC favorable.
We find that the column densities of HC13CCN, H13CCCN, and HCC13CN are quite
consistent with each other, suggesting that such an effect is not significant
for HC3N isotopolgues.

Presumably, the isotopic abundance ratios of very heavy elements are not
altered during the AGB evolution. We do not find a significant difference of
the S, Si, and Mg isotopic abundance ratios between CRL 2688, IRC+10216, and
the Sun, being consistent with the prediction of stellar theory. The only
exception is the CS $J=2\rightarrow 1$ line which gives a relatively low
32S/34S ratio. But considering that this line is likely to be optically thick,
the resultant S isotopic ratio only represents a lower limit.

##  4 Discussion

###  4.1 Temporal variations

Evidences of temporal variations of molecular lines in IRC+10216 were
presented by Cernicharo et al. (2014), who found that most of the abundant
molecules, except SiC2, exhibit strong variations in line intensities and the
variations are more pronounced for high-$J$ lines. This can be attributed to
the variations in the IR pumping rate. In this scenario, optically thick
lines, such as low-$J$ lines of SiC2, are insensitive to the IR pumping. Such
studies are relevant to the reliability of the intensities of molecular lines
in AGB envelopes as the flux calibrator and the tracer of mass loss rate.

CRL 2688 is a commonly used standard source for flux calibration. The
brightness of its lobes has been found to increase by $-0.22$ V magnitude
during 1994–2008 (Hrivnak et al., 2010). Klochkova et al. (2000) found that
the radial velocity of the envelope varies with time. Presumably, the rapid
variable stellar brightness and circumstellar dynamics may lead to temporal
variations of the intensities of some molecular lines in the CSE. The C4H
$J=21\rightarrow 19$, C4H $J=19\rightarrow 17$, SiC2
$J_{K_{a},K_{c}}=4_{2,2}\rightarrow 3_{2,1}$, and H13CCCN $J=11\rightarrow 10$
lines in CRL 2688 were also observed in May 1985 by Lucas et al. (1986) using
the same observational configurations as ours. We investigated the intensity
variations using the data sets spanning over 32 years. The velocity-integrated
intensity ratios of the four lines between the measurements of Lucas et al.
(1986) and ours are $1.6\pm 0.1$, $1.7\pm 0.1$, $0.6\pm 0.2$, and $1.2\pm
0.1$, respectively. It is notable that the two C4H lines show a consistently
increasing velocity-integrated intensity ratio, whereas no significant
variation is found for the SiC2 and H13CCCN lines. Although we cannot
conclusively attribute this to the luminosity variations, it would be
interesting to study the different variational behaviors of these molecular
lines in the future.

###  4.2 A comparison with previous line survey observations of IRC+10216

A $\lambda$ = 2 mm line survey of IRC+10216, the prototype of carbon-rich
CSEs, has been presented by Cernicharo et al. (2000) with the IRAM 30 m
telescope. An overlap of their and our spectra occurs in the spectral range
from 160.8 to 168.6 GHz. To investigate the alteration of physical and
chemical environments during the AGB–PPN evolution, we make a comparison of
the velocity-integrated intensities of the lines in the overlapping range
between IRC+10216 and CRL 2688. The line intensities, normalized such that
$I$(HC3N $J$=18$\rightarrow$17) = 1, are shown in Fig. 6, which reveals
systematic differences. The 13C isotopologues of HC3N shows slightly
strengthened intensities in CRL 2688 with an unknown reason (see Section 3.5).
CRL 2688 also exhibits enriched cyanides, such as HC5N and CH3CN, which can be
ascribed to the enhancement of CN fragments resulted from the photolysis of
HCN during the AGB–PPN transition. The vibrational excitation line of HC3N is
largely strengthened in CRL 2688, suggesting a higher temperature in the inner
regions of the PPN where this line arises. In CRL 2688, the lines of C-rich
molecules, such as C3N, $l$-C3H, and C4H, and of Si-bearing molecules, such as
SiS and SiC2, appear to be consistently weaker than those in IRC+10216, This
can be interpreted if IRC+10216 is intrinsically richer in carbon abundance
and the gas-phase refractory-element molecules in CRL 2688 have been heavily
depleted onto grains during the PPN expansion. The depletion of SiC2 of CRL
2688 relative to IRC+10216 is consistent with the 3 mm observations of Lucas
et al. (1986). However, Lucas et al. (1986) suggested an enhanced C4H in CRL
2688, contrary to our result and that of Zhang et al. (2013). This might be
due to the effect of excitation conditions and spatial distribution of this
molecules, as pointed out by Zhang et al. (2013). We find that the column
density of $l$-C3H is higher than that of $c$-C3H by a factor of 3 in CRL
2688, where the $l$-C3H/$c$-C3H ratio is close to that of IRC+10216 ($\sim$2;
Cernicharo et al., 2011). The higher $l$-C3H/$c$-C3H ratio in IRC+10216
relative to that in the dark cloud TMC-1 has been regarded as an evidence of
atom-neutral reactions (Kaiser et al., 1996).

###  4.3 Modelling and circumstellar chemistry

Chemical modelling is a powerful tool for deepening our understanding of
circumstellar chemistry (e.g. Millar & Herbst, 1994; Agúndez et al., 2020).
The abundance patterns of many AGB molecules can be satisfactorily reproduced
by state-of-the-art astrochemistry models, which, however, seemingly perform
less well at explaining the formation of complex molecules in PPNs. New
observations continue to show unexpected results, prompting the development of
models. To investigate the shortcomings of the models in studying PPN
chemistry, we compare the observations with model predictions. For the
modelling, we use the package presented by McElroy et al. (2013), which is
based on an updated UMIST Database for Astrochemistry (RATE12). The reaction
network consists of 6173 gas-phase reactions and 195 reactions on grain
surfaces involving 467 atomic and molecular species. We assume an uniform
loss-rate of 3.0 $\times$ 10-5 $M_{\odot}$ yr-1 (Lo & Bechis, 1976) and an
spherically symmetrical expansion velocity of 16.5 km s-1 (Crampton et al.,
1975). The gas density radially decreases as 1/$r^{2}$ with an inner radius of
$1.5\times 10^{16}$ cm ($\sim$1$\arcsec$ at $D=1$ kpc) where the parent
species are injected into the CSE and an outer radius of 1.5 $\times$ 1018 cm
($\sim$100$\arcsec$ at $D=1$ kpc) where the majority of molecules are
dissociated by a standard interstellar ultraviolet radiation field. The
radiation field of the central star is ignored.

The modelling results are listed in Table 4 and are compared with the
observations in Fig. 7. We find that for 19 out of the 26 detected molecular
species, the predicted and observed column densities show agreement within one
order of magnitude. An excellent agreement is shown for the abundance patterns
of cyanopolyynes (HC2n-1N: $n=1$–4); the only exception is HCN, which exhibits
a much lower observed abundance and could be attributed to the effect of
optical depth. For other C-rich species, CnH ($n=2$–6) and CnS ($n=1$–3), the
models can reproduce the observations to a reasonable accuracy; there are only
slight disagreements for C5H and and C6H. Based on the 20 GHz band data
obtained with the 100 m Green Bank Telescope (GBT), Gupta et al. (2009)
derived the C6H column density of $4.5\times 10^{12}$ cm-2, which is about six
times lower than our result, and even worse when comparing with the modelling
result. Thus the disagreements are likely to result from the uncertainties of
the model. The other molecules that cannot be modelled well are PN, H2CS,
H2CO, and SiO, which show the observed-to-calculated abundance ratios of 826,
42, 151, and 0.001, respectively. Figure 8 shows the modelled abundances of
the molecules in CRL 2688 as a function of radius as well as the observed
values. It is obvious that the observed abundances of PN, H2CS, and H2CO are
higher than the modelled results at any radius.

Apart from the uncertainties introduced by observations and data analysis, the
possible causes of the disagreements include poor knowledge of the physical
conditions, missing routes in the modelling, and inaccurate reaction rates. To
decouple the affects of uncertain physical conditions, we compare the
observed-to-calculated abundance ratios of the molecules in IRC+10216, the
progenitor object of CRL 2688. The results are shown in Fig. 7. The modelled
and observed data of IRC+10216 are taken from McElroy et al. (2013) and the
references therein. Generally, the agreement of the molecules in IRC+10216 is
slightly better than those in CRL 2688. However, it is interesting to note
that the models overestimate the abundances PN, H2CS, and H2CO in CRL 2688 and
IRC+10216 by about the same degree, suggesting that significant reaction
routes for forming these molecules might been neglected or undervalued in the
modelling.

The enhancement of SiO has been commonly observed in C-rich AGB stars, and can
be attributed to shock-induced chemistry (Schöier et al., 2006) that has not
been taken into account in this model. CRL 2688 shows rich shock features
caused by fast outflows. However, the largely overestimated SiO abundances by
the model are against with the predictions of shock chemistry. We infer that
the model might substantially underestimate the freeze-out of SiO in the PPN
stage. Alternatively, because the fast outflows in CRL 2688 are highly
collimated, the shock chemistry processes only in rather small regions and
does not play an important role on the enhancement of gas-phase SiO. Severe
depletion of SiO and SiS has been also found in another PPN IRAS $22272+5435$
(Zhang, 2020), which exhibits strong 21 $\micron$ feature. The CO observations
suggest that the material ejection of IRAS $22272+5435$ is dominated by the
spherical wind, but a jet is developing and interacting with ambient materials
(Nakashima et al., 2012). CRL 2688 has well developed jets and is thought to
just metamorphose from a ‘21 $\micron$ phase’ (Geballe et al., 1992).
Therefore, we reasonably conjecture that the depletion of Si-bearing molecules
and the development of jets may be associated to the phenomenon of the 21
$\micron$ feature in PPNs. Sensitive observations of a large PPN sample are
required to draw firm conclusion.

###  4.4 Metal-bearing molecules

CSEs surrounding evolved stars are characterized by unambiguous identification
of metal-bearing (Na-, Mg-, Al-, K-, Ca-, and Fe-bearing) molecules. These
molecules have never been discovered in other types of sources, even in the
rich molecular cloud TMC-1, which might be attributed to severe depletion onto
dust grains (Turner et al., 2005). So far, the detected metal-bearing
molecules in IRC+10216 include NaCl, NaCN, MgCN, MgNC, HMgNC, MgC2H, MgC3N,
MgC4H, MgC5H, MgC6H, AlNC, AlF, AlCl, KCl, KCN, CaNC, and FeCN (Cernicharo et
al., 2019; Pardo et al., 2021, and references therein), indicating a rich gas-
phase metal chemistry in C-rich circumstellar environments. The first
identification of metal halides was made by Cernicharo & Guélin (1987), who
also reported a tentative detection of AlF, later confirmed by Ziurys et al.
(1994). According to the chemical equilibrium calculations (Tsuji, 1973),
these stable closed-shell diatomic molecules are mostly formed in the warn
inner shell, where the LTE condition holds. Kawaguchi et al. (1993) identified
MgNC for the first time in the spectrum of IRC+10216 presented by Guélin et
al. (1986). The metal cyanide has an open-shell structure and should be formed
in the outer envelope along with other radicals, facilitating the successful
detection of NaCN in IRC+10216 (Guélin et al., 1993; Turner et al., 1994). The
different spatial distributions of metal halides and metal cyanides are
revealed by line profiles and high spatial resolution maps of IRC+10216
(Ziurys, 2006, and references therein). When the CSE expands, materials move
from the inner to outer regions. As a result, one could expect that the
AlF/MgNC abundance ratio decreases through the transition from the AGB to the
PPN phase. This can be tested by measuring the abundance of metal-bearing
molecules in CRL 2688.

Our observations result in the detection of AlF, MgNC, and NaCN in CRL 2688.
All the three Metal-bearing molecules have already been discovered by
Highberger et al. (2001) in the 1.2 and 2 mm windows. Apart from the three
species, the only metal-bearing molecule detected in CRL 2688 is NaCl
(Highberger et al., 2003). Unfortunately, there is no NaCl line lying within
our frequency range. Following Highberger et al. (2001), we plot the rotation
diagram of AlF, NaCN, and MgNC by assuming the source sizes to be respectively
5″, 5″, and 30″for the corrections of the beam-dilution effect, as displayed
in Fig. 9. For this we use the $J=4\rightarrow 3$, $5\rightarrow 4$, and
$8\rightarrow 7$ transitions for AlF, the $N=8\rightarrow 7$, $14\rightarrow
13$, and $15\rightarrow 14$ transitions for MgNC, and the
$J_{K_{a},K_{c}}=5_{0,5}\rightarrow 4_{0,4}$, $5_{1,4}\rightarrow 4_{1,3}$,
$6_{0,6}\rightarrow 5_{0,5}$, $6_{2,4}\rightarrow 5_{2,3}$,
$6_{1,5}\rightarrow 5_{1,4}$, $10_{1,9}\rightarrow 9_{1,8}$,
$10_{1,11}\rightarrow 10_{1,10}$, $10_{0,11}\rightarrow 9_{0,10}$, and
$10_{1,10}\rightarrow 9_{1,9}$ transitions for NaCN. Furthermore, we perform
the rotation-diagram analysis by assuming various source sizes, as listed in
Table 7. It is clear that AlF and MgCN have the highest and lowest excitation
temperatures, respectively, supporting their origins respectively from the
inner and outer shells. The excitation temperature of NaCN is closer to that
of MgCN, as opposed to the theoretical expectation that NaCN is an inner-layer
molecule.

For comparison, Figure 9 and Table 7 show the results of Highberger et al.
(2001) who used the same transitions for AlF, the $N=11\rightarrow 10$,
$12\rightarrow 11$, and $13\rightarrow 12$ transitions for MgNC, and the
$J_{K_{a},K_{c}}=9_{0,9}\rightarrow 8_{0,8}$, $10_{0,10}\rightarrow 9_{0,9}$,
$10_{3,8}\rightarrow 9_{3,7}$, $10_{3,7}\rightarrow 9_{3,6}$, and
$10_{2,8}\rightarrow 9_{2,7}$ transitions for NaCN. We find that the results
of AlF are consistent with those of Highberger et al. (2001), whereas there a
slight and significant disagreement for MgNC and NaCN, respectively. Under the
assumption of a source size of 5$\arcsec$, we obtain the column density of
NaCN an order of magnitude lower than that by Highberger et al. (2001).
However, the difference can be alleviated if NaCN arises from a more extended
shell. Based on observed line profiles, Highberger et al. (2003) suggested
that NaCN in CRL 2688 should be sited much farther away from the central star
than that in IRC+10216. In Fig. 10, we show the column densities calculated by
using different source sizes for the beam-dilution corrections. We can see
that if the source sizes of AlF and MgNC are 5$\arcsec$ and 20$\arcsec$,
respectively, our results are perfectly consistent with those of Highberger et
al. (2001). However, even for source size of 30$\arcsec$, the column density
of NaCN derived by Highberger et al. (2001) is still 6 times larger than our
value, suggesting that the disagreement cannot be completely ascribed to the
uncertainty of the source size.

The fractional abundances are determined following the approach described in
Highberger et al. (2001) and Highberger & Ziurys (2003). The input parameters
and the results are listed in Table 8. The assumed geometries are the same as
those used by Highberger et al. (2001) and Highberger & Ziurys (2003); AlF is
sited in a spherical envelope while NaCN and MgNC are confined on spherical
shells. We derive a AlF/MgNC abundance ratio of $\sim$0.7, significantly lower
than that in IRC+10216 ($\sim$3), supporting the theoretical expectation of
the CSE expansion. Further support for the chemical evolution comes from the
observations of CRL 618, the presumable descendant of CRL 2688, in which MgNC
is the only discovered metal-bearing molecule (Highberger & Ziurys, 2003).

The fractional abundances of these metal-bearing molecules can provide a lower
limit of the gas-phase elemental abundance. Our calculations show that
magnesium locked in MgNC has a Mg/H ratio of $2\times 10^{-8}$ (Table 8).
According to the element depletion parameters given by Jenkins (2009), one can
estimate that the gas-phase Mg/H ratio in the ISM is larger than 2 $\times$
10-6. If assuming that solar abundances can well reflect the total elemental
abundance and the depletion factor in the PPN is similar to that of the ISM,
we could conclude that less than $1\%$ gas-phase magnesium is in the form of
MgNC, and thus MgNC may not be the dominant Mg-bearing molecule in CRL 2688.

Fluorine has a cosmic abundance much lower than aluminium, and thus is the key
element of AlF. The astrophysical origin of fluorine remains a widely debated
problem (see, e.g., Indelicato et al., 2017; Spitoni et al., 2018, and
references therein). Because the only stable fluorine isotope, 19F, is fragile
in hot stellar interiors, the most possible explanation for cosmic fluorine
abundance is that most of the F escapes from the harsh stellar interiors
immediately after its production. The AGB star is an ideal source of F, where
19F is synthesized and brought to stellar surface during the He-burning
thermal pulses and the third dredge-up (Forestini et al., 1992; Jorissen et
al., 1992). The F enhancement in AGB stars has been confirmed by the
observations of F ions in PNs (e.g. Zhang & Liu, 2005). The most abundant
interstellar F-bearing molecule is HF, which, however, is inaccessible from
ground telescopes. CF+ has been found to be the second abundant one in
photodissociation regions (Neufeld et al., 2006). Our research group has
systematically searched for CF+ in CSEs, but failed to see any positive
detection (Zhang et al., 2014). Therefore, AlF provides an unique opportunity
to constrain the F abundance in CSEs. A discussion on this aspect has been
made by Highberger et al. (2001). We reexamine this problem using our data of
CRL 2688. The derived AlF/H2 abundance ratio is 2.7 $\times$ 10-8 (Table 8),
suggesting a F/H abundance of $>$1.4 $\times$ 10-8. The obtained lower limit
of F/H is only 2.4 times lower than the solar value of Lodders (2003), and is
presumably far below the fluorine abundance in CRL 2688 since AlF is not the
dominant F-bearing molecule in circumstellar environments. Although the
$\alpha$ element Al in the PPN can be enhanced through the third drudge-up,
given the fact that available hydrogen is much richer than aluminium in
circumstellar environments (solar Al/H =3 $\times$ 10-6; Lodders, 2003), the
abundance of HF should be larger than AlF by orders of magnitude. As a result,
we infer that CRL 2688 has a significant F enhancement. Combined with the
C-rich nature of CRL 2688, our results further confirm the third drudge-up in
AGB stars as the source of F production.

##  5 Summary

CRL 2688 is a prototypical source suitable for studying the chemical
alteration of CSEs during the transition from AGB to PPN phase. It might hold
key clues to investigate the puzzle of the 21 $\micron$ feature. This paper
reports a new observation towards this PPN at the 3 mm and 2 mm bands, which
results in clear detections of 196 transition lines arising from 38 molecules.
Although no new circumstellar molecule is discovered, 153 transition lines and
13 molecules are new detections in this object. We compile a comprehensive
list of the gas-phase molecules discovered in CRL 2688 so far, providing a
valuable reference for the community of circumstellar chemistry. By comparing
the line intensities observed by different telescopes, we estimate a Gaussian
brightness FWHM of 15$\hbox to0.0pt{.\hss}\arcsec$5 or a source size of
27$\hbox to0.0pt{.\hss}\arcsec$7 for a disk geometry, although the exact sizes
are different from species to species. The standard chemical modelling is
unable to reproduce the observed abundances of PN, H2CS, and H2CO, suggesting
that some key reaction routes may have been missed in the chemical network.
Through a comparison between the molecular abundances in IRC+10216, CRL 2688,
and CRL 618, we conclude that the differences can be partially attributed to
the effect of CSE evolution. The three CSEs are the only sources where metal-
bearing molecules are detected. The results of the analysis support the
theoretical expectations that AlF is confined in the inner shell while MgNC
and NaCN are located on extended regions. An enhancement of F in the PPN is
revealed by the fractional abundance of AlF relative to H2.

The chemical processes in CSEs are complex. In-depth explorations of this
research field call for high-sensitivity and broad-frequency-coverage
observations of a large sample, which, however, are relatively scarce, and
have been limited to only a few bright CSEs. In the future, such efforts
should be vigorously pursued to decouple the effects of various factors,
including stellar and circumstellar properties (stellar mass, metallicity,
binary system, mass-loss rate, wind velocity, radiation field, dust formation,
shocks, etc.), on the circumstellar chemistry.

We would like to thank an anonymous referee whose comments significantly
improved the paper. This work was supported by National Science Foundation of
China (NSFC, Grant No. 11973099) awarded to YZ. JJQ and JSZ thanks the support
of NSFC (Grant Nos. 12003080, 12041302, and 12073088), the China Postdoctoral
Science Foundation funded project (No. 2019M653144), the Guangdong Basic and
Applied Basic Research Foundation (No. 2019A1515110588), and the Fundamental
Research Funds for the Central Universities (Sun Yat-sen University, No.
2021qntd28 and 19lgpy284). We also acknowledge the science research grants
from the China Manned Space Project with NO. CMS-CSST-2021-A09 and CMS-
CSST-2021-A10. We wish to express my gratitude to the staff at the IRAM 30 m
telescope for their kind help and support during our observations. IRAM is
supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). This work
presents results from the European Space Agency (ESA) space mission Gaia. Gaia
data are being processed by the Gaia Data Processing and Analysis Consortium
(DPAC). Funding for the DPAC is provided by national institutions, in
particular the institutions participating in the Gaia MultiLateral Agreement
(MLA). The Gaia mission website is https://www.cosmos.esa.int/gaia. The Gaia
archive website is https://archives.esac.esa.int/gaia.

## References

  * Agúndez et al. (2020) Agúndez, M., Martínez, J. I., de Andres, P. L., et al. 2020, A&A, 637, A59
  * Bachiller et al. (1997) Bachiller, R., Forveille, T., Huggins, P. J., et al. 1997, A&A, 324, 1123
  * Balick et al. (2012) Balick, B., Gomez, T., Vinković, D., et al. 2012, ApJ, 745, 188
  * Bloecker (1995) Bloecker, T. 1995, A&A, 299, 755
  * Brinch & Hogerheijde (2010) Brinch, C. & Hogerheijde, M. R. 2010, A&A, 523, A25
  * Bujarrabal et al. (1988) Bujarrabal, V., Gomez-Gonzalez, J., Bachiller, R., et al. 1988, A&A, 204, 242
  * Buss et al. (1993) Buss, R. H., Tielens, A. G. G. M., Cohen, M., et al. 1993, ApJ, 415, 250
  * Cernicharo (2004) Cernicharo, J. 2004, ApJ, 608, L41
  * Cernicharo et al. (2011) Cernicharo, J., Agúndez, M., & Guélin, M. 2011, The Molecular Universe, 280, 237
  * Cernicharo et al. (2019) Cernicharo, J., Cabezas, C., Pardo, J. R., et al. 2019, A&A, 630, L2
  * Cernicharo & Guélin (1987) Cernicharo, J. & Guélin, M. 1987, A&A, 183, L10
  * Cernicharo et al. (2000) Cernicharo, J., Guélin, M., & Kahane, C. 2000, A&AS, 142, 181
  * Cernicharo et al. (2010) Cernicharo, J., Waters, L. B. F. M., Decin, L., et al. 2010, A&A, 521, L8
  * Cernicharo et al. (2014) Cernicharo, J., Teyssier, D., Quintana-Lacaci, G., et al. 2014, ApJ, 796, L21
  * Cohen & Kuhi (1977) Cohen, M. & Kuhi, L. V. 1977, ApJ, 213, 79
  * Cox et al. (2000) Cox, P., Lucas, R., Huggins, P. J., et al. 2000, A&A, 353, L25
  * Crampton et al. (1975) Crampton, D., Cowley, A. P., & Humphreys, R. M. 1975, ApJ, 198, L135
  * Dinh-V-Trung et al. (2020) Dinh-V-Trung, Lim, J., & Ohyama, Y. 2020, ApJ, 896, 91
  * Forestini et al. (1992) Forestini, M., Goriely, S., Jorissen, A., et al. 1992, A&A, 261, 157
  * Fukasaku et al. (1994) Fukasaku, S., Hirahara, Y., Masuda, A., et al. 1994, ApJ, 437, 410
  * Geballe et al. (1992) Geballe, T. R., Tielens, A. G. G. M., Kwok, S., & Hrivnak, B. J. 1992, ApJ, 387, L89
  * Gong et al. (2015) Gong, Y., Henkel, C., Spezzano, S., et al. 2015, A&A, 574, A56
  * Giesen et al. (2020) Giesen, T. F., Mookerjea, B., Fuchs, G. W., et al. 2020, A&A, 633, A120
  * Ginsburg & Mirocha (2011) Ginsburg, A. & Mirocha, J. 2011, Astrophysics Source Code Library. ascl:1109.001
  * Goldsmith & Langer (1999) Goldsmith, P. F. & Langer, W. D. 1999, ApJ, 517, 209
  * Guélin et al. (1986) Guélin, M., Cernicharo, J., Kahane, C., et al. 1986, A&A, 157, L17
  * Guélin et al. (1993) Guélin, M., Lucas, R., & Cernicharo, J. 1993, A&A, 280, L19
  * Gupta et al. (2009) Gupta, H., Gottlieb, C. A., McCarthy, M. C., et al. 2009, ApJ, 691, 1494
  * He et al. (2008) He, J. H., Dinh-V-Trung, Kwok, S., et al. 2008, ApJS, 177, 275
  * Herwig (2005) Herwig, F. 2005, ARA&A, 43, 435
  * Highberger et al. (2003) Highberger, J. L., Thomson, K. J., Young, P. A., et al. 2003, ApJ, 593, 393
  * Highberger & Ziurys (2003) Highberger, J. L. & Ziurys, L. M. 2003, ApJ, 597, 1065
  * Highberger et al. (2001) Highberger, J. L., Savage, C., Bieging, J. H., et al. 2001, ApJ, 562, 790
  * Hony et al. (2002) Hony, S., Waters, L. B. F. M., & Tielens, A. G. G. M. 2002, A&A, 390, 533
  * Howe et al. (1992) Howe, D. A., Millar, T. J., & Williams, D. A. 1992, MNRAS, 255, 217
  * Hrivnak et al. (2010) Hrivnak, B. J., Lu, W., Maupin, R. E., et al. 2010, ApJ, 709, 1042
  * Huggins et al. (1984) Huggins, P. J., Carlson, W. J., & Kinney, A. L. 1984, A&A, 133, 347
  * Höfner & Olofsson (2018) Höfner, S. & Olofsson, H. 2018, A&A Rev., 26, 1
  * Indelicato et al. (2017) Indelicato, I., La Cognata, M., Spitaleri, C., et al. 2017, ApJ, 845, 19
  * Jaminet et al. (1992) Jaminet, P. A., Danchi, W. C., Sandell, G., et al. 1992, ApJ, 400, 535
  * Jenkins (2009) Jenkins, E. B. 2009, ApJ, 700, 1299
  * Jorissen et al. (1992) Jorissen, A., Smith, V. V., & Lambert, D. L. 1992, A&A, 261, 164
  * Kahane et al. (2000) Kahane, C., Dufour, E., Busso, M., et al. 2000, A&A, 357, 669
  * Kaiser et al. (1996) Kaiser, R. I., Ochsenfeld, C., Head-Gordon, M., et al. 1996, Science, 274, 1508
  * Kasuga et al. (1997) Kasuga, T., Yamamura, I., & Deguchi, S. 1997, A&A, 320, 575
  * Kawabe et al. (1987) Kawabe, R., Ishiguro, M., Kasuga, T., et al. 1987, ApJ, 314, 322
  * Kawaguchi et al. (1993) Kawaguchi, K., Kagi, E., Hirano, T., et al. 1993, ApJ, 406, L39
  * Klochkova et al. (2000) Klochkova, V. G., Szczerba, R., & Panchuk, V. E. 2000, Astronomy Letters, 26, 439
  * Knapp et al. (1994) Knapp, G. R., Bowers, P. F., Young, K., et al. 1994, ApJ, 429, L33
  * Kwok (1993) Kwok, S. 1993, ARA&A, 31, 63
  * Langer (1992) Langer, W. D. 1992, Astrochemistry of Cosmic Phenomena, 150, 193
  * Lo & Bechis (1976) Lo, K. Y. & Bechis, K. P. 1976, ApJ, 205, L21
  * Lodders (2003) Lodders, K. 2003, ApJ, 591, 1220
  * Lopez & Perrin (2000) Lopez, B. & Perrin, J.-M. 2000, A&A, 354, 657
  * Lovas (2004) Lovas, F. J. 2004, Journal of Physical and Chemical Reference Data, 33, 177
  * Lucas et al. (1986) Lucas, R., Omont, A., Guilloteau, S., et al. 1986, A&A, 154, L12
  * Matthews & Sears (1983) Matthews, H. E. & Sears, T. J. 1983, ApJ, 267, L53
  * McElroy et al. (2013) McElroy, D., Walsh, C., Markwick, A. J., et al. 2013, A&A, 550, A36
  * McGuire (2018) McGuire, B. A. 2018, ApJS, 239, 17
  * McGuire (2021) McGuire, B. A. 2021, arXiv:2109.13848
  * Milam et al. (2008) Milam, S. N., Halfen, D. T., Tenenbaum, E. D., et al. 2008, ApJ, 684, 618
  * Milam et al. (2005) Milam, S. N., Savage, C., Brewster, M. A., et al. 2005, ApJ, 634, 1126
  * Millar & Herbst (1994) Millar, T. J. & Herbst, E. 1994, A&A, 288, 561
  * Müller et al. (2001) Müller, H. S. P., Thorwirth, S., Roth, D. A., et al. 2001, A&A, 370, L49
  * Müller et al. (2005) Müller, H. S. P., Schlöder, F., Stutzki, J., et al. 2005, Journal of Molecular Structure, 742, 215
  * Nakashima et al. (2012) Nakashima, J.-. ichi ., Koning, N., Volgenau, N. H., et al. 2012, ApJ, 759, 61
  * Neufeld et al. (2006) Neufeld, D. A., Schilke, P., Menten, K. M., et al. 2006, A&A, 454, L37.
  * Ney et al. (1975) Ney, E. P., Merrill, K. M., Becklin, E. E., et al. 1975, ApJ, 198, L129
  * Nguyen-Q-Rieu et al. (1984) Nguyen-Q-Rieu, Graham, D., & Bujarrabal, V. 1984, A&A, 138, L5
  * Olofsson (1997) Olofsson, H. 1997, IAU Symposium, 178, 457
  * Omont et al. (1995) Omont, A., Moseley, S. H., Cox, P., et al. 1995, ApJ, 454, 819
  * Pardo et al. (2021) Pardo, J. R., Cabezas, C., Fonfría, J. P., et al. 2021, A&A, 652, L13. doi:10.1051/0004-6361/202141671
  * Pardo et al. (2007) Pardo, J. R., Cernicharo, J., Goicoechea, J. R., et al. 2007, ApJ, 661, 250
  * Park et al. (2008) Park, J. A., Cho, S.-H., Lee, C. W., et al. 2008, AJ, 136, 2350
  * Patel et al. (2011) Patel, N. A., Young, K. H., Gottlieb, C. A., et al. 2011, ApJS, 193, 17
  * Qiu et al. (2020) Qiu, J., Zhang, J., Zhang, Y., et al. 2020, A&A, 634, A125
  * Saberi et al. (2017) Saberi, M., Maercker, M., De Beck, E., et al. 2017, A&A, 599, A63
  * Sahai et al. (1998a) Sahai, R., Hines, D. C., Kastner, J. H., et al. 1998a, ApJ, 492, L163
  * Sahai et al. (1998b) Sahai, R., Trauger, J. T., Watson, A. M., et al. 1998b, ApJ, 493, 301
  * Schmidt & Ziurys (2018) Schmidt, D. R. & Ziurys, L. M. 2018, IAU Symposium, 332, 218
  * Schmidt & Ziurys (2016) Schmidt, D. R. & Ziurys, L. M. 2016, ApJ, 817, 175
  * Schöier et al. (2006) Schöier, F. L., Olofsson, H., & Lundgren, A. A. 2006, A&A, 454, 247
  * Schöier et al. (2005) Schöier, F. L., van der Tak, F. F. S., van Dishoeck, E. F., et al. 2005, A&A, 432, 369
  * Sopka et al. (1989) Sopka, R. J., Olofsson, H., Johansson, L. E. B., et al. 1989, A&A, 210, 78
  * Spitoni et al. (2018) Spitoni, E., Matteucci, F., Jönsson, H., et al. 2018, A&A, 612, A16
  * Takano et al. (2019) Takano, S., Nakajima, T., & Kohno, K. 2019, PASJ, 71, S20
  * Tenenbaum et al. (2009) Tenenbaum, E. D., Milam, S. N., Woolf, N. J., et al. 2009, ApJ, 704, L108
  * Truong-Bach et al. (1993) Truong-Bach, Graham, D., & Nguyen-Q-Rieu 1993, A&A, 277, 133
  * Truong-Bach et al. (1988) Truong-Bach, Graham, D., & Rieu, N. Q. 1988, A&A, 199, 291
  * Truong-Bach et al. (1990) Truong-Bach, Morris, D., Nguyen-Q-Rieu, et al. 1990, A&A, 230, 431
  * Tsuji (1973) Tsuji, T. 1973, A&A, 23, 411
  * Turner et al. (2005) Turner, B. E., Petrie, S., Dunbar, R. C., et al. 2005, ApJ, 621, 817
  * Turner et al. (1994) Turner, B. E., Steimle, T. C., & Meerts, L. 1994, ApJ, 426, L97
  * Ueta et al. (2006) Ueta, T., Murakawa, K., & Meixner, M. 2006, ApJ, 641, 1113
  * Volk et al. (2020) Volk, K., Sloan, G. C., & Kraemer, K. E. 2020, Ap&SS, 365, 88
  * Watson et al. (1976) Watson, W. D., Anicich, V. G., & Huntress, W. T. 1976, ApJ, 205, L165
  * Woods et al. (2003a) Woods, P. M., Millar, T. J., Herbst, E., et al. 2003a, A&A, 402, 189
  * Woods et al. (2003b) Woods, P. M., Schöier, F. L., Nyman, L.-Å., et al. 2003b, A&A, 402, 617
  * Woods et al. (2005) Woods, P. M., Nyman, L.-Å., Schöier, F. L., et al. 2005, A&A, 429, 977
  * Yamamura et al. (1995) Yamamura, I., Onaka, T., Kamijo, F., et al. 1995, ApJ, 439, L13
  * Young et al. (1992) Young, K., Serabyn, G., Phillips, T. G., et al. 1992, ApJ, 385, 265
  * Zhang & Liu (2005) Zhang, Y. & Liu, X.-W. 2005, ApJ, 631, L61
  * Zhang (2017) Zhang, Y. 2017, IAU Symposium, 323, 141
  * Zhang (2020) Zhang, Y. 2020, ApJ, 898, 151
  * Zhang et al. (2020) Zhang, Y., Chau, W., Nakashima, J., et al. 2020, PASJ, 72, 46
  * Zhang et al. (2009) Zhang, Y., Kwok, S., & Nakashima, J. 2009, ApJ, 700, 1262
  * Zhang et al. (2013) Zhang, Y., Kwok, S., Nakashima, J., et al. 2013, ApJ, 773, 71
  * Zhang et al. (2014) Zhang, Y., Kwok, S., & Strom, A. 2014, ATNF Proposal, M664
  * Zhang et al. (2017) Zhang, X.-Y., Zhu, Q.-F., Li, J., et al. 2017, A&A, 606, A74
  * Ziurys (2006) Ziurys, L. M. 2006, Proceedings of the National Academy of Science, 103, 12274
  * Ziurys et al. (1994) Ziurys, L. M., Apponi, A. J., & Phillips, T. G. 1994, ApJ, 433, 729

Figure 1:  Overview of the full spectra of CRL 2688 obtained in current
observations with strong lines marked.

Figure 2:  Spectra of our observations with all lines marked. The RMS are 6.6,
11.0, 10.4, and 24.7 mK after smoothing to a frequency resolution of 13.5,
10.9, 6.5, and 5.8 MHz at the rest frequency of 78.0, 96.9, 163.1, and 182.0
GHz, respectively. Newly detected transition lines are indicated in blue.

Figure 2 (Cont.):

Figure 2 (Cont.):

Figure 2 (Cont.):

Figure 3:  The emission lines of CS and its 13C, 33S, and 34C isotopologues.
The emission line of other molecules can be found in the online figure set.
The red and blue curves represent the stellar-shell fitting of each
transition. The green curve represents the total stellar-shell fitting of
blended transitions. The cyan dashed curve represents the line profile
resulting from the LIME modelling. The vertical lines mark the positions and
relative intensities of hyperfine components. The panels marked with
‘smoothed’ show the smoothed spectra. (The complete figure set (116 images) is
available in the online journal.)

Fig. Set3. The emission lines of molecular species and their isotopologues.
The red and blue curves represent the stellar-shell fitting of each
transition. The green curve represents the total stellar-shell fitting of
blended transitions. The cyan dashed curve represents the line profile
resulting from the LIME modelling. The vertical lines mark the positions and
relative intensities of hyperfine components. The panels marked with
‘smoothed’ show the smoothed spectra. (See the published version for the
figure set.)

Figure 4: Upper: The $F$ factor versus the source size ($\theta_{\rm s}$ or
$\theta_{\rm d}$), where the solid and dashed curves represent those
calculated assuming Gaussian and disk brightness distributions, respectively.
Lower: The velocity-integrated intensity ratios of the lines detected by the
IRAM 30 m and ARO 12 m telescopes, after correcting for the beam-dilution
effect. See the text for the details.

Figure 5:  The rotation diagrams. The filled and open cycles represent the
data obtained in this work and those taken from the literature (see the text
for details), respectively.  Figure 5 (Cont.):  Figure 5 (Cont.):  Figure 5
(Cont.):  Figure 6:  Comparison of molecular line intensities between CRL 2688
(red) and IRC+10216 (blue), which is ordered by increasing carbon atoms from
top to bottom.  Figure 7: Comparison between the observed (black) and modelled
(red) column densities of the molecules in CRL 2688 (left panel), and the
observed-to-modelled column density ratios of the molecules in CRL 2688 and
IRC+10216 (right panel). For some molecules in IRC+10216, diverse abundances
have been reported in different observations, which are denoted by open
circles linked by straight lines. The vertical lines in the right panel mark
the ranges within which the observed and modelled values are in agreement
within one order of magnitude.  Figure 8: The modelled abundances of molecules
in CRL 2688 as a function of radius. The observed abundances are denoted by
squares, which are located at different radii for the convenience of view. The
open and solid squares represent those within and beyond the modelling results
in the radius range from 1.5 $\times$ 1016–1.5 $\times$ 1018 cm, respectively.
The vertical lines show the level of disagreement between the observed and the
maximum modelled abundances.  Figure 9:  Rotation diagram of AlF, NaCN, and
MgNC, for which the source sizes have been assumed to be 5$\arcsec$,
5$\arcsec$, and 30$\arcsec$, respectively. The measurements of AlF
(triangles), NaCN (cycles), and MgNC (squares) taken by us and those by
Highberger et al. (2001) are denoted by filled and open symbols, respectively.
Figure 10: Column densities as a function of assumed source sizes. The
measurements of AlF (triangles), NaCN (cycles), and MgNC (squares) taken by us
and those by Highberger et al. (2001) are denoted by filled and open symbols,
respectively.

Table 1: Stellar and circumstellar parameters of CRL 2688.

Parameter Description | Parameter | Unit | Value | Reference | Note
---|---|---|---|---|---
Stellar radius | $R_{\ast}$ | $R_{\odot}$ |  51 $\pm$ 5 | (1) | 1
Stellar effective temperature | $T_{\rm eff}$ | K | 7000 | (1) | 2
Stellar spectrum type |  $\cdots$ |  $\cdots$ | F5 Ia | (2) | 3
Distance | $D$ | pc | 1500 | (3) | 4
|  |  | 420 | (4) | 5
|  |  | 300–350 | (1) | 6
|  |  | 1000 | (2) | 7
|  |  |  350 $\pm$ 53 | (5) | 8
|  |  |  3200 $\pm$ 4260 | (6) | 9
Luminosity of the central star | $L_{\ast}$ | $L_{\odot}$ | 18000 | (7) | 10
|  |  | 3300 | (4) | 11
|  |  |  5500 $\pm$ 1100 | (1) | 12
Mass-loss | $\dot{M}$ |  $M_{\odot}$ yr-1 |  3.0 $\times$ 10-5 | (8) | 13
|  |  |  $<$3.6 $\times$ 10-3 | (4) | 14
|  |  |  1.7 $\times$ 10-4 | (9) | 15
|  |  |  2.0 $\times$ 10-5 | (10) | 16

Reference:
††footnotetext: (1) Balick et al. (2012); (2) Crampton et al. (1975); (3)
Cohen & Kuhi (1977); (4) Ueta et al. (2006); (5) Gaia TGAS; (6) Gaia DR2; (7)
Ney et al. (1975); (8) Lo & Bechis (1976); (9) Truong-Bach et al. (1990); (10)
Jaminet et al. (1992).  Note:
11footnotetext: Obtained based on the luminosity, stellar temperature, and
distance in Balick et al. (2012). 22footnotetext: Obtained under an assumption
of the blackbody spectrum. 33footnotetext: Obtained based on a fitting of the
spectrum in the range 4100–5000 Å. 44footnotetext: Obtained based on the
source size at 10–20 $\mu$m (Ney et al., 1975). 55footnotetext: Obtained based
on the proper motion of the nebula at the tips of the bipolar lobes.
66footnotetext: Obtained based on the proper motion of the nebular rings.
77footnotetext: Obtained by assuming that this source is associated with
Cygnus OB4. 88footnotetext: Derived from the archive date of Gaia TGAS, which
gives a parallax of 2.78 $\pm$ 0.26 mas. 99footnotetext: Derived from the
archive date of Gaia DR2, which gives a parallax of 0.31 $\pm$ 0.41 mas.
1010footnotetext:  Obtained by assuming a distance of 1 kpc (Crampton et al.,
1975) and a isotropic IR emission. 1111footnotetext:  Obtained based on a
distance of 420 pc and a bolometric flux of 6 $\times$ 10-7 ergs s-1 cm-2
(Knapp et al., 1994). 1212footnotetext:  Obtained based on a distance of 420
pc and a fitting of the spectral energy distribution in the range of
$\sim$0.2–1000 $\mu$m. 1313footnotetext:  Obtained based on the 13CO
$J=1\rightarrow 0$ observations with the NRAO 11 m telescope at Kitt Peak.
1414footnotetext:  Obtained based on a distance of 420 pc from and a radiation
transfer modeling of Lopez & Perrin (2000). 1515footnotetext:  Obtained based
on the CO $J=1\rightarrow 0$ and CO $J=2\rightarrow 1$ observations with the
IRAM 30 m telescope. 1616footnotetext:  Obtained based on the CO
$J=3\rightarrow 2$ observations with the JCMT 15 m telescope.

Table 2: Summary of the observations.

Band Range |  $F_{\rm eff}$a |  $B_{\rm eff}$a | HPBW
---|---|---|---
(GHz) |  |  | (″)
75.70–83.48 | 95 | 81 | 32.5–29.5
91.38–99.16 | 95 | 81 | 26.9–24.8
160.84–168.62 | 93 | 73 | 15.3–14.6
176.52–184.30 | 94 | 63 | 13.9–13.3

aafootnotetext: Taken from the IRAM website.

Table 3: A summary of the lines in CRL 2688 detected by our observations and in the literature. Frequency | Species | Transition | $V_{\rm exp}$ | Horn/Center | $T_{\rm mb}$ |  $\int T_{\rm mb}$d$V$ | RMS | Dv | FWHM | Telescope | Ref. | Mark
---|---|---|---|---|---|---|---|---|---|---|---|---
(MHz) |  | (upper–lower) | (km s-1) |  | (mK) | (mK km s-1) | (mK) | (km s-1) | (″) |  |  |
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) | (12) | (13)
113144.2 | CN |  $N$=1–0 | ? | ? | ? | 97000 $\pm$ 2000 | ? | ? | 21.7 | IRAM-30m | 1 | a
113144.2 | CN |  $N$=1–0, $J$=1/2–1/2, $F$=1/2–3/2 | ? | ? | 111.0 | 3575.9 $\pm$ 999.5 | 44.4 | ? | 47.7 | TRAO-14m | 2 |
113170.5 | CN |  $N$=1–0, $J$=1/2–1/2, $F$=3/2–1/2 | ? | ? | 266.6 | 5997.5 $\pm$ 733.0 | 44.4 | ? | 47.7 | TRAO-14m | 2 |
113191.3 | CN |  $N$=1–0, $J$=1/2–1/2, $F$=3/2–3/2 | ? | ? | 177.7 | 4020.9 $\pm$ 688.6 | 44.4 | ? | 47.7 | TRAO-14m | 2 |
113488.1 | CN |  $N$=1–0, $J$=3/2–1/2, $F$=3/2–1/2 | ? | ? | 400.4 | 9342.0 $\pm$ 311.4 | 66.7 | ? | 47.5 | TRAO-14m | 2 | b
113491.0 | CN |  $N$=1–0, $J$=3/2–1/2, $F$=5/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 47.5 | TRAO-14m | 2 | b
113499.6 | CN |  $N$=1–0, $J$=3/2–1/2, $F$=1/2–1/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 47.5 | TRAO-14m | 2 | b
113508.9 | CN |  $N$=1–0, $J$=3/2–1/2, $F$=3/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 47.5 | TRAO-14m | 2 | b
113520.4 | CN |  $N$=1–0, $J$=3/2–1/2, $F$=1/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 47.5 | TRAO-14m | 2 | b
226287.4 | CN |  $N$=2–1 | ? | ? | ? | 58000 $\pm$ 2000 | ? | ? | 10.9 | IRAM-30m | 1 | c
226287.4 | CN |  $N$=2–1, $J$=3/2–3/2, $F$=1/2–1/2 | ? | ? | 78 | 2860 | 6.2 | ? | 33.4 | SMT-10m | 3 | b
226298.9 | CN |  $N$=2–1, $J$=3/2–3/2, $F$=1/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.4 | SMT-10m | 3 | b
226303.0 | CN |  $N$=2–1, $J$=3/2–3/2, $F$=3/2–1/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.4 | SMT-10m | 3 | b
226314.5 | CN |  $N$=2–1, $J$=3/2–3/2, $F$=3/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.4 | SMT-10m | 3 | b
226332.5 | CN |  $N$=2–1, $J$=3/2–3/2, $F$=3/2–5/2 | ? | ? | 102 | 3693 | 6.2 | ? | 33.4 | SMT-10m | 3 | b
226341.9 | CN |  $N$=2–1, $J$=3/2–3/2, $F$=5/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.4 | SMT-10m | 3 | b
226359.9 | CN |  $N$=2–1, $J$=3/2–3/2, $F$=5/2–5/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.4 | SMT-10m | 3 | b
226616.6 | CN |  $N$=2–1, $J$=3/2–1/2, $F$=1/2–3/2 | ? | ? | 99 | 2756 | 5.4 | ? | 33.3 | SMT-10m | 3 | b
226632.2 | CN |  $N$=2–1, $J$=3/2–1/2, $F$=3/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.3 | SMT-10m | 3 | b
226659.6 | CN |  $N$=2–1, $J$=3/2–1/2, $F$=5/2–3/2 | ? | ? | 279 | 6460 | 5.4 | ? | 33.3 | SMT-10m | 3 | b
226663.7 | CN |  $N$=2–1, $J$=3/2–1/2, $F$=1/2–1/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.3 | SMT-10m | 3 | b
226679.3 | CN |  $N$=2–1, $J$=3/2–1/2, $F$=3/2–1/2 | ? | ? | 113 | 1917 | 4.5 | ? | 33.3 | SMT-10m | 3 | b
226874.2 | CN |  $N$=2–1, $J$=5/2–3/2, $F$=5/2–3/2 | ? | ? | 566 | 12804 | 4.5 | ? | 33.3 | SMT-10m | 3 | b
226874.8 | CN |  $N$=2–1, $J$=5/2–3/2, $F$=7/2–5/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.3 | SMT-10m | 3 | b
226875.9 | CN |  $N$=2–1, $J$=5/2–3/2, $F$=3/2–1/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.3 | SMT-10m | 3 | b
226887.4 | CN |  $N$=2–1, $J$=5/2–3/2, $F$=3/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.3 | SMT-10m | 3 | b
226892.1 | CN |  $N$=2–1, $J$=5/2–3/2, $F$=5/2–5/2 | ? | ? | 198 | 3007 | 4.5 | ? | 33.3 | SMT-10m | 3 | b
226905.4 | CN |  $N$=2–1, $J$=5/2–3/2, $F$=3/2–5/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 33.3 | SMT-10m | 3 | b
339446.8 | CN |  $N$=3–2 | ? | ? | ? | 29000 $\pm$ 500 | ? | ? | 22.2 | CSO-10m | 1 | d
340033.4 | CN |  $N$=3–2, $J$=5/2–3/2 | ? | ? | ? | 1.5: | ? | ? | 14.8 | JCTM-15m | 4 | b
340248.0 | CN |  $N$=3–2, $J$=7/2–5/2 | ? | ? | ? | 2.7: | ? | ? | 14.8 | JCTM-15m | 4 | b
108651.3 |  13CN |  $N$=1–0, $J$=1/2–1/2, $F_{1}$=1–0, $F$=2–1 | ? | ? | 26 | 352: | 6.0 | ? | 58.4 | ARO-12m | 3 | b
108657.6 |  13CN |  $N$=1–0, $J$=3/2–1/2, $F_{1}$=1–1, $F$=2–2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 58.4 | ARO-12m | 3 | b
108658.9 |  13CN |  $N$=1–0, $J$=3/2–1/2, $F_{1}$=1–1, $F$=1–2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 58.4 | ARO-12m | 3 | b
108780.2 |  13CN |  $N$=1–0, $J$=3/2–1/2, $F_{1}$=2–1, $F$=3–2 | ? | ? | 38 | 1702 | 6.0 | ? | 58.3 | ARO-12m | 3 | b
108782.4 |  13CN |  $N$=1–0, $J$=3/2–1/2, $F_{1}$=2–1, $F$=2–1 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 58.3 | ARO-12m | 3 | b
108787.0 |  13CN |  $N$=1–0, $J$=3/2–1/2, $F_{1}$=2–1, $F$=1–0 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 58.3 | ARO-12m | 3 | b
108793.8 |  13CN |  $N$=1–0, $J$=3/2–1/2, $F_{1}$=2–1, $F$=1–1 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 58.3 | ARO-12m | 3 | b
108796.4 |  13CN |  $N$=1–0, $J$=3/2–1/2, $F_{1}$=2–1, $F$=2–2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 58.3 | ARO-12m | 3 | b
115271.2 | CO |  $J$=1–0 | ? | ? | 10100 | 266000 | 200 | ? | 21.3 | IRAM-30m | 5 |
115271.2 | CO |  $J$=1–0 | ? | ? | 2106.0 | 57602.3 $\pm$ 1187.5 | 67.2 | ? | 46.8 | TRAO-14m | 2 |
115271.2 | CO |  $J$=1–0 | ? | ? | 3500.0 | ? | 700.0 | ? | 33 | OSO-20m | 6 |
230538.0 | CO |  $J$=2–1 | ? | ? | ? | 312500 $\pm$ 1300 | ?? | ? | 10.7 | IRAM-30m | 1 |
230538.0 | CO |  $J$=2–1 | ? | ? | 3087 | 83055 | 5.4 | ? | 32.8 | SMT-10m | 3 |
345796.0 | CO |  $J$=3–2 | ? | ? | ? | 15.5: | ? | ? | 14.6 | JCTM-15m | 4 |
110201.4 |  13CO |  $J$=1–0 | ? | ? | 800 | 27000 | 100 | ? | 22.3 | IRAM-30m | 5 |
110201.4 |  13CO |  $J$=1–0 | ? | ? | ? | 35400 $\pm$ 600 | ? | ? | 22.3 | IRAM-30m | 1 |
110201.4 |  13CO |  $J$=1–0 | ? | ? | 307.3 | 7813.3 $\pm$ 482.8 | 43.9 | ? | 49.0 | TRAO-14m | 2 |
110201.4 |  13CO |  $J$=1–0 | ? | ? | 500.0 | ? | 100.0 | ? | 33 | OSO-20m | 6 |
220398.7 |  13CO |  $J$=2–1 | ? | ? | ? | 112000 $\pm$ 3000 | ? | ? | 11.2 | IRAM-30m | 1 |
330588.0 |  13CO |  $J$=3–2 | ? | ? | ? | 5: | ? | ? | 15.2 | JCTM-15m | 4 |
220398.7 |  13CO |  $J$=2–1 | ? | ? | 671 | 20398 | 7.3 | ? | 34.3 | SMT-10m | 3 |
224714.4 | C17O |  $J$=2–1 | ? | ? | 71 | 2248 | 5.8 | ? | 33.6 | SMT-10m | 3 |
337061.1 | C17O |  $J$=3–2 | ? | ? | ? | 0.7: | ? | ? | 15.2 | JCTM-15m | 4 |
109782.1 | C18O |  $J$=1–0 | ? | ? | 32 | 983 | 3.8 | ? | 57.8 | ARO-12m | 3 |
219560.4 | C18O |  $J$=2–1 | ? | ? | 70 | 2015 | 4.9 | ? | 34.4 | SMT-10m | 3 |
97981.0 | CS |  $J$=2–1 | ? | ? | 209.2 | 5020.6 $\pm$ 669.4 | 41.8 | ? | 55.1 | TRAO-14m | 2 |
97981.0 | CS |  $J$=2–1 | ? | ? | 158 | 5252 | 3.5 | ? | 64.8 | ARO-12m | 3 |
97981.0 | CS |  $J$=2–1 | 15.7 $\pm$ 0.1 | -0.46 $\pm$ 0.03 | 877.3 | 23364.0 $\pm$ 289.7 | 6.4 | 0.60 | 25.1 | IRAM-30m | 7 | e
146969.0 | CS |  $J$=3–2 | ? | ? | 1500 | 43400 | 40 | ? | 16.7 | IRAM-30m | 5 |
146969.0 | CS |  $J$=3–2 | ? | ? | 360.6 | 9170.0 $\pm$ 515.2 | 51.5 | ? | 36.7 | TRAO-14m | 2 |
244935.6 | CS |  $J$=5–4 | ? | ? | 449 | 13162 | 4.5 | ? | 30.8 | SMT-10m | 3 |
342882.9 | CS |  $J$=7–6 | ? | ? | ? | 3: | ? | ? | 14.7 | JCTM-15m | 4 |
92494.3 |  13CS |  $J$=2–1 | 16.5 $\pm$ 0.3 | 0.08 $\pm$ 0.19 | 24.6 | 831.0 $\pm$ 46.6 | 5.6 | 1.90 | 26.6 | IRAM-30m | 7 | f
138739.3 |  13CS |  $J$=3–2 | ? | ? | 18.0 | ? | 1.0 | ? | 45.7 | ARO-12m | 8 |
231221.0 |  13CS |  $J$=5–4 | ? | ? | 34 | 720 | 5.2 | ? | 32.7 | SMT-10m | 3 |
97172.1 | C33S |  $J$=2–1 | 15.1 $\pm$ 0.6 | -0.25 $\pm$ 0.26 | 19.0 | 524.9 $\pm$ 42.5 | 4.4 | 3.01 | 25.3 | IRAM-30m | 7 |
242913.6 | C33S |  $J$=5–4 | ? | ? | 9 | 162: | 3.5 | ? | 31.1 | SMT-10m | 3 |
96413.0 | C34S |  $J$=2–1 | 16.5 $\pm$ 0.1 | -0.24 $\pm$ 0.05 | 71.5 | 2168.2 $\pm$ 38.5 | 6.7 | 0.61 | 25.5 | IRAM-30m | 7 |
96412.9 | C34S |  $J$=2–1 | ? | ? | 15 | 385 | 3.5 | ? | 65.8 | ARO-12m | 3 |
241016.1 | C34S |  $J$=5–4 | ? | ? | 79 | 1950 | 10.0 | ? | 31.3 | SMT-10m | 3 |
86847.0 | SiO |  $J$=2–1 | ? | ? | 23 | 397 | 4.9 | ? | 73.1 | ARO-12m | 3 |
130268.7 | SiO |  $J$=3–2 | ? | ? | ? | 9000 $\pm$ 400 | ? | ? | 18.9 | IRAM-30m | 1 |
260518.0 | SiO |  $J$=6–5 | ? | ? | 75 | 3013 | 7.2 | ? | 29.0 | SMT-10m | 3 |
90771.5 | SiS |  $J$=5–4 | ? | ? | 34 | 682 | 6.6 | ? | 69.9 | ARO-12m | 3 |
90771.5 | SiS |  $J$=5–4 | ? | ? | 2433.3 | 60818.2 | 97.0 | ? | 18.5 | NRO-45m | 9 |
108924.3 | SiS |  $J$=6–5 | ? | ? | 33 | 1310 | 3.2 | ? | 58.3 | ARO-12m | 3 |
163376.8 | SiS |  $J$=9–8 | 16.8 $\pm$ 0.1 | 0.65 $\pm$ 0.05 | 215.0 | 8803.7 $\pm$ 86.6 | 11.0 | 0.36 | 15.1 | IRAM-30m | 7 |
181525.2 | SiS |  $J$=10–9 | 17.3 $\pm$ 0.1 | 0.53 $\pm$ 0.07 | 225.2 | 9158.6 $\pm$ 139.7 | 32.7 | 0.32 | 13.6 | IRAM-30m | 7 |
235961.1 | SiS |  $J$=13–12 | ? | ? | 64 | 1931 | 5.1 | ? | 32.0 | SMT-10m | 3 |
254102.9 | SiS |  $J$=14–13 | ? | ? | 63 | 1957 | 6.6 | ? | 29.7 | SMT-10m | 3 |
143175.3 | Si33S |  $J$=8–7 | ? | ? | ? | ? | 2.9 | ? | 17.2 | IRAM-30m | 10 | g, p
176554.7 | Si34S |  $J$=10–9 | 20.5 $\pm$ 0.7 | 0.46 $\pm$ 0.54 | 13.7 | 650.8 $\pm$ 73.5 | 16.5 | 2.98 | 13.9 | IRAM-30m | 7 |
178190.2 |  29SiS |  $J$=10–9 | 19.1 $\pm$ 0.8 | 0.18 $\pm$ 0.48 | 15.5 | 628.2 $\pm$ 72.7 | 6.74 | 2.96 | 13.8 | IRAM-30m | 7 |
161297.2 | NS |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=9/2–7/2, $l$=e | 26.2 $\pm$ 0.5 | -0.49 $\pm$ 0.25 | 6.5 | 285.0 $\pm$ 29.8 | 2.1 | 4.36 | 15.3 | IRAM-30m | 7 | b
161298.4 | NS |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=7/2–5/2, $l$=e | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.3 | IRAM-30m | 7 | b
161301.7 | NS |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=5/2–3/2, $l$=e | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.3 | IRAM-30m | 7 | b
161697.3 | NS |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=9/2–7/2, $l$=f | ? | ? | 7.0: | 369.7 | 2.1 | 8.69 | 15.2 | IRAM-30m | 7 | b, s
161703.4 | NS |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=7/2–5/2, $l$=f | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.2 | IRAM-30m | 7 | b, s
161704.0 | NS |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=5/2–3/2, $l$=f | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.2 | IRAM-30m | 7 | b, s
93979.8 | PN |  $J$=2–1 | 21.2 $\pm$ 0.8 | 0.37 $\pm$ 0.44 | 7.8 | 369.0 $\pm$ 30.9 | 1.8 | 5.61 | 26.2 | IRAM-30m | 7 |
93979.8 | PN |  $J$=2–1 | 25.5 $\pm$ 6.4 | ? | 3 | 61 | 2.0 | ? | 67.0 | ARO-12m | 11 |
140967.8 | PN |  $J$=3–2 | 36.0: | ? | 7: | 118 | ? | ? | 44.0 | ARO-12m | 11 | h
234935.7 | PN |  $J$=5–4 | 32.0 $\pm$ 2.6 | ? | 9 | 253 | 5.0 | ? | 32.0 | SMT-10m | 11 |
131899.0 | AlF |  $J$=4–3 | ? | ? | 8.0 | ? | 1.3 | ? | 20.0 | IRAM-30m | 8 | i
164868.0 | AlF |  $J$=5–4 | ? | ? | 11.0 | ? | 1.7 | ? | 15.0 | IRAM-30m | 8 |
164867.8 | AlF |  $J$=5–4 | 26.0 $\pm$ 4.1 | -0.62 $\pm$ 0.95 | 7.9 | 328.0 $\pm$ 120.0 | 3.4 | 4.26 | 14.9 | IRAM-30m | 7 |
263749.5 | AlF |  $J$=8–7 | ? | ? | 27.0 | ? | 2.3 | ? | 9.0 | IRAM-30m | 8 | j
102031.9 | AlCl |  $J$=7–6 | ? | ? |  $<$3.9 | ? | ? | ? | 24.0 | IRAM-30m | 10 |
160312.1 | AlCl |  $J$=11–10 | ? | ? |  $<$6.1 | ? | ? | ? | 15.0 | IRAM-30m | 10 |
91169.9 | NaCl |  $J$=7–6 | 17.7 $\pm$ 1.8 | ? | 7.8 | 192.2 $\pm$ 63.6 | 1.3 | ? | 27.0 | IRAM-30m | 10 |
143237.4 | NaCl |  $J$=11–10 | 14.6 $\pm$ 0.4 | ? | 15.9 | 358.0 $\pm$ 72.5 | 2.9 | ? | 17.0 | IRAM-30m | 10 |
156248.6 | NaCl |  $J$=12–11 | 13.3 $\pm$ 0.5 | ? | 17.9 | 352.2 $\pm$ 52.2 | 4.5 | ? | 16.0 | IRAM-30m | 10 |
156248.6 | NaCl |  $J$=12–11 | ? | ? | 4.1 | ? | 1.4 | ? | 40.0 | ARO-12m | 10 |
234251.9 | NaCl |  $J$=18–17 | 13.3 $\pm$ 0.2 | ? | 19.2 | 386.5 $\pm$ 42.3 | 7.7 | ? | 11.0 | IRAM-30m | 10 |
101961.6 | Na37Cl |  $J$=8–7 | 13.9 $\pm$ 1.3 | ? | 5.3 | 107.9 $\pm$ 15.8 | 2.6 | ? | 24.0 | IRAM-30m | 10 |
77836.7 | NaCN |  $J(Ka,Kc)$=5(0,5)–4(0,4) | 17.3 $\pm$ 1.1 | 0.02 $\pm$ 0.07 | 15.1 | 528.6 $\pm$ 52.5 | 10.4 | 3.761 | 31.6 | IRAM-30m | 7 | k
80846.9 | NaCN |  $J(Ka,Kc)$=5(1,4)–4(1,3) | 16.8 $\pm$ 0.5 | 3.54 $\pm$ 3.42 | 3.6 | 263.0 $\pm$ 50.4 | 11.3 | 2.17 | 30.4 | IRAM-30m | 7 |
93206.1 | NaCN |  $J(Ka,Kc)$=6(0,6)–5(0,5) | 15.2 $\pm$ 0.4 | 2.21 $\pm$ 1.37 | 5.8 | 304.6 $\pm$ 36.6 | 4.3 | 2.51 | 26.4 | IRAM-30m | 7 |
94334.8 | NaCN |  $J(Ka,Kc)$=6(2,4)–5(2,3) | ?? $\pm$ ? | ?? $\pm$ ?? | ?? | ?? $\pm$ ?? | ?? | ?? | 26.1 | IRAM-30m | 7 |
96959.8 | NaCN |  $J(Ka,Kc)$=6(1,5)–5(1,4) | 17.7 $\pm$ 2.7 | 4.74 $\pm$ 18.03 | 3.7 | 337.9 $\pm$ 48.5 | 4.2 | 3.62 | 25.4 | IRAM-30m | 7 |
138651.9 | NaCN |  $J(Ka,Kc)$=9(0,9)–8(0,8) | ? | ? | 5.0 | ? | 1.0 | ? | 46.0 | ARO-12m | 8 |
140299.5 | NaCN |  $J(Ka,Kc)$=9(6,4)–8(6,3) | 17.8 $\pm$ 0.6 | ? | 11.4 | 295.7 $\pm$ 61.4 | 5.7 | ? | 18.0 | IRAM-30m | 10 | g
140299.5 | NaCN |  $J(Ka,Kc)$=9(6,3)–8(6,2) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 18.0 | IRAM-30m | 10 | g
140347.1 | NaCN |  $J(Ka,Kc)$=9(2,8)–8(2,7) | 15.2 $\pm$ 0.8 | ? | 22.9 | 545.7 $\pm$ 58.6 | 5.7 | ? | 18.0 | IRAM-30m | 10 |
140483.8 | NaCN |  $J(Ka,Kc)$=9(5,5)–8(5,4) | 12.9 $\pm$ 0.5 | ? | 14.3 | 280.0 $\pm$ 40.0 | 5.7 | ? | 18.0 | IRAM-30m | 10 | g
140483.8 | NaCN |  $J(Ka,Kc)$=9(5,4)–8(5,3) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 18.0 | IRAM-30m | 10 | g
140666.3 | NaCN |  $J(Ka,Kc)$=9(4,6)–8(4,5) | 13.5 $\pm$ 0.4 | ? | 22.9 | 554.3 $\pm$ 45.7 | 5.7 | ? | 18.0 | IRAM-30m | 10 | g
140667.5 | NaCN |  $J(Ka,Kc)$=9(4,5)–8(4,4) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 18.0 | IRAM-30m | 10 | g
153557.7 | NaCN |  $J(Ka,Kc)$=10(0,10)–9(0,9) | ? | ? | 8.0 | ? | 1.7 | ? | 41.0 | ARO-12m | 8 |
156106.2 | NaCN |  $J(Ka,Kc)$=10(5,6)–9(5,5) | 13.6 $\pm$ 0.8 | ? | 19.4 | 461.2 $\pm$ 49.3 | 4.5 | ? | 16.0 | IRAM-30m | 10 | g
156106.2 | NaCN |  $J(Ka,Kc)$=10(5,5)–9(5,4) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 16.0 | IRAM-30m | 10 | g
156322.9 | NaCN |  $J(Ka,Kc)$=10(4,7)–9(4,6) | 17.1 $\pm$ 0.5 | ? | 26.9 | 698.5 $\pm$ 43.3 | 4.5 | ? | 16.0 | IRAM-30m | 10 | g
156325.5 | NaCN |  $J(Ka,Kc)$=10(4,6)–9(4,5) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 16.0 | IRAM-30m | 10 | g
156541.2 | NaCN |  $J(Ka,Kc)$=10(3,8)–9(3,7) | ? | ? | 4.0 | ? | 1.0 | ? | 40.0 | ARO-12m | 8 |
156684.0 | NaCN |  $J(Ka,Kc)$=10(3,7)–9(3,6) | ? | ? | 5.0 | ? | 1.0 | ? | 40.0 | ARO-12m | 8 | l
158616.4 | NaCN |  $J(Ka,Kc)$=10(2,8)–9(2,7) | ? | ? | 5.0 | ? | 1.3 | ? | 40.0 | ARO-12m | 8 |
161014.8 | NaCN |  $J(Ka,Kc)$=10(1,9)-9(1,8) | 15.6 $\pm$ 0.6 | -0.10 $\pm$ 0.32 | 17.4 | 525.8 $\pm$ 47.9 | 10.8 | 2.55 | 15.3 | IRAM-30m | 7 |
165083.1 | NaCN |  $J(Ka,Kc)$=11(1,11)–10(1,10) | 17.5 $\pm$ 0.4 | ? | 18.2 | 487.9 $\pm$ 37.9 | 7.6 | ? | 15.0 | IRAM-30m | 10 |
165083.3 | NaCN |  $J(Ka,Kc)$=10(1,11)–10(1,10) | 13.8 $\pm$ 0.6 | 0.46 $\pm$ 0.76 | 13.2 | 419.0 $\pm$ 62.5 | 8.4 | 1.42 | 14.9 | IRAM-30m | 7 |
168345.2 | NaCN |  $J(Ka,Kc)$=11(0,11)–10(0,10) | 15.9 $\pm$ 0.8 | 0.15 $\pm$ 0.58 | 15.6 | 519.5 $\pm$ 72.6 | 4.0 | 2.43 | 14.6 | IRAM-30m | 7 |
176890.0 | NaCN |  $J(Ka,Kc)$=11(1,10)–10(1,9) | 14.4 $\pm$ 0.4 | 0.92 $\pm$ 0.59 | 16.7 | 626.4 $\pm$ 56.3 | 5.4 | 3.31 | 13.9 | IRAM-30m | 7 |
234289.2 | NaCN |  $J(Ka,Kc)$=15(5,11)–14(5,10) | 11.4 $\pm$ 0.3 | ? | 21.2 | 351.9 $\pm$ 42.3 | 7.7 | ? | 11.0 | IRAM-30m | 10 | g
234290.2 | NaCN |  $J(Ka,Kc)$=15(5,10)–14(5,9) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 11.0 | IRAM-30m | 10 | g
95454.1 | MgNC |  $N$=8–7, $J$=15/2–13/2 | 16.3 $\pm$ 0.2 | 0.81 $\pm$ 0.25 | 20.7 | 855.9 $\pm$ 36.7 | 4.5 | 1.84 | 25.8 | IRAM-30m | 7 |
95469.3 | MgNC |  $N$=8–7, $J$=17/2–15/2 | 16.0 $\pm$ 0.2 | 1.00 $\pm$ 0.26 | 22.5 | 958.4 $\pm$ 35.6 | 4.5 | 1.84 | 25.8 | IRAM-30m | 7 |
131241.6 | MgNC |  $N$=11–10, $J$=21/2–19/2 | ? | ? | 5.0 | ? | 1.0 | ? | 48.0 | ARO-12m | 8 | m, n
131256.8 | MgNC |  $N$=11–10, $J$=23/2–21/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 48.0 | ARO-12m | 8 | m, n
143168.7 | MgNC |  $N$=12–11, $J$=23/2–21/2 | ? | ? | 28.6 | ? | 2.9 | ? | 17.2 | IRAM-30m | 10 | m, g, p
143183.9 | MgNC |  $N$=12–11, $J$=25/2–23/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 17.2 | IRAM-30m | 10 | m, g, p
143168.7 | MgNC |  $N$=12–11, $J$=23/2–21/2 | ? | ? | 4.0 | ? | 0.7 | ? | 44.0 | ARO-12m | 8 | m
143183.9 | MgNC |  $N$=12–11, $J$=25/2–23/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 44.0 | ARO-12m | 8 | m
155094.6 | MgNC |  $N$=13–12, $J$=25/2–23/2 | ? | ? | 3.0 | ? | 0.3 | ? | 40.0 | ARO-12m | 8 | m
155109.8 | MgNC |  $N$=13–12, $J$=27/2–25/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 40.0 | ARO-12m | 8 | m
167019.1 | MgNC |  $N$=14–13, $J$=27/2–25/2 | 16.1 $\pm$ 0.8 | -0.24 $\pm$ 0.35 | 11.4 | 337.7 $\pm$ 37.5 | 3.9 | 2.45 | 14.7 | IRAM-30m | 7 | m
167034.4 | MgNC |  $N$=14–13, $J$=29/2–27/2 | 16.7 $\pm$ 0.7 | -0.48 $\pm$ 0.23 | 16.7 | 467.7 $\pm$ 37.5 | 3.9 | 2.45 | 14.7 | IRAM-30m | 7 | m
178942.3 | MgNC |  $N$=15–14, $J$=29/2–27/2 | ? | ? | 10.0: | 694.8 | 6.2 | 2.95 | 13.7 | IRAM-30m | 7 | m, o
178957.6 | MgNC |  $N$=15–14, $J$=31/2–29/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 13.7 | IRAM-30m | 7 | m, o
80522.3 |  26MgNC |  $N$=7–6, $J$=13/2–11/2 | ? | ? | 5.0: | 479.4 | 6.2 | 4.36 | 30.6 | IRAM-30m | 7 | m, q
80535.1 |  26MgNC |  $N$=7–6, $J$=15/2–13/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 30.6 | IRAM-30m | 7 | m, q
92023.7 |  26MgNC |  $N$=8–7, $J$=15/2–13/2 | ? | ? | 5.0: | 408.2 | 2.2 | 5.73 | 26.7 | IRAM-30m | 7 | m, q
92038.4 |  26MgNC |  $N$=8–7, $J$=17/2–15/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 26.7 | IRAM-30m | 7 | m, q
93063.6 | SiC2 |  $J(Ka,Kc)$=4(0,4)–3(0,3) | ? | ? | 17 | 410 | 4.0 | ? | 68.2 | ARO-12m | 3 |
93063.6 | SiC2 |  $J(Ka,Kc)$=4(0,4)–3(0,3) | 16.4 $\pm$ 0.1 | 0.91 $\pm$ 0.14 | 48.0 | 2053.4 $\pm$ 47.9 | 9.3 | 0.63 | 26.4 | IRAM-30m | 7 |
94245.4 | SiC2 |  $J(Ka,Kc)$=4(2,3)–3(2,2) | 16.5 $\pm$ 0.1 | 0.73 $\pm$ 0.14 | 35.0 | 1432.1 $\pm$ 35.9 | 9.1 | 0.62 | 26.1 | IRAM-30m | 7 |
95579.3 | SiC2 |  $J(Ka,Kc)$=4(2,2)–3(2,1) | ? | ? | ? | 973.0 $\pm$ 313.9 | ? | ? | 25.7 | IRAM-30m | 12 |
95579.4 | SiC2 |  $J(Ka,Kc)$=4(2,2)–3(2,1) | 16.0 $\pm$ 0.1 | 0.92 $\pm$ 0.15 | 37.8 | 1579.1 $\pm$ 38.1 | 7.1 | 0.61 | 25.7 | IRAM-30m | 7 |
137180.8 | SiC2 |  $J(Ka,Kc)$=6(0,6)–5(0,5) | ? | ? | ? | 5700 $\pm$ 600 | ? | ? | 17.9 | IRAM-30m | 1 |
158499.2 | SiC2 |  $J(Ka,Kc)$=7(0,7)–6(0,6) | ? | ? | 34: | 320: | 8.4 | ? | 40.0 | ARO-12m | 3 |
164069.1 | SiC2 |  $J(Ka,Kc)$=7(2,6)–6(2,5) | 16.4 $\pm$ 0.1 | 0.62 $\pm$ 0.06 | 126.9 | 5019.2 $\pm$ 73.2 | 22.8 | 0.36 | 15.0 | IRAM-30m | 7 |
164770.5 | SiC2 |  $J(Ka,Kc)$=7(6,1)–6(6,0) | 16.7 $\pm$ 0.1 | 0.76 $\pm$ 0.10 | 60.6 | 2533.6 $\pm$ 45.7 | 9.4 | 0.36 | 14.9 | IRAM-30m | 7 | g
164770.5 | SiC2 |  $J(Ka,Kc)$=7(6,2)–6(6,1) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 14.9 | IRAM-30m | 7 | g
164770.5 | SiC2 |  $J(Ka,Kc)$=7(6,1)–6(6,0) | ? | ? | 127.0 | ? | 3.3 | ? | 14.9 | IRAM-30m | 8 | g
164770.5 | SiC2 |  $J(Ka,Kc)$=7(6,2)–6(6,1) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 14.9 | IRAM-30m | 8 | g
165511.0 | SiC2 |  $J(Ka,Kc)$=7(4,4)–6(4,3) | ? | ? | 100.0: |  $<$8930.9 | 9.5 | 0.35 | 14.9 | IRAM-30m | 7 | g, r
165523.9 | SiC2 |  $J(Ka,Kc)$=7(4,3)–6(4,2) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 14.9 | IRAM-30m | 7 | g, r
179446.5 | SiC2 |  $J(Ka,Kc)$=8(0,8)–7(0,7) | 16.6 $\pm$ 0.1 | 0.47 $\pm$ 0.07 | 144.0 | 5544.6 $\pm$ 87.1 | 23.4 | 0.33 | 13.7 | IRAM-30m | 7 |
220773.7 | SiC2 |  $J(Ka,Kc)$=10(0,10)–9(0,9) | ? | ? | 54 | 1333 | 7.4 | ? | 34.2 | SMT-10m | 3 |
222009.4 | SiC2 |  $J(Ka,Kc)$=9(2,7)–8(2,6) | ? | ? | 30 | 923 | 7.2 | ? | 34.0 | SMT-10m | 3 |
232534.1 | SiC2 |  $J(Ka,Kc)$=10(2,9)–9(2,8) | ? | ? | 31 | 930 | 5.1 | ? | 32.5 | SMT-10m | 3 |
234534.0 | SiC2 |  $J(Ka,Kc)$=10(8,2)–9(8,1) | ? | ? | 19 | 298 | 4.8 | ? | 32.2 | SMT-10m | 3 | g
234534.0 | SiC2 |  $J(Ka,Kc)$=10(8,3)–9(8,2) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 32.2 | SMT-10m | 3 | g
235713.0 | SiC2 |  $J(Ka,Kc)$=10(6,5)–9(6,4) | ? | ? | 36 | 1016 | 5.9 | ? | 32.0 | SMT-10m | 3 | g
235713.1 | SiC2 |  $J(Ka,Kc)$=10(6,4)–9(6,3) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 32.0 | SMT-10m | 3 | g
237150.0 | SiC2 |  $J(Ka,Kc)$=10(4,7)–9(4,6) | ? | ? | 29 | 505 | 4.5 | ? | 31.8 | SMT-10m | 3 |
237331.3 | SiC2 |  $J(Ka,Kc)$=10(4,6)–9(4,5) | ? | ? | 23 | 567 | 4.5 | ? | 31.8 | SMT-10m | 3 |
241367.7 | SiC2 |  $J(Ka,Kc)$=11(0,11)–10(0,10) | ? | ? | 43 | 903 | 10.8 | ? | 31.3 | SMT-10m | 3 |
247529.1 | SiC2 |  $J(Ka,Kc)$=10(2,8)–9(2,7) | ? | ? | 44 | 1177 | 6.0 | ? | 30.5 | SMT-10m | 3 |
254981.5 | SiC2 |  $J(Ka,Kc)$=11(2,10)–10(2,9) | ? | ? | 40 | 1091 | 6.4 | ? | 29.6 | SMT-10m | 3 |
258065.0 | SiC2 |  $J(Ka,Kc)$=11(8,3)–10(8,2) | ? | ? | 23 | 716 | 6.1 | ? | 29.3 | SMT-10m | 3 | g
258065.0 | SiC2 |  $J(Ka,Kc)$=11(8,4)–10(8,3) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 29.3 | SMT-10m | 3 | g
259433.3 | SiC2 |  $J(Ka,Kc)$=11(6,6)–10(6,5) | ? | ? | 41 | 800 | 6.8 | ? | 29.1 | SMT-10m | 3 | g
259433.3 | SiC2 |  $J(Ka,Kc)$=11(6,5)–10(6,4) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 29.1 | SMT-10m | 3 | g
261150.7 | SiC2 |  $J(Ka,Kc)$=11(4,8)–10(4,7) | ? | ? | 46 | 902 | 7.6 | ? | 28.9 | SMT-10m | 3 |
261509.3 | SiC2 |  $J(Ka,Kc)$=11(4,7)–10(4,6) | ? | ? | 25 | 631 | 7.9 | ? | 28.9 | SMT-10m | 3 |
261990.7 | SiC2 |  $J(Ka,Kc)$=12(0,12)–11(0,11) | ? | ? | ? | ? | ? | ? | 28.8 | SMT-10m | 3 | s
76842.1 | Si13CC |  $J(Ka,Kc)$=16(3,13)–16(3,14) | 26.6 $\pm$ 2.4 | 0.68 $\pm$ 0.95 | 6.7 | 438.1 $\pm$ 70.2 | 3.7 | 6.86 | 32.0 | IRAM-30m | 7 |
161345.3 | Si13CC |  $J(Ka,Kc)$=7(5,3)–6(5,2) | ? | ? | 2.0: | 64.3 | 2.1 | 6.53 | 15.2 | IRAM-30m | 7 | g, t
161345.5 | Si13CC |  $J(Ka,Kc)$=7(5,2)–6(5,1) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.2 | IRAM-30m | 7 | g, t
161734.2 |  29SiC2 |  $J(Ka,Kc)$=7(2,6)–6(2,5) | ? | ? | 12.0: | 407.3 | 3.8 | 3.25 | 15.2 | IRAM-30m | 7 | g, t
162372.1 |  29SiC2 |  $J(Ka,Kc)$=7(6,1)–6(6,0) | ? | ? | 6.0: | 217.9 | 3.8 | 3.25 | 15.2 | IRAM-30m | 7 | g, t
162372.1 |  29SiC2 |  $J(Ka,Kc)$=7(6,2)–6(6,1) | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.2 | IRAM-30m | 7 | g, t
168051.5 |  29SiC2 |  $J(Ka,Kc)$=7(2,5)–6(2,4) | 14.4 $\pm$ 0.5 | 2.0 $\pm$ 1.3 | 4.9 | 233.9 $\pm$ 29.3 | 3.2 | 3.14 | 14.6 | IRAM-30m | 7 |
81505.2 | C2S |  $N$=6–5, $J$=7–6 | 17.2 $\pm$ 0.8 | 9.23 $\pm$ 12.79 | 2.0 | 280.0 $\pm$ 56.2 | 4.0 | 4.31 | 30.2 | IRAM-30m | 7 |
93870.1 | C2S |  $N$=7–6, $J$=8–7 | 15.7 $\pm$ 0.7 | 3.27 $\pm$ 2.64 | 5.3 | 349.7 $\pm$ 50.6 | 4.0 | 3.12 | 26.2 | IRAM-30m | 7 | u
103640.8 | C2S |  $N$=8–7, $J$=8–7 | ? | ? | 149.7 | 7783.5 $\pm$ 727.0 | 42.8 | ? | 52.1 | TRAO-14m | 2 |
87284.2 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–1 | ? | ? | ? | 1615.4 $\pm$ 343.7 | ? | ? | 28.2 | IRAM-30m | 12 |
87316.9 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=2–1 | ? | ? | ? | 10238.6 $\pm$ 343.6 | ? | ? | 28.2 | IRAM-30m | 12 |
87328.6 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–0 | ? | ? | ? | 5359.1 $\pm$ 343.5 | ? | ? | 28.2 | IRAM-30m | 12 |
87402.0 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=1–1 | ? | ? | ? | 6933.5 $\pm$ 343.2 | ? | ? | 28.1 | IRAM-30m | 12 | b
87407.2 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=0–1 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 28.1 | IRAM-30m | 12 | b
87446.4 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=1–0 | ? | ? | ? | 1029.2 $\pm$ 343.1 | ? | ? | 28.1 | IRAM-30m | 12 |
87284.6 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–1 | ? | ? | 30: | ? | ? | ? | 78.7 | NRAO-11m | 13 | b
87317.1 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=2–1 | ? | ? | 40: | ? | ? | ? | 78.6 | NRAO-11m | 13 | b
87284.6 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–1 | ? | ? | 1684.9 | 39666.7 | 84.8 | ? | 19.2 | NRO-45m | 9 |
87317.1 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=2–1 | ? | ? | 1151.5 | 23575.8 | 84.8 | ? | 19.2 | NRO-45m | 9 |
87328.7 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–0 | ? | ? | 1333.3 | 27424.2 | 84.8 | ? | 19.2 | NRO-45m | 9 |
87402.5 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=1–1 | ? | ? | 303.0 | 4060.6 | 84.8 | ? | 19.2 | NRO-45m | 9 |
87407.2 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=0–1 | ? | ? | 724.2 | 13909.1 | 90.9 | ? | 19.2 | NRO-45m | 9 |
87316.9 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=2–1 | ? | ? | 100.5 | 3537.1 $\pm$ 562.7 | 40.2 | ? | 61.8 | TRAO-14m | 2 |
87328.6 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–0 | ? | ? | 100.5 | 4964.2 $\pm$ 623.0 | 40.2 | ? | 61.8 | TRAO-14m | 2 |
87412.0 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–1 | ? | ? | 100.5 | 10152.6 $\pm$ 944.9 | 40.2 | ? | 61.7 | TRAO-14m | 2 |
87284.6 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–1 | ? | ? | 20 | 580: | 9.7 | ? | 72.7 | ARO-12m | 3 | b
87317.1 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=2–1 | ? | ? | 97 | 4670 | 9.7 | ? | 72.7 | ARO-12m | 3 | b
87328.7 | C2H |  $N$=1–0, $J$=3/2–1/2, $F$=1–0 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 72.7 | ARO-12m | 3 | b
87402.1 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=1–1 | ? | ? | 67 | 1820 | 9.7 | ? | 72.6 | ARO-12m | 3 | b
87407.2 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=0–1 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 72.6 | ARO-12m | 3 | b
87446.4 | C2H |  $N$=1–0, $J$=1/2–1/2, $F$=1–0 | ? | ? | 19 | 510： | 9.7 | ? | 72.6 | ARO-12m | 3 |
262005.3 | C2H |  $N$=3–2, $J$=7/2–5/2 | ? | ? | 232 | 7621 | 8.2 | ? | 28.8 | SMT-10m | 3 | s
262066.1 | C2H |  $N$=3–2, $J$=5/2–3/2 | ? | ? | 147 | 4630 | 8.2 | ? | 28.8 | SMT-10m | 3 |
262208.4 | C2H |  $N$=3–2, $J$=5/2–5/2 | ? | ? | 30 | 540： | 8.2 | ? | 28.8 | SMT-10m | 3 |
168274.5 |  13CCH |  $N$=2–1, $J$=5/2–3/2, $F_{1}$=3–2, $F$=7/2–5/2 | 16.9 $\pm$ 0.4 | 0.98 $\pm$ 0.64 | 9.3 | 415.2 $\pm$ 41.3 | 4.3 | 2.44 | 14.6 | IRAM-30m | 7 | b
168276.6 |  13CCH |  $N$=2–1, $J$=5/2–3/2, $F_{1}$=3–2, $F$=5/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 14.6 | IRAM-30m | 7 | b
168303.6 |  13CCH |  $N$=2–1, $J$=5/2–3/2, $F_{1}$=2–1, $F$=3/2–1/2 | 22.2 $\pm$ 0.1 | 0.34 $\pm$ 0.35 | 11.7 | 581.0 $\pm$ 46.9 | 4.3 | 2.44 | 14.6 | IRAM-30m | 7 | b
168307.7 |  13CCH |  $N$=2–1, $J$=3/2–1/2, $F_{1}$=2–1, $F$=5/2–3/2 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 14.6 | IRAM-30m | 7 | b
88631.8 | HCN |  $J$=1–0 | ? | ? | 6500 | 175000 | 100 | ? | 27.8 | IRAM-30m | 5 |
88631.8 | HCN |  $J$=1–0 | ? | ? | ? | 177000 $\pm$ 1000 | ? | ? | 27.8 | IRAM-30m | 1 |
88631.8 | HCN |  $J$=1–0 | ? | ? | 727.0 | 18660.1 $\pm$ 403.9 | 20.2 | ? | 60.9 | TRAO-14m | 2 |
88631.6 | HCN |  $J$=1–0 | ? | ? | 990 | 29090 | 4.9 | ? | 71.6 | ARO-12m | 3 |
88631.6 | HCN |  $J$=1–0 | ? | ? | 2500.0 | ? | 500.0 | ? | 42 | OSO-20m | 6 |
177261.1 | HCN |  $J$=2–1 | 18.8 $\pm$ 0.1 | -1.00 $\pm$ 0.03 | 11280.1 | 282548.0 $\pm$ 2211.1 | 16.3 | 0.33 | 13.9 | IRAM-30m | 7 | v
177238.7 | HCN |  $\nu_{2}$=1, $J$=2–1, $l$=e | $\times$ | $\times$ | 500: | $\times$ | 16.3 | 0.33 | 13.9 | IRAM-30m | 7 | v
265886.4 | HCN |  $J$=3–2 | ? | ? | 2878 | 84870 | 10.6 | ? | 28.4 | SMT-10m | 3 | g
354505.5 | HCN |  $J$=4–3 | ? | ? | ? | 13.5: | ? | ? | 14.2 | JCTM-15m | 4 |
265852.8 | HCN |  $\nu_{2}$=1, $J$=3–2, $l$=e | $\times$ | $\times$ | $\times$ |  | $\times$ | $\times$ | 28.4 | SMT-10m | 3 | g
178136.5 | HCN |  $\nu_{2}$=1, $J$=2–1, $l$=f | 13.9 $\pm$ 0.1 | -0.90 $\pm$ 0.02 | 503.2 | 9846.3 $\pm$ 74.2 | 19.4 | 0.33 | 13.8 | IRAM-30m | 7 |
86340.2 | H13CN |  $J$=1–0 | ? | ? | 320.4 | 8310.4 $\pm$ 300.4 | 20.0 | ? | 62.5 | TRAO-14m | 2 |
86340.2 | H13CN |  $J$=1–0 | ? | ? | 315 | 8710 | 5.0 | ? | 73.5 | ARO-12m | 3 |
86340.2 | H13CN |  $J$=1–0 | ? | ? | 850.0 | ? | 170.0 | ? | 42 | OSO-20m | 6 |
259011.8 | H13CN |  $J$=3–2 | ? | ? | 1106 | 30313 | 6.9 | ? | 29.2 | SMT-10m | 3 |
345340.0 | H13CN |  $J$=4–3 | ? | ? | ? | 5.8: | ? | ? | 14.6 | JCTM-15m | 4 |
258154.7 | HC15N |  $J$=3–2 | ? | ? | 140 | 3593 | 5.5 | ? | 29.3 | SMT-10m | 3 |
167453.3 | H13C15N |  $J$=2–1 | 14.2 $\pm$ 1.0 | -0.37 $\pm$ 0.33 | 12.4 | 307.8 $\pm$ 35.0 | 3.8 | 3.15 | 14.7 | IRAM-30m | 7 |
90663.6 | HNC |  $J$=1–0 | ? | ? | ? | 26700 $\pm$ 300 | ? | ? | 27.1 | IRAM-30m | 1 |
90663.5 | HNC |  $J$=1–0 | ? | ? | 81.4 | 2930.1 $\pm$ 468.0 | 40.7 | ? | 59.5 | TRAO-14m | 2 |
90663.6 | HNC |  $J$=1–0 | ? | ? | 128 | 3720 | 4.9 | ? | 70.0 | ARO-12m | 3 |
90663.6 | HNC |  $J$=1–0 | ? | ? | 309.1 | 5121.2 | 84.8 | ? | 18.5 | NRO-45m | 9 |
90663.6 | HNC |  $J$=1–0 | ? | ? | 190.0 | ? | 38.0 | ? | 42 | OSO-20m | 6 |
181324.8 | HNC |  $J$=2–1 | 16.0 $\pm$ 0.1 | -0.11 $\pm$ 0.02 | 1751.3 | 63186.0 $\pm$ 149.3 | 31.7 | 0.32 | 13.6 | IRAM-30m | 7 |
261263.4 | HN13C |  $J$=3–2 | ? | ? | 27 | 618 | 7.5 | ? | 28.9 | SMT-10m | 3 |
89188.5 | HCO+ |  $J$=1–0 | ? | ? | ? |  $<$700 | ? | ? | 27.6 | IRAM-30m | 1 |
89188.5 | HCO+ |  $J$=1–0 | ? | ? | 10 | 267: | 5.0 | ? | 71.2 | ARO-12m | 3 |
178375.1 | HCO+ |  $J$=2–1 | ? | ? | 40: |  $<$3317.6 | 5.8 | 2.95 | 13.8 | IRAM-30m | 7 | w, x
159802.6 | HCP |  $J$=4–3 | 29.5 $\pm$ 3.8 | ? | 12 | 354 | 6.0 | ? | 39.0 | ARO-12m | 11 |
239693.8 | HCP |  $J$=4–3 | 35.0 $\pm$ 2.5 | ? | 19 | 463 | 9.0 | ? | 31.0 | SMT-10m | 11 |
279634.7 | HCP |  $J$=4–3 | 28.9 $\pm$ 3.2 | ? | 28 | 758 | 10.0 | ? | 27.0 | SMT-10m | 11 |
225697.8 | H2CO | 3(1,2)–2(1,1) | ? | ? | 23 | 739 | 6.0 | ? | 33.5 | SMT-10m | 3 |
244047.8 | H2CS | 7(1,6)–6(1,5) | ? | ? | 10 | 239: | 3.7 | ? | 30.9 | SMT-10m | 3 |
131762.8 | HC2N |  $N$=6–5, $J$=7–6 | ? | ? | 24.0 | ? | 1.3 | ? | 18.7 | IRAM-30m | 8 |
131833.3 | HC2N |  $N$=6–5, $J$=6–5 | ? | ? | 22.0 | ? | 1.3 | ? | 18.7 | IRAM-30m | 8 |
131956.2 | HC2N |  $N$=6–5, $J$=5–4 | ? | ? | 17.0 | ? | 1.3 | ? | 18.6 | IRAM-30m | 8 |
263683.6 | HC2N |  $N$=12–11, $J$=11–10 | ? | ? | 24.0 | ? | 2.3 | ? | 9.3 | IRAM-30m | 8 | y
92488.5 | C3S |  $J$=16–15 | 15.7 $\pm$ 0.4 | 1.53 $\pm$ 0.84 | 9.8 | 464.3 $\pm$ 45.9 | 5.6 | 1.90 | 26.6 | IRAM-30m | 7 | f
79151.1 | C3N |  $N$=8–7, $J$=17/2–15/2 | ? | ? | 38 | 935 | 5.3 | ? | 80.2 | ARO-12m | 3 |
79151.0 | C3N |  $N$=8–7, $J$=17/2–15/2 | 16.3 $\pm$ 0.1 | 0.50 $\pm$ 0.06 | 166.1 | 6311.5 $\pm$ 73.2 | 10.0 | 0.74 | 31.1 | IRAM-30m | 7 |
79170.0 | C3N |  $N$=8–7, $J$=15/2–13/2 | ? | ? | 47 | 1084 | 5.3 | ? | 80.2 | ARO-12m | 3 |
79169.8 | C3N |  $N$=8–7, $J$=15/2–13/2 | 16.3 $\pm$ 0.1 | 0.47 $\pm$ 0.06 | 153.7 | 5807.5 $\pm$ 73.0 | 10.0 | 0.74 | 31.1 | IRAM-30m | 7 |
89045.6 | C3N |  $N$=9–8, $J$=19/2–17/2 | ? | ? | ? | 6199.1 $\pm$ 235.8 | ? | ? | 27.6 | IRAM-30m | 12 |
89064.4 | C3N |  $N$=9–8, $J$=17/2–15/2 | ? | ? | ? | 5288.3 $\pm$ 235.8 | ? | ? | 27.6 | IRAM-30m | 12 |
89045.7 | C3N |  $N$=9–8, $J$=19/2–17/2 | ? | ? | 44 | 1454 | 5.6 | ? | 71.3 | ARO-12m | 3 |
89064.4 | C3N |  $N$=9–8, $J$=17/2–15/2 | ? | ? | 39 | 1212 | 5.6 | ? | 71.3 | ARO-12m | 3 |
98940.0 | C3N |  $N$=10–9, $J$=21/2–19/2 | ? | ? | 41 | 1170 | 3.3 | ? | 64.1 | ARO-12m | 3 |
98940.0 | C3N |  $N$=10-9, $J$=21/2–19/2 | 16.3 $\pm$ 0.1 | 0.33 $\pm$ 0.04 | 209.6 | 7574.4 $\pm$ 76.3 | 9.4 | 0.59 | 24.9 | IRAM-30m | 7 |
98958.8 | C3N |  $N$=10–9, $J$=19/2–17/2 | ? | ? | 37 | 1048 | 3.3 | ? | 64.1 | ARO-12m | 3 |
98958.8 | C3N |  $N$=10–9, $J$=19/2–17/2 | 16.0 $\pm$ 0.1 | 0.46 $\pm$ 0.05 | 178.9 | 6621.5 $\pm$ 75.6 | 9.4 | 0.59 | 24.9 | IRAM-30m | 7 |
108834.3 | C3N |  $N$=11–10, $J$=21/2–19/2 | ? | ? | 43 | 1442 | 3.2 | ? | 58.3 | ARO-12m | 3 |
108853.0 | C3N |  $N$=11–10, $J$=19/2–17/2 | ? | ? | 44 | 1610 | 3.2 | ? | 58.3 | ARO-12m | 3 |
138534.5 | C3N |  $N$=14–13, $u$ | ? | ? | 32.0 | ? | 1.0 | ? | 45.8 | ARO-12m | 8 |
168194.9 | C3N |  $N$=17–16, $J$=35/2–33/2 | 16.1 $\pm$ 0.2 | -0.24 $\pm$ 0.08 | 65.9 | 1955.7 $\pm$ 52.3 | 6.5 | 1.39 | 14.6 | IRAM-30m | 7 | m
168213.7 | C3N |  $N$=17–16, $J$=33/2–31/2 | 15.8 $\pm$ 0.1 | -0.32 $\pm$ 0.08 | 63.9 | 1802.6 $\pm$ 51.4 | 6.5 | 1.39 | 14.6 | IRAM-30m | 7 | m
178087.4 | C3N |  $N$=18–17, $J$=37/2–35/2 | ? | ? | 50.0: |  $<$3913.6 | 6.4 | 2.30 | 13.8 | IRAM-30m | 7 | m, z
178106.1 | C3N |  $N$=18–17, $J$=35/2–33/2 | ? | ? | 50.0: | $\times$ | 6.4 | 2.30 | 13.8 | IRAM-30m | 7 | m, z
91494.3 |  $c$-C3H |  $N_{K_{-1},K_{+1}}$=21,2–11,1, $J$=5/2–3/2, $F$=3–2 | ? | ? | 5.0: | 318.9 | 1.9 | 11.52 | 26.9 | IRAM-30m | 7 | b, x
91497.6 |  $c$-C3H |  $N_{K_{-1},K_{+1}}$=21,2–11,1, $J$=5/2–3/2, $F$=2–1 | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 26.9 | IRAM-30m | 7 | b, x
76198.7 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=4–3, $l$=f | 16.1 $\pm$ 0.3 | 1.45 $\pm$ 0.57 | 24.6 | 1179.2 $\pm$ 82.8 | 8.0 | 2.31 | 32.3 | IRAM-30m | 7 | b
76199.9 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=3–2, $l$=f |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 32.3 | IRAM-30m | 7 | b
76204.2 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=4–3, $l$=e | 17.5 $\pm$ 0.2 | 1.05 $\pm$ 0.42 | 31.6 | 1493.4 $\pm$ 83.1 | 8.0 | 2.31 | 32.3 | IRAM-30m | 7 | b
76205.1 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=7/2–5/2, $F$=3–2, $l$=e |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 32.3 | IRAM-30m | 7 | b
80388.1 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=7/2–5/2, $F$=4–3, $l$=f | 19.0 $\pm$ 0.2 | 0.73 $\pm$ 0.30 | 16.3 | 770.4 $\pm$ 37.6 | 5.1 | 1.46 | 30.6 | IRAM-30m | 7 | b
80389.4 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=7/2–5/2, $F$=3–2, $l$=f |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 30.6 | IRAM-30m | 7 | b
80420.6 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=7/2–5/2, $F$=4–3, $l$=e | 16.8 $\pm$ 0.2 | 0.69 $\pm$ 0.26 | 16.0 | 660.8 $\pm$ 35.0 | 4.8 | 1.46 | 30.6 | IRAM-30m | 7 | b
80422.1 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=7/2–5/2, $F$=3–2, $l$=e |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 30.6 | IRAM-30m | 7 | b
97995.2 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=9/2–7/2, $F$=5–4, $l$=e | 17.6 $\pm$ 0.2 | 1.47 $\pm$ 0.70 | 48.5 | 2541.9 $\pm$ 219.0 | 6.2 | 0.60 | 25.1 | IRAM-30m | 7 | b, n
97996.0 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=9/2–7/2, $F$=4–3, $l$=e |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 25.1 | IRAM-30m | 7 | b, n
98011.6 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=9/2–7/2, $F$=5–4, $l$=f | 17.0 $\pm$ 0.2 | 1.22 $\pm$ 0.71 | 44.0 | 2105.4 $\pm$ 213.3 | 6.2 | 0.60 | 25.1 | IRAM-30m | 7 | b
98012.6 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=9/2–7/2, $F$=4–3, $l$=f |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 25.1 | IRAM-30m | 7 | b
98011.6 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=9/2–7/2, $F$=5–4, $l$=f | ? | ? | 19 | 545 | 3.3 | ? | 64.7 | ARO-12m | 3 | b
98012.5 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=9/2–7/2, $F$=4–3, $l$=f | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 64.7 | ARO-12m | 3 | b
103319.3 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=9/2–7/2, $F$=5–4, $l$=f | ? | ? | 12 | 367 | 4.0 | ? | 61.4 | ARO-12m | 3 | b
103319.8 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=9/2–7/2, $F$=4–3, $l$=f | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 61.4 | ARO-12m | 3 | b
103372.5 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=9/2–7/2, $F$=5–4, $l$=e | ? | ? | 13 | 419 | 4.0 | ? | 61.4 | ARO-12m | 3 | b
103373.1 |  $l$-C3H |  ${}^{2}\Pi_{3/2}$, $J$=9/2–7/2, $F$=4–3, $l$=e |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 61.4 | ARO-12m | 3 | b
163491.0 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=15/2-13/2, $F$=8-7, $l$=e | 16.3 $\pm$ 0.1 | 1.77 $\pm$ 0.22 | 30.3 | 1572.6 $\pm$ 38.1 | 11.1 | 0.36 | 15.0 | IRAM-30m | 7 | b
163491.6 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=15/2-13/2, $F$=7-6, $l$=e |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.0 | IRAM-30m | 7 | b
163597.2 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=15/2-13/2, $F$=8-7, $l$=f | 16.3 $\pm$ 0.1 | 1.47 $\pm$ 0.17 | 34.2 | 1672.1 $\pm$ 35.5 | 11.6 | 0.36 | 15.0 | IRAM-30m | 7 | b
163597.9 |  $l$-C3H |  ${}^{2}\Pi_{1/2}$, $J$=15/2-13/2, $F$=7-6, $l$=f |  | $\times$ | $\times$ | $\times$ | $\times$ | $\times$ | 15.0 | IRAM-30m | 7 | b
23694.5 | NH3 |  $(J,K)$=(1,1) | ? | ? | 36 | 851 $\pm$ 529 | 7 | ? | 70.8 | NRO-45m | 14 |
23722.6 | NH3 |  $(J,K)$=(2,2) | ? | ? | 18 | 574: | 7 | ? | 70.6 | NRO-45m | 14 |
23870.1 | NH3 |  $(J,K)$=(3,3) | ? | ? | 25 | 862 $\pm$ 460 | 7 | ? | 70.3 | NRO-45m | 14 |
24139.4 | NH3 |  $(J,K)$=(4,4) | ? | ? | 14 | 494: | 7 | ? | 69.5 | NRO-45m | 14 |
23694.5 | NH3 |  $(J,K)$=(1,1) | ? | ? | 67.0 | ? | 5.0 | ? | 31.9 | Eff-100m | 15 |
23722.6 | NH3 |  $(J,K)$=(2,2) | ? | ? | 48.0 | ? | 5.0 | ? | 31.8 | Eff-100m | 15 |
23870.1 | NH3 |  $(J,K)$=(3,3) | ? | ? | 74 | ? | 4 | ? | 31.6 | Eff-100m | 16 |
24139.4 | NH3 |  $(J,K)$=(4,4) | ? | ? | 34 | ? | 6 | ? | 31.3 | Eff-100m | 16 |
23098.8 | NH3 |  $(J,K)$=(2,1) | ? | ? | 11 | ? | 5 | ? | 32.7 | Eff-100m | 16 |
22834.2 | NH3 |  $(J,K)$=(3,2) | ? | ? | 12 | ? | 4 | ? | 33.1 | Eff-100m | 16 |
45490.3 | HC3N |  $J$=5–4 | ? | ? | 765.7 | 19955.2 | 68.7 | ? | 36.9 | NRO-45m | 9 |
72783.8 | HC3N |  $J$=8–7 | ? | ? | 2500 | 71600 | 40 | ? | 33.8 | IRAM-30m | 5 |
72783.8 | HC3N |  $J$=8–7 | ? | ? | 158 | 4840 | 4.8 | ? | 87.2 | ARO-12m | 3 |
81881.5 | HC3N |  $J$=9–8 | 16.5 $\pm$ 0.1 | -0.41 $\pm$ 0.03 | 1046.2 | 29809.9 $\pm$ 327.1 | 15.6 | 0.72 | 30.0 | IRAM-30m | 7 |
81881.5 | HC3N |  $J$=9–8 | ? | ? | 198 | 5475 | 4.8 | ? | 77.5 | ARO-12m | 3 |
90979.0 | HC3N |  $J$=10–9 | ? | ? | 203.7 | 6987.6 $\pm$ 590.8 | 40.7 | ? | 59.3 | TRAO-14m | 2 |
90979.0 | HC3N |  $J$=10–9 | ? | ? | 211 | 6860 | 3.9 | ? | 69.8 | ARO-12m | 3 |
90979.0 | HC3N |  $J$=10–9 | ? | ? | 3857.6 | 102818.0 | 221.2 | ? | 18.4 | NRO-45m | 9 |
90979.0 | HC3N |  $J$=10–9 | ? | ? | 300.0 | ? | 60.0 | ? | 42 | OSO-20m | 6 |
100076.4 | HC3N |  $J$=11–10 | ? | ? | 210.9 | 6368.7 $\pm$ 379.6 | 21.1 | ? | 53.9 | TRAO-14m | 2 |
100076.4 | HC3N |  $J$=11–10 | ? | ? | 235 | 7335 | 4.1 | ? | 63.4 | ARO-12m | 3 |
109173.6 | HC3N |  $J$=12–11 | ? | ? | 2400 | 75200 | 40 | ? | 22.5 | IRAM-30m | 5 |
109173.6 | HC3N |  $J$=12–11 | ? | ? | 174.9 | 4371.4 $\pm$ 306.0 | 21.9 | ? | 49.4 | TRAO-14m | 2 |
109173.6 | HC3N |  $J$=12–11 | ? | ? | 284 | 9560 | 3.6 | ? | 58.1 | ARO-12m | 3 | $\alpha$
127367.7 | HC3N |  $J$=14–13 | ? | ? | 330.1 | 9901.9 $\pm$ 330.1 | 23.6 | ? | 42.4 | TRAO-14m | 2 |
136464.4 | HC3N |  $J$=15–14 | ? | ? | 319.0 | 9080.2 $\pm$ 490.8 | 49.1 | ? | 39.5 | TRAO-14m | 2 |
145560.9 | HC3N |  $J$=16–15 | ? | ? | ? | 67500 $\pm$ 600 | ? | ? | 16.9 | IRAM-30m | 1 |
145560.9 | HC3N |  $J$=16–15 | ? | ? | 358.2 | 11207.6 $\pm$ 870.0 | 76.8 | ? | 37.1 | TRAO-14m | 2 |
154657.3 | HC3N |  $J$=17–16 | ? | ? | 1300 | 40000 | 40 | ? | 15.9 | IRAM-30m | 5 |
163753.4 | HC3N |  $J$=18–17 | 22.7 $\pm$ 0.2 | -1.00 $\pm$ 0.02 | 1921.6 | 58306.6 $\pm$ 410.1 | 41.7 | 0.36 | 15.0 | IRAM-30m | 7 |
181944.9 | HC3N |  $J$=20–19 | 23.3 $\pm$ 0.1 | -0.99 $\pm$ 0.01 | 1834.0 | 57487.8 $\pm$ 382.9 | 40.9 | 0.32 | 13.5 | IRAM-30m | 7 |
227418.9 | HC3N |  $J$=25–24 | ? | ? | 258 | 8069 | 5.6 | ? | 33.2 | SMT-10m | 3 |
236512.8 | HC3N |  $J$=26–25 | ? | ? | 248 | 7686 | 5.0 | ? | 31.9 | SMT-10m | 3 |
245606.3 | HC3N |  $J$=27–26 | ? | ? | 292 | 9496 | 4.4 | ? | 30.8 | SMT-10m | 3 |
254699.5 | HC3N |  $J$=28–27 | ? | ? | 274 | 8724 | 6.5 | ? | 29.7 | SMT-10m | 3 |
263792.3 | HC3N |  $J$=29–28 | ? | ? | 243 | 7705 | 6.9 | ? | 28.6 | SMT-10m | 3 |
263792.4 | HC3N |  $J$=29–28 | ? | ? | 1800.0 | ? | 100.0 | ? | 9.3 | IRAM-30m | 8 | j
345609.0 | HC3N |  $J$=38–37 | ? | ? | ? | 5: | ? | ? | 14.6 | JCTM-15m | 4 |
354697.5 | HC3N |  $J$=39–38 | ? | ? | ? | 5: | ? | ? | 14.2 | JCTM-15m | 4 |
82082.8 | HC3N |  $\nu_{7}$=1, $J$=9–8, $l$=1e | 14.1 $\pm$ 0.3 | -0.07 $\pm$ 0.19 | 20.1 | 554.3 $\pm$ 30.6 | 4.8 | 1.43 | 30.0 | IRAM-30m | 7 |
82200.4 | HC3N |  $\nu_{7}$=1, $J$=9–8, $l$=1f | 10.6 $\pm$ 0.3 | 0.64 $\pm$ 0.46 | 18.7 | 481.5 $\pm$ 38.7 | 5.3 | 2.14 | 29.9 | IRAM-30m | 7 |