11institutetext: Grupo de Astrofísica Molecular, Instituto de Física
Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain. 11email:
<EMAIL_ADDRESS>22institutetext: Centro de Desarrollos Tecnológicos,
Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain. 33institutetext:
Observatorio Astronómico Nacional (OAN, IGN), Madrid, Spain.

# Discovery of HCCCO and C5O in TMC-1 with the QUIJOTE1 line survey††thanks:
Based on observations carried out with the Yebes 40m telescope (projects
19A003, 20A014, 20D023, and 21A011) and the Institut de Radioastronomie
Millimétrique (IRAM) 30m telescope. The 40m radiotelescope at Yebes
Observatory is operated by the Spanish Geographic Institute (IGN, Ministerio
de Transportes, Movilidad y Agenda Urbana). IRAM is supported by INSU/CNRS
(France), MPG (Germany), and IGN (Spain).

J. Cernicharo 11 M. Agúndez 11 C. Cabezas 11 B. Tercero 2233 N. Marcelino 33
R. Fuentetaja JR. Pardo 1111 P. deVicente 22

(Received; accepted)

We report on the detection, for the first time in space, of the radical HCCCO
and of pentacarbon monoxide, C5O. The derived column densities are
(1.6$\pm$0.2)$\times$1011 cm-2and (1.5$\pm$0.2)$\times$1010 cm-2,
respectively. We have also analysed the data for all the molecular species of
the families HCnO and CnO within our QUIJOTE1’s line survey. Upper limits are
obtained for HC4O, HC6O, C4O, and C6O. We report a robust detection of HC5O
and HC7O based on 14 and 12 rotational lines detected with a signal-to-noise
ratio $\geq$30 and $\geq$5, respectively. The derived N(HC3O)/N(HC5O)
abundance ratio is 0.09$\pm$0.03, while N(C3O)/N(C5O) is 80$\pm$2, and
N(HC5O)/N(HC7O) is 2.2$\pm$0.3. As opposed to the cyanopolyyne family,
HC2n+1N, which shows a continuous decrease in the abundances with increasing
$n$, the CnO and HCnO species show a clear abundance maximum for $n$=3 and 5,
respectively. They also show an odd and even abundance alternation, with odd
values of n being the most abundant, which is reminiscent of the behaviour of
CnH radicals, where in that case species with even values of $n$ are more
abundant. We explored the formation of these species through two mechanisms
previously proposed, which are based on radiative associations between
CnH${}_{m}^{+}$ ions with CO and reactions of C${}_{n}^{-}$ and CnH- anions
with O atoms, and we found that several species, such as C5O, HC4O, and HC6O,
are significantly overestimated. Our understanding of how these species are
formed is incomplete as of yet. Other routes based on neutral-neutral
reactions such as those of Cn and CnH carbon chains with O, OH, or HCO, could
be behind the formation of these species.

###### Key Words.:

molecular data — line: identification — ISM: molecules — ISM: individual
(TMC-1) — — astrochemistry

##  1 Introduction

Recent observations of the molecular cloud TMC-1 with the Yebes 40m
(Cernicharo et al., 2021a) and Green Bank (McGuire et al. , 2018) radio
telescopes have shown that this source presents a paramount challenge to our
understanding of the chemical processes in cold prestellar cores. The
detection of cyclopentadiene and indene (Cernicharo et al., 2021b), ortho-
benzyne (Cernicharo et al., 2021a), the cyano derivatives of cyclopentadiene,
benzene and naphthalene (McGuire et al. , 2018; McCarthy et al., 2021; Lee et
al., 2021; McGuire et al. , 2021), the ethynyl derivatives of cyclopentandiene
(Cernicharo et al., 2021c), together with the discovery of extremely abundant
hydrocarbons such as propargyl (Agúndez et al. , 2021a), vinyl acetylene
(Cernicharo et al., 2021d), and allenyl acetylene (Cernicharo et al., 2021e)
suggest that key chemical processes involving hydrocarbons (radical, neutral,
and/or ionic species) were not considered in previous chemical networks. Most
of these species were not predicted to have a significant abundance. Hence, a
new chemistry is emerging from these observations of TMC-1.

TMC-1 is also a source that is particularly rich in sulphur bearing molecules.
In addition to the well-known species CS, CCS, and C3S in this source (Saito
et al. , 1987; Yamamoto et al. , 1987), eight new S-bearing species have been
discovered in the last year with the QUIJOTE line survey: HCCCS+, NCS, HCCS,
H2CCS, H2CCCS, C4S, HCSCN, and HCSCCH (Cernicharo et al., 2021f, g, h). In
addition, C5S, which has previously only been detected in evolved stars, has
also been detected in TMC-1 (Cernicharo et al., 2021g). Also complex O-bearing
molecules, such as CH2CHCHO, CH2CHOH, HCOOCH3, and CH3OCH3, have been detected
(Agúndez et al. , 2021b) in this cloud. Moreover, the protonated species of
tricarbon monoxide and ketene, HC3O+ and CH3CO+, have also been detected
recently (Cernicharo et al., 2020a, 2021i). On the other hand, one of the most
intriguing results concerning O-bearing species has been the detection of the
long carbon chain radicals HC5O and HC7O which are particularly abundant
(McGuire et al. , 2017; Cordiner et al. , 2017). Therefore, similar molecules
of the type CnO and HCnO could be present in TMC-1. The detection of these
species could provide insights on the oxygen chemistry in this source.

In this Letter, we report the discovery of the radical HCCCO (trans-
propynonyl) and of the O-bearing carbon chain C5O (pentacarbon monoxide). Line
surveys with the Yebes 40m telescope (QUIJOTE111Q-band Ultrasensitive
Inspection Journey to the Obscure TMC-1 Environment; see Cernicharo et al.
2021a) and the IRAM 30m radiotelescope have been used for this work. We report
a detailed study of the HCnO and CnO families of O-bearing species. The
formation of these species is investigated with the aid of a chemical model. A
robust detection of HC5O and HC7O is also presented.

Figure 1: Observed transitions of HCCCO in TMC-1. The abscissa corresponds to
the rest frequency of the lines. Frequencies and intensities for the observed
lines are given in Table LABEL:obs_line_parameters. The ordinate is the
antenna temperature, corrected for atmospheric and telescope losses, in milli
Kelvin. The quantum numbers of each transition are indicated in the
corresponding panel. The red line shows the computed synthetic spectrum for
this species for Tr=7 K and a column density of 1.3$\times$1011 cm-2. Blank
channels correspond to negative features produced in the folding of the
frequency switching data.

##  2 Observations

New receivers, built within the Nanocosmos
project222https://nanocosmos.iff.csic.es/ and installed at the Yebes 40m
radiotelescope, were used for the observations of TMC-1
($\alpha_{J2000}=4^{\rm h}41^{\rm m}41.9^{\rm s}$ and
$\delta_{J2000}=+25^{\circ}41^{\prime}27.0^{\prime\prime}$). A detailed
description of the system is given by Tercero et al. (2021). The receiver
consists of two cold high electron mobility transistor amplifiers covering the
31.0-50.3 GHz band with horizontal and vertical polarisations. Receiver
temperatures in the runs achieved during 2020 vary from 22 K at 32 GHz to 42 K
at 50 GHz. Some power adaptation in the down-conversion chains have reduced
the receiver temperatures during 2021 to 16 K at 32 GHz and 30 K at 50 GHz.
The backends are $2\times 8\times 2.5$ GHz fast Fourier transform
spectrometers with a spectral resolution of 38.15 kHz, providing the whole
coverage of the Q-band in both polarisations. All observations were performed
in the frequency switching mode with frequency throws of 8 and 10 MHz. A
detailed description of the QUIJOTE line survey is provided in Cernicharo et
al. (2021a). The main beam efficiency varies from 0.6 at 32 GHz to 0.43 at 50
GHz. Pointing corrections were derived from nearby quasars and SiO masers, and
errors were always within 2-3′′. The telescope beam size is 56′′ and 31′′ at
31 and 50 GHz, respectively. The intensity scale used in this work, that is to
say the antenna temperature ($T_{A}^{*}$), was calibrated using two absorbers
at different temperatures and the atmospheric transmission model (ATM;
Cernicharo 1985; Pardo et al. 2001). Calibration uncertainties were adopted to
be 10 %.

The IRAM 30m data come from a line survey performed towards TMC-1 and B1 and
the observations have been described by Marcelino et al. (2007) and Cernicharo
et al. (2012). All data were analysed using the GILDAS
package333http://www.iram.fr/IRAMFR/GILDAS.

##  3 Results

The sensitivity of our observations towards TMC-1, between 0.19-0.35 mK
(1$\sigma$), is much larger than previously published line surveys of this
source at the same frequencies (Kaifu et al., 2004). In fact, it has been
possible to detect many individual lines from molecular species that were
previously reported by only stacking techniques (Marcelino et al., 2021;
Cernicharo et al., 2021b, c). Line identification in this work has been
performed using the MADEX catalogue (Cernicharo , 2012), the Cologne Database
of Molecular Spectroscopy (CDMS) catalogue (Müller et al. 2005), and the Jet
Propulsion Laboratory (JPL) catalogue (JPL; Pickett et al.  1998).

###  3.1 First detection of trans-propynonyl (HCCCO)

Rotational spectroscopy for this radical is available from the microwave (Chen
et al. , 1996) and millimetre and sub-millimetre domains (Cooksy et al. ,
1992a, b). Frequency predictions as provided by the CDMS catalogue (Müller et
al., 2005) are rather accurate in the 31-50 GHz domain. They have been
implemented in the MADEX code. The dipole moment adopted for this species is
$\mu_{a}$=2.39 D (Cooksy et al. , 1995). The ground electronic state of the
molecule corresponds to a ${}^{2}A^{\prime}$ quasilinear species. Due to the
large $A$ rotational constant, $\sim$261 GHz (Cooksy et al. , 1992a), all
rotational levels with energies below 12.2 K have $K_{a}$=0 up to $J$=12. The
first $K_{a}$=1 level is the 111 $J$=1/2 $F$=1, which has an energy of 12.4 K.
Hence, the strongest lines for a cold dark cloud with TK=10 K are those
connecting levels with $K_{a}$=0, with weaker emission from $K_{a}$=1
transitions. Two of the $K_{a}$=0 rotational transitions, the 303-202 and
404-303, are in the domain of the QUIJOTE line survey and are predicted to
have intensities above 1 mK for column densities $\geq$1011 cm-2. These
transitions exhibit a fine ($J=N\pm 1/2$) and hyperfine structure, and they
have been detected with QUIJOTE. Fig. 1 shows the observed lines and the
derived line parameters provided in Table LABEL:obs_line_parameters. The
averaged vLSR of the lines is 5.75$\pm$0.04 km s-1, which agrees within
2$\sigma$ with the value obtained for the cyanopolyynes family of
5.83$\pm$0.03 km s-1 (Cernicharo et al., 2020b).

The two observed rotational transitions have very close upper level energies
and thus cannot provide a reasonable estimate for Trot. Moreover, no
collisional rates are available for this species. Hence, in order to derive a
column density, we need to assume a rotational temperature. We have assumed
the value derived for C3O in Appendix C.2 (Trot=7 K). Adopting a source of
uniform brightness and a diameter of 80′′ (Fossé et al., 2001), which means
that the source practically fills the beam at all of the frequencies of the
survey, we derived a column density for HCCCO of (1.3$\pm$0.2)$\times$1011
cm-2. The column densities of other members of the HCnO family are derived in
Appendix B and are given in Table 1. We note that HC5O appears to be the most
abundant species of the HCnO family of O-bearing carbon chains, with a
relative abundance of 1.3$\pm$0.3, 1.8$\pm$0.3, 11$\pm$3, and 2.2$\pm$0.3 with
respect to HCO, HCCO, HC3O, and HC7O, respectively. Also, HC4O and HC6O have
been searched, but only upper limits have been obtained (see Appendix B.3).

Figure 2: Similar to Fig. 1, but for the observed transitions of C5O in TMC-1.
The red line shows the computed synthetic spectrum for this species for Tr=10
K and a column density of 1.5$\times$1010 cm-2.

###  3.2 First detection of pentacarbon monoxide (C5O)

The O-bearing chains CCO and C3O have been observed in TMC-1 with large column
densities (see Appendices C.1 and C.2). Hence, other similar chains such as
C4O, C5O, and C6O could be present in this cloud.

We note that C5O has been observed in the microwave laboratory up to 24.6 GHz
and $J_{max}$=9 by Ogata et al.  (1995). The uncertainty of the frequency
measurements is 1 kHz. We adopted a dipole moment of 4.06 D as derived by
Botschwina et al.  (1995) from ab initio calculations. Six lines of C5O from
$J=12-11$ up to $J=17-16$ are clearly detected at a vLSR of 5.38-5.66 km s-1.
They are shown in Fig. 2. All the lines of C5O are shifted by 15-40 kHz with
respect the predictions if a vLSR of 5.75 km s-1 is adopted. This velocity is
the average value of the vLSR velocities observed for the lines of HCCCO, CCO,
C3O, HC5O, and HC7O (see Table LABEL:obs_line_parameters and Appendices B, C).
Consequently, we adopted a velocity of 5.75 km s-1 to derive the rest
frequencies of the observed lines of C5O. We merged them with the laboratory
data of Ogata et al.  (1995) to derive improved rotational and distortion
constants. The obtained values are $B$=1366.847016$\pm$0.000058 MHz and
$D$=0.03413$\pm$0.00045 kHz, while the laboratory data alone provide
$B$=1366.847112$\pm$0.000065 MHz and $D$=0.03509$\pm$0.00056 kHz.

A rotational diagram of the observed lines of C5O provides a rotational
temperature of 10$\pm$0.5 K and a column density of (1.5$\pm$0.2)$\times$1010
cm-2. In order to check the excitation conditions for the rotational lines of
this species, we assumed that the collisional rates of C5O could be similar to
those of HC5N with $p$-H2 (kindly provided by Francois Lique, private
communication). For a linewidth of 0.6 km s-1, a volume density of
4$\times$104 cm-3 (Cernicharo & Guélin, 1987; Fossé et al., 2001), and a
kinetic temperature of 10 K, large velocity gradient (LVG) calculations
performed with the MADEX code indicate that all rotational lines up to
$J_{u}$=20 will have excitation temperatures close to thermalisation (between
9.5 and 10 K). This result is similar to that obtained for HC5N and its
isotopologues from the data in our line survey (Cernicharo et al., 2020b). The
synthetic spectra computed with a rotational temperature of 10 K and the
column density derived from the rotational diagram are shown in Fig. 2 and are
in excellent agreement with observations.

Figure 3: Calculated fractional abundances for several CnO and HCnO species as
a function of time. Observed abundances in TMC-1 are indicated by horizontal
dashed lines, while horizontal dotted lines represent upper limits for non-
detected species.

Assuming a column density for H2 of 1022 cm-2 (Cernicharo & Guélin, 1987), we
derived an abundance for C5O of 2$\times$10-12. This is 50$\pm$15 and
80$\pm$20 times lower than that of CCO and C3O, respectively (see Appendices
C.1, C.2, and Table 1). We searched for the species C4O and C6O (see Appendix
C.3), but only upper limits have been obtained. They are given in Table 1. It
is important to note that C3O appears to be the most abundant species of the
CnO family (n$\geq$2).

##  4 Discussion

The derived abundance for the different members of the CnO ($n\geq 2$) and
HCnO ($n\geq 1$) families permit one to study the possible routes for their
formation. It is worth noting that HC5O is the more abundant species of both
series, and that the heavy species HC7O is only slightly lower in abundance.
This is a peculiar result indeed as it is clearly opposite to the behaviour of
the HC2n+1N family, where the abundance decreases with $n$, and the different
molecules are formed by the consecutive addition of CCH or CN (Cernicharo et
al. , 1987; Agúndez et al. , 2008). Nevertheless, the peculiarities of HC5O
and HC7O are not restricted to the physical conditions of interstellar clouds.
In the laboratory experiments devoted to the rotational characterisation of
these molecules, Mohamed et al.  (2005) reported that HC5O was the most
abundant species generated in their discharges for different precursors. In
the Stardust and AROMA experimental setups of the Nanocosmos project (Martínez
et al. , 2020; Santoro et al. , 2020; Sabbah et al. , 2017), it has been
observed that in experiments using pure carbon as a seed for the growth of
nanoparticles, masses associated with HC5O and HC7O are clearly detected, with
that of HC7O being rather prominent and comparable in intensity to C7H2
(Martínez et al. , 2020). In similar experiments, but adding C2H2 to the
growth chamber, HC3O, HC5O, and HC7O are also detected. The source of the
oxygen for the formation of these molecules is probably related to air
contamination (O2) during transportation of the samples (Martínez et al. ,
2020; Santoro et al. , 2020). No other oxidised molecules were detected in
these experiments. Although the physical conditions between the interstellar
clouds and these experiments are very different, it seems that these molecules
are easily formed under a variety of conditions.

Two main mechanisms have been proposed to account for the formation of CnO and
HCnO species in interstellar space. The first involves the radiative
association of Cn-1H+, Cn-1H${}_{2}^{+}$, and Cn-1H${}_{3}^{+}$ ions with CO
(Adams et al. , 1989), followed by dissociative recombination with electrons,

$\displaystyle\rm C_{\textit{n}-1}H^{+}+CO\rightarrow
HC_{\textit{n}}O^{+}+{\textit{h}\nu},$ (1a) $\displaystyle\rm
C_{\textit{n}-1}H_{2}^{+}+CO\rightarrow
H_{2}C_{\textit{n}}O^{+}+{\textit{h}\nu},$ $\displaystyle\rm
C_{\textit{n}-1}H_{3}^{+}+CO\rightarrow H_{3}C_{\textit{n}}O+{\textit{h}\nu},$
$\displaystyle\rm HC_{\textit{n}}O^{+}+e^{-}\rightarrow C_{\textit{n}}O+H,$
(1b) $\displaystyle\rm H_{2}C_{\textit{n}}O^{+}+e^{-}\rightarrow
HC_{\textit{n}}O+H,$ $\displaystyle\rm
H_{3}C_{\textit{n}}O^{+}+e^{-}\rightarrow HC_{\textit{n}}O+H+H.$

The second consists of direct formation through reactions of C${}_{n}^{-}$ and
CnH- anions with O atoms (Eichelberger et al. , 2007; Cordiner & Charnley ,
2012),

$\displaystyle\rm C_{\textit{n}}^{-}+O\rightarrow C_{\textit{n}}O+e^{-},$ (2)
$\displaystyle\rm C_{\textit{n}}H^{-}+O\rightarrow HC_{\textit{n}}O+e^{-}.$

The two mechanisms have problems reproducing some of the abundances observed
for CnO and HCnO in TMC-1. McGuire et al.  (2017) implemented the mechanism
(1) in a chemical model and found that the abundance HC4O is overestimated by
at least two orders of magnitude, while including the two mechanisms (1) and
(2) resulted in an overestimation of the abundances of HC6O, C6O, and C7O by
at least one or two orders of magnitude (Cordiner & Charnley , 2012; Cordiner
et al. , 2017).

Here we included the reactions relevant for mechanisms (1) and (2) in a
chemical model of a cold dark cloud similar to those presented in previous
studies of TMC-1 (e.g. Agúndez et al.  2021a). We adopted rate coefficients
for radiative associations in scheme (1a) from Adams et al.  (1989); for
dissociative recombinations in scheme (1b) from the estimations of Loison et
al. (2017) for HC3O+, H2C3O+, and H3C3O+; and for reactions of negative ions
with O atoms in scheme (2) from Eichelberger et al.  (2007) and Cordiner &
Charnley  (2012). We assume that CnO and HCnO species react with the most
abundant cations (H+, C+, He+, HCO+, and H3O+) and with C atoms, but we
consider that only open shell species (all HCnO species and CnO with $n$ even)
react with H, N, and O atoms.

The results are shown in Fig. 3. The observed abundances of HC3O, HC5O, and
HC7O are reasonably reproduced. However, HC4O and HC6O are overestimated by
one order of magnitude. That is, the chemical model does not make a strong
differentiation between HCnO species with an even $n$ and an odd $n$, in
contrast to observations which indicate that HCnO species with $n$ odd are
favoured. Concerning the carbon chains C$n$O, the chemical model predicts a
clear differentiation between CnO with an even $n$ and an odd $n$, as a result
of the different reactivity with H, N, and O atoms. However, the calculated
abundances are too large. This is especially dramatic in the case of C5O,
whose calculated abundance is almost four orders of magnitude too high. The
chemical mechanisms (1) and (2) are too efficient at producing O-bearing
carbon chains, and they lack specificity between an even $n$ and and an odd
$n$ for HCnO species. The assumptions behind these mechanisms must be revised,
in particular the rate coefficients adopted for the radiative associations in
scheme (1a) and the branching ratios leading to CnO in the reactions between
C${}_{n}^{-}$ and O atoms in scheme (2). In addition, it would be desirable to
know the true low-temperature reactivity of these O-bearing carbon chains with
neutral atoms.

The overabundance of some of the CnO and HCnO species in the chemical model
including the chemical mechanisms (1) and (2), together with the fact that
HC3O, HC5O, and HC7O are observed in the Nanocosmos experiments, suggest that
other chemical routes may be behind the formation of these species. In
particular, the reactions of Cn and CnH carbon chains with O atoms and the
radicals OH and HCO are potential sources of CnO and HCnO species. The
alternation in the abundances of consecutive CnH radicals, with those with $n$
even being more abundant that those with $n$ odd, could produce, in their
reaction with HCO, a similar behaviour for the HCnO species in which case
those with $n$ odd would be more abundant than those with $n$ even. However,
little is known about these reactions.

##  5 Conclusions

We detected, for the first time in space, the species HC3O and C5O. In
addition, we have provided an exhaustive list of abundances and upper limits
for O-bearing carbon chains CnO and HCnO in TMC-1. The species with $n$ odd
are found to be more abundant than those with $n$ even. In particular, C3O and
HC5O are especially abundant. Formation mechanisms for these species based on
radiative associations of CnH${}_{m}^{+}$ ions with CO and reactions between
C${}_{n}^{-}$ and CnH- anions with O atoms reproduce some features, but result
in abundances that are too large for some species such as C5O, HC4O, and HC6O.
Alternative chemical routes for these species involving neutral-neutral
reactions should be explored.

Table 1: Column densities for relevant O-bearing species in TMC-1

Molecule | $N^{a}$ |  $T_{rot}$b | $X^{c}$
---|---|---|---
C2O |  (7.5$\pm$0.3)$\times$1011 |  7-10.0d |  7.5$\times$10-11
C3O |  (1.2$\pm$0.2)$\times$1012 |  7.0$\pm$1.0 |  1.2$\times$10-10
C4O |  $\leq$9.0$\times$1010 | 10.0 |  $\leq$9.0$\times$10-12
C5O |  (1.5$\pm$0.2)$\times$1010 |  10.0$\pm$0.5 |  1.5$\times$10-12
C6O |  $\leq$1.1$\times$1011 | 10.0 |  $\leq$1.1$\times$10-11
HCO |  (1.1$\pm$0.1)$\times$1012 | 7.0 |  1.1$\times$10-10
HC2O |  (7.7$\pm$0.7)$\times$1011 | 7.0 |  7.7$\times$10-11
HC3O |  (1.3$\pm$0.2)$\times$1011 | 7.0 |  1.3$\times$10-11
HC4O |  $\leq$9.0$\times$1010 | 10.0 |  $\leq$9.0$\times$10-12
HC5O |  (1.4$\pm$0.2)$\times$1012 | 10.0 |  1.4$\times$10-10
HC6O |  $\leq$1.8$\times$1011 | 10.0 |  $\leq$1.8$\times$10-11
HC7O |  (6.5$\pm$0.5)$\times$1011 | 10.0 |  6.5$\times$10-11
444
$a$$a$footnotetext: Column density in cm-2. Upper limits correspond to
3$\sigma$ values.
$b$$b$footnotetext: Rotational temperature. When the uncertainty is not
provided, it corresponds to an assumed value.
$c$$c$footnotetext: Relative abundance to H2, assuming a total column density
of molecular hydrogen of 1022 cm-2 (Cernicharo & Guélin, 1987).
$d$$d$footnotetext: Column density derived from a large velocity gradient
model (see Appendix C.1).

###### Acknowledgements.

We thank Ministerio de Ciencia e Innovación of Spain (MICIU) for funding
support through projects PID2019-106110GB-I00, PID2019-107115GB-C21 / AEI /
10.13039/501100011033, and PID2019-106235GB-I00. We also thank ERC for funding
through grant ERC-2013-Syg-610256-NANOCOSMOS. M.A. thanks MICIU for grant
RyC-2014-16277. We would like to thank C. Joblin for her usefull comments on
the laboratory experiments of formation of carbon dust analogues in which
HC3O, HC5O and HC7O are observed. We would like also to thank Evelyne Roueff
and Octavio Roncero for useful comments on the possible reactions leading to
the formation of HCnO.

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##  Appendix A Observed line parameters

Line parameters for the different molecules studied in this work were obtained
by fitting a Gaussian line profile to the observed data. A window of $\pm$ 15
km s-1 around the vLSR of the source was considered for each transition. The
derived line parameters for all the molecular species studied in this work are
given in Table LABEL:obs_line_parameters.

Table 2: Observed line parameters for the species studied in this work Transition |  $\nu_{rest}$ a |  $\int T_{A}^{*}dv$ b |  $\Delta v$ c |  $\Delta$vd |  $T_{A}^{*}$e |
---|---|---|---|---|---|---
| (MHz) |  (mK km s-1) |  (km s-1) |  (km s-1) | (mK) |
HCCCO1
$4_{0,4}-3_{0,3}$ 9/2-7/2 5-4 |  36261.666$\pm$0.004 |  0.90$\pm$0.12 |  5.73$\pm$0.04 |  0.74$\pm$0.11 |  1.14$\pm$0.15 |
$4_{0,4}-3_{0,3}$ 9/2-7/2 4-3 |  36261.863$\pm$0.004 |  1.41$\pm$0.14 |  5.83$\pm$0.05 |  1.15$\pm$0.15 |  1.14$\pm$0.15 |
$4_{0,4}-3_{0,3}$ 7/2-5/2 4-3 |  36269.344$\pm$0.003 |  0.75$\pm$0.14 |  5.66$\pm$0.08 |  0.82$\pm$0.16 |  0.85$\pm$0.15 |
$4_{0,4}-3_{0,3}$ 7/2-5/2 3-2 |  36269.666$\pm$0.003 |  |  |  |  | A
$5_{0,5}-4_{0,4}$ 11/2-9/2 6-5 |  45327.712$\pm$0.004 |  0.63$\pm$0.19 |  5.74$\pm$0.20 | 0.60 |  0.99$\pm$0.22 | B
$5_{0,5}-4_{0,4}$ 11/2-9/2 5-4 |  45327.843$\pm$0.004 |  0.61$\pm$0.18 |  5.92$\pm$0.20 | 0.60 |  0.96$\pm$0.22 | B
$5_{0,5}-4_{0,4}$ 9/2-7/2 5-4 |  45335.424$\pm$0.004 |  1.35$\pm$0.15 |  5.79$\pm$0.04 |  0.62$\pm$0.07 |  2.06$\pm$0.22 |
$5_{0,5}-4_{0,4}$ 9/2-7/2 4-3 |  45335.613$\pm$0.004 |  0.73$\pm$0.13 |  5.82$\pm$0.05 |  0.44$\pm$0.09 |  1.57$\pm$0.22 |
CCO2
$1_{1}-0_{1}$ |  32623.449$\pm$0.007 |  2.71$\pm$0.30 |  5.53$\pm$0.05 |  0.87$\pm$0.11 |  2.94$\pm$0.27 |
$2_{1}-1_{1}$ |  32738.613$\pm$0.005 |  1.23$\pm$0.23 |  5.73$\pm$0.06 |  0.70$\pm$0.13 |  1.65$\pm$0.24 |
$2_{3}-1_{2}$ |  45826.734$\pm$0.002 |  9.11$\pm$0.47 |  5.75$\pm$0.07 |  0.75$\pm$0.03 |  11.36$\pm$0.46 |
$2_{2}-1_{1}$ |  46182.187$\pm$0.002 |  2.73$\pm$0.30 |  5.70$\pm$0.03 |  0.49$\pm$0.07 |  5.26$\pm$0.44 |
$4_{5}-3_{4}$ |  92227.870$\pm$0.003 |  6.34$\pm$0.35 |  5.71$\pm$0.03 |  0.53$\pm$0.09 |  11.34$\pm$1.53 |
$4_{4}-3_{3}$ |  92363.257$\pm$0.004 |  3.37$\pm$0.47 |  5.64$\pm$0.05 |  0.32$\pm$0.05 |  11.06$\pm$1.37 |
$4_{3}-3_{2}$ |  92718.775$\pm$0.003 |  4.31$\pm$0.38 |  5.73$\pm$0.03 |  0.64$\pm$0.07 |  6.29$\pm$0.80 |
C3O3
4- 3 |  38486.891$\pm$0.001 |  38.76$\pm$0.34 |  5.74$\pm$0.01 |  0.59$\pm$0.01 |  61.78$\pm$0.43 |
5- 4 |  48108.474$\pm$0.001 |  40.83$\pm$0.15 |  5.74$\pm$0.01 |  0.65$\pm$0.01 |  58.61$\pm$0.23 |
8- 7 |  76972.587$\pm$0.001 |  30.31$\pm$0.53 |  5.75$\pm$0.01 |  0.55$\pm$0.01 |  51.56$\pm$1.10 |
9- 8 |  86593.685$\pm$0.001 |  17.30$\pm$1.40 |  5.75$\pm$0.02 |  0.41$\pm$0.04 |  39.26$\pm$3.00 |
10- 9 |  96214.614$\pm$0.001 |  8.47$\pm$0.22 |  5.75$\pm$0.01 |  0.50$\pm$0.02 |  16.07$\pm$0.55 |
11-10 |  105835.358$\pm$0.002 |  4.85$\pm$0.71 |  5.78$\pm$0.03 |  0.38$\pm$0.07 |  12.04$\pm$2.30 |
C5O3,4
12-11 |  32804.100$\pm$0.010 |  0.69$\pm$0.21 | 5.75 |  0.57$\pm$0.19 |  0.68$\pm$0.17 |
13-12 |  35527.730$\pm$0.010 |  0.53$\pm$0.15 | 5.75 |  0.51$\pm$0.20 |  0.97$\pm$0.18 |
14-13 |  38271.346$\pm$0.010 |  0.55$\pm$0.13 | 5.75 |  0.67$\pm$0.19 |  0.78$\pm$0.18 |
15-14 |  41004.959$\pm$0.020 |  0.79$\pm$0.14 | 5.75 |  0.69$\pm$0.13 |  1.07$\pm$0.22 |
16-15 |  43738.562$\pm$0.030 |  0.57$\pm$0.19 | 5.75 |  0.95$\pm$0.33 |  0.56$\pm$0.23 |
17-16 |  46472.169$\pm$0.030 |  0.42$\pm$0.12 | 5.75 |  0.56$\pm$0.11 |  0.96$\pm$0.26 |
HCO1
$1_{01}-0_{00}$ 3/2-1/2 2-1 |  86670.760$\pm$0.060 |  42.52$\pm$1.97 |  5.94$\pm$0.02 |  0.66$\pm$0.04 |  60.69$\pm$3.91 |
$1_{01}-0_{00}$ 3/2-1/2 1-0 |  86708.360$\pm$0.040 |  |  |  |  | C
$1_{01}-0_{00}$ 1/2-1/2 1-1 |  86777.460$\pm$0.040 |  23.94$\pm$1.67 |  5.90$\pm$0.02 |  0.60$\pm$0.06 |  37.57$\pm$3.10 |
$1_{01}-0_{00}$ 1/2-1/2 0-1 |  86805.780$\pm$0.100 |  11.84$\pm$1.70 |  5.95$\pm$0.04 |  0.58$\pm$0.10 |  19.25$\pm$3.27 |
HCCO1
$2_{02}-1_{01}$ 5/2-3/2 3-2 |  43317.667$\pm$0.004 |  5.80$\pm$0.19 |  5.71$\pm$0.02 |  0.68$\pm$0.03 |  8.00$\pm$0.22 |
$2_{02}-1_{01}$ 5/2-3/2 2-1 |  43321.145$\pm$0.004 |  4.22$\pm$0.27 |  5.66$\pm$0.04 |  0.92$\pm$0.08 |  4.30$\pm$0.22 |
$2_{02}-1_{01}$ 3/2-1/2 2-1 |  43329.542$\pm$0.003 |  5.07$\pm$0.26 |  5.69$\pm$0.02 |  0.73$\pm$0.04 |  6.56$\pm$0.22 |
$2_{02}-1_{01}$ 3/2-1/2 1-0 |  43335.463$\pm$0.004 |  2.23$\pm$0.25 |  5.70$\pm$0.08 |  0.98$\pm$0.13 |  2.14$\pm$0.22 |
$2_{02}-1_{01}$ 5/2-3/2 2-2 |  43336.861$\pm$0.004 |  0.98$\pm$0.25 |  5.66$\pm$0.08 |  0.63$\pm$0.15 |  1.45$\pm$0.22 |
$2_{02}-1_{01}$ 3/2-3/2 1-1 |  43337.304$\pm$0.006 |  1.54$\pm$0.23 |  5.67$\pm$0.08 |  0.75$\pm$0.15 |  1.93$\pm$0.22 |
$4_{04}-3_{03}$ 9/2-7/2 5-4 |  86642.342$\pm$0.006 |  3.16$\pm$1.00 |  5.64$\pm$0.08 |  0.22$\pm$0.11 |  13.71$\pm$3.80 |
$4_{04}-3_{03}$ 9/2-7/2 4-3 |  86643.848$\pm$0.005 |  4.27$\pm$1.09 |  5.67$\pm$0.08 |  0.26$\pm$0.11 |  15.31$\pm$3.80 |
$4_{04}-3_{03}$ 7/2-5/2 4-3 |  86655.831$\pm$0.005 |  7.23$\pm$1.45 |  5.69$\pm$0.08 |  0.40$\pm$0.13 |  16.97$\pm$3.80 |
$4_{04}-3_{03}$ 7/2-5/2 3-2 |  86657.485$\pm$0.005 |  2.96$\pm$0.97 |  5.64$\pm$0.12 |  0.24$\pm$0.12 |  11.80$\pm$3.80 |
HC5O5
25/2-23/2 $e$ 13-12 |  32267.964$\pm$0.002 |  3.86$\pm$0.42 |  5.75$\pm$0.03 |  0.67$\pm$0.06 |  5.38$\pm$0.19 |
25/2-23/2 $e$ 12-11 |  32268.049$\pm$0.002 |  4.08$\pm$0.42 |  5.75$\pm$0.04 |  0.75$\pm$0.06 |  5.10$\pm$0.19 |
25/2-23/2 $f$ 13-12 |  32271.760$\pm$0.002 |  4.11$\pm$0.29 |  5.67$\pm$0.03 |  0.75$\pm$0.05 |  5.14$\pm$0.19 |
25/2-23/2 $f$ 12-11 |  32271.848$\pm$0.002 |  3.21$\pm$0.30 |  5.72$\pm$0.02 |  0.65$\pm$0.06 |  5.38$\pm$0.19 |
27/2-25/2 $e$ 14-13 |  34849.461$\pm$0.002 |  4.15$\pm$0.11 |  5.74$\pm$0.01 | 0.60 |  6.49$\pm$0.19 | D
27/2-25/2 $e$ 13-12 |  34849.540$\pm$0.002 |  3.18$\pm$0.11 |  5.73$\pm$0.02 | 0.60 |  4.99$\pm$0.19 | D
27/2-25/2 $f$ 14-13 |  34853.387$\pm$0.002 |  3.55$\pm$0.09 |  5.71$\pm$0.01 | 0.60 |  5.55$\pm$0.19 | D
27/2-25/2 $f$ 13-12 |  34854.469$\pm$0.002 |  2.79$\pm$0.10 |  5.67$\pm$0.01 | 0.60 |  4.37$\pm$0.19 | D
29/2-27/2 $e$ 15-14 |  37430.945$\pm$0.003 |  3.67$\pm$0.15 |  5.74$\pm$0.02 | 0.60 |  5.74$\pm$0.21 | D
29/2-27/2 $e$ 14-13 |  37431.020$\pm$0.003 |  3.42$\pm$0.14 |  5.73$\pm$0.02 | 0.60 |  5.35$\pm$0.21 | D
29/2-27/2 $f$ 15-14 |  37435.011$\pm$0.003 |  3.70$\pm$0.14 |  5.72$\pm$0.02 | 0.60 |  5.79$\pm$0.21 | D
29/2-27/2 $f$ 14-13 |  37435.088$\pm$0.003 |  3.29$\pm$0.14 |  5.70$\pm$0.02 | 0.60 |  5.14$\pm$0.21 | D
31/2-29/2 $e$ |  40012.451$\pm$0.004 |  6.25$\pm$0.19 |  5.73$\pm$0.01 |  0.94$\pm$0.03 |  6.26$\pm$0.25 |
31/2-29/2 $f$ |  40016.669$\pm$0.004 |  6.13$\pm$0.19 |  5.73$\pm$0.02 |  0.95$\pm$0.03 |  6.05$\pm$0.25 |
33/2-31/2 $e$ |  42593.906$\pm$0.005 |  4.76$\pm$0.17 |  5.75$\pm$0.02 |  0.89$\pm$0.04 |  5.03$\pm$0.23 |
33/2-31/2 $f$ |  42598.284$\pm$0.005 |  5.42$\pm$0.17 |  5.75$\pm$0.02 |  0.90$\pm$0.03 |  5.69$\pm$0.23 |
35/2-33/2 $e$ |  45175.347$\pm$0.005 |  4.88$\pm$0.23 |  5.79$\pm$0.02 |  0.87$\pm$0.04 |  5.31$\pm$0.28 |
35/2-33/2 $f$ |  45179.895$\pm$0.005 |  5.51$\pm$0.26 |  5.77$\pm$0.02 |  1.00$\pm$0.06 |  5.20$\pm$0.28 |
37/2-35/2 $e$ |  47746.772$\pm$0.007 |  3.59$\pm$0.33 |  5.82$\pm$0.04 |  0.71$\pm$0.09 |  4.36$\pm$0.30 |
37/2-35/2 $f$ |  47761.502$\pm$0.007 |  3.89$\pm$0.36 |  5.72$\pm$0.04 |  0.72$\pm$0.07 |  5.05$\pm$0.30 |
HC7O5
57/2-55/2 $e$ |  31274.822$\pm$0.003 |  0.90$\pm$0.25 |  5.81$\pm$0.08 |  0.84$\pm$0.21 |  1.01$\pm$0.19 |
57/2-55/2 $f$ |  31276.500$\pm$0.003 |  1.05$\pm$0.15 |  5.89$\pm$0.06 |  0.78$\pm$0.12 |  1.28$\pm$0.19 |
59/2-57/2 $e$ |  32372.184$\pm$0.003 |  |  |  |  | E
59/2-57/2 $f$ |  32373.871$\pm$0.003 |  1.29$\pm$0.10 |  5.89$\pm$0.04 |  0.87$\pm$0.08 |  1.39$\pm$0.19 |
61/2-59/2 $e$ |  33469.542$\pm$0.004 |  1.02$\pm$0.11 |  6.00$\pm$0.04 |  0.74$\pm$0.02 |  1.30$\pm$0.20 |
61/2-59/2 $f$ |  33471.241$\pm$0.004 |  |  |  |  | E
63/2-61/2 $e$ |  34566.898$\pm$0.004 |  |  |  |  | F
63/2-61/2 $f$ |  34568.607$\pm$0.004 |  0.53$\pm$0.08 |  5.73$\pm$0.08 |  0.65$\pm$0.16 |  0.76$\pm$0.20 |
65/2-63/2 $e$ |  35664.244$\pm$0.005 |  0.37$\pm$0.07 |  5.78$\pm$0.18 |  0.58$\pm$0.21 |  0.59$\pm$0.18 |
65/2-63/2 $f$ |  35665.972$\pm$0.005 |  0.54$\pm$0.07 |  5.68$\pm$0.25 |  1.22$\pm$0.32 |  0.43$\pm$0.18 |
67/2-65/2 $e$ |  36761.602$\pm$0.005 |  0.43$\pm$0.12 |  5.49$\pm$0.16 |  1.11$\pm$0.35 |  0.41$\pm$0.21 |
67/2-65/2 $f$ |  36763.333$\pm$0.005 |  0.59$\pm$0.10 |  5.69$\pm$0.07 |  0.79$\pm$0.12 |  0.70$\pm$0.21 |
69/2-67/2 $e$ |  37858.949$\pm$0.005 |  0.33$\pm$0.10 |  6.00$\pm$0.12 |  0.43$\pm$0.25 |  0.71$\pm$0.23 |
69/2-67/2 $f$ |  37860.691$\pm$0.005 |  0.55$\pm$0.13 |  6.04$\pm$0.09 |  0.74$\pm$0.19 |  0.70$\pm$0.23 |
Table 2: continued. 555
$a$$a$footnotetext: Rest frequency of the transition. See text for the
laboratory data used for the frequency predictions of each molecular species.
$b$$b$footnotetext: Integrated line intensity in mK km s-1.
$c$$c$footnotetext: vLSR in km s-1.
$d$$d$footnotetext: Linewidth at half intensity derived by fitting a Gaussian
function to the observed line profile (in km s-1).
$e$$e$footnotetext: Antenna temperature in milli Kelvin.
$1$$1$footnotetext: Quantum numbers are $N,K_{a},K_{c},J$, and $F$.
$2$$2$footnotetext: Quantum numbers are $N$ and $J$.
$3$$3$footnotetext: The quantum number is $J$.
$4$$4$footnotetext: Improved rest frequencies were derived assuming a vLSR of
5.75 km s-1 (see Sect. 3.2).
$5$$5$footnotetext: Quantum numbers are $J$, $\Lambda$-doubling component, and
$F$. If the hyperfine structure is not resolved in our data, then $F$ is not
given. In this case, the adopted rest frequency corresponds to the average of
those of the hyperfine components.
$A$$A$footnotetext: This line is contaminated by the frequency switching
negative feature of the transition $4_{0,4}-3_{0,3}$ 9/2-7/2 5-4 of the same
species after data folding since they are separated by exactly 8.0 MHz, i.e.
the frequency throw used in the observations.
$B$$B$footnotetext: This doublet appears unresolved in our data (see Fig. 1).
The linewidth has been fixed to 0.6 km s-1. The resulting frequencies and
intensities have slightly larger uncertainties than those of other transitions
for this molecule.
$C$$C$footnotetext: The $J$=3/2-1/2 $F$=1-0 component is heavily blended with
the $J$=15-14 transition of C3S avoiding a reliable determination of the line
parameters (see Fig. 4).
$D$$D$footnotetext: The hyperfine components are only partially resolved. To
obtain reliable parameters for each hyperfine component, their linewidths are
fixed to 0.6 km s-1.
$E$$E$footnotetext: Heavily affected by a negative feature produced in the
folding of the frequency switching data (see Fig. 7).
$F$$F$footnotetext: This transition is heavily blended with the $J$=31-30
transition of HC613CN at 34566.972 MHz, which has an intensity of 2.1 mK (see
Fig. 7)

##  Appendix B HCnO species

###  B.1 HCO

The HCO radical has an electronic ground state ${}^{2}A^{\prime}$ and has been
observed in the laboratory by different authors (Saito , 1972; Austin et al. ,
1974; Pickett & Boyd , 1978; Blake et al. , 1984). Frequency predictions are
available in the JPL catalogue (Pickett et al. , 1998). We implemented the
molecule in the MADEX catalogue and searched for its lines in our 3mm line
survey of TMC-1. No lines of HCO appear in the QUIJOTE line survey. Four
hyperfine components of the 101-000 transition are detected and they are shown
in Fig. 4. Their line parameters are given in Table LABEL:obs_line_parameters.
One of these lines is heavily blended with the $J$=15-14 line of C3S. The
observed velocities are systematically shifted by 0.15 km s-1 with respect the
velocity of CCO, C3O, HCCO, and HC3O. This is a result of the low accuracy,
40-100 kHz, of the laboratory measurements for this transition (see Table
LABEL:obs_line_parameters and Pickett & Boyd  (1978)). We adopted a rotational
temperature of 7 K and derived a column density of (1.1$\pm$0.1)$\times$1012
cm-2.

Figure 4: Observed lines of the $1_{01}-0_{00}$ transition of HCO in TMC-1.
The abscissa corresponds to the rest frequency of the lines. The vLSR,
linewidth, and integrated intensity of each line are provided in Table
LABEL:obs_line_parameters. The ordinate is the antenna temperature, corrected
for atmospheric and telescope losses, in milli Kelvin. The noise of this
spectrum is 3.7 mK. The quantum numbers for each transition are indicated. The
red line shows the computed synthetic spectrum for this species (see Appendix
B.1).

###  B.2 HCCO

We note that HCCO has an electronic ground state ${}^{2}A^{\prime\prime}$ and
its rotational transitions with $K_{a}$=0 have been observed in the laboratory
by different authors (Endo & Hirota , 1987; Ohshima & Endo , 1993; Chantzos et
al. , 2019). The dipole moment of the molecule has been estimated through ab
initio calculations to be $\mu_{a}$=1.59 D, with a small component along the
$b$ axis of the molecule (Szalay et al. , 1996; Müller et al., 2005).
Predictions for the transitions of this molecule are provided in the CDMS
catalogue (Müller et al., 2005) and have been implemented in MADEX. The
molecule has been detected towards several cold dark clouds by Agúndez et al.
(2015). The $N=2-1$ transition has been observed with the QUIJOTE line survey.
The six fine and hyperfine components are shown in Fig. 5. The derived line
parameters are given in Table LABEL:obs_line_parameters. Two transitions of
HCCO, the $N=4-3$ and $N=5-4$, have also been observed in our 3mm data. The
four strongest hyperfine components of the $N=4-3$ are detected at the
3$\sigma$ level and their paramaters are given in Table
LABEL:obs_line_parameters. For the $N=5-4$ transition, only upper limits of 11
mK (3$\sigma$) are obtained. Adopting the rotational temperature of CCO (7 K),
and a source of uniform brightness with a diameter of 80′′ (Fossé et al.,
2001), we derived a column density of (7.7$\pm$0.7)$\times$1011 cm-2. The
synthetic spectrum for the $N=2-1$ transition is shown in red in Fig. 5 and
matches the observed lines rather well. The predicted intensities for the
$N=4-3$ lines are in agreement with the derived parameters for the four
strongest hyperfine components of this transition.

Figure 5: Observed lines of the $2_{02}-1_{01}$ transition of HCCO in TMC-1.
The abscissa corresponds to the rest frequency of the lines. The vLSR,
linewidth, and integrated intensity of each line are given in Table
LABEL:obs_line_parameters. The ordinate is the antenna temperature, corrected
for atmospheric and telescope losses, in milli Kelvin. The noise of this
spectrum is 0.22 mK. The quantum numbers for each transition are indicated.
The hyperfine components of the fine structure transitions $J$=5/2-3/2 and
$J$=3/2-1/2 are indicated in violet and blue, respectively. The red line shows
the computed synthetic spectrum for this species (see Appendix B.2). Blank
channels correspond to negative features produced in the folding of the
frequency switching data.

###  B.3 HC5O and HC7O

It is important to note that HC5O is a linear molecule with a ${}^{2}\Pi_{r}$
state and its ${}^{2}\Pi_{1/2}$ rotational spectrum has been observed in the
laboratory by Mohamed et al.  (2005), who also provided an estimate of the
dipole moment of 2.16 D from ab initio calculations. The molecule has been
detected in TMC-1 by McGuire et al.  (2017) through the detection of the two
$\Lambda$-components, each one splitted in two hyperfine components, of the
$J$=17/2-15/2 rotational transition at 21.85 GHz. In Fig. 6 we report the two
$\Lambda$-components of seven rotational transitions from $J$=25/2-23/2 up to
$J$=37/2-35/2. The signal-to-noise ratio for each individual line is larger
than 30. Line parameters for the observed lines are given in Table
LABEL:obs_line_parameters. The derived velocities of the observed transitions
are $\sim$5.75 km s-1, that is similar to those of the lines of C3O. We
adopted a rotational temperature of 10 K and derived a column density for HC5O
of (1.4$\pm$0.2)$\times$1012 cm-2. This value is in good agreement with the
column density of 1.7$\times$1012 cm-2 derived by McGuire et al.  (2017).

We note that HC7O also has a ${}^{2}\Pi_{r}$ ground electronic state and its
rotational spectrum has also been derived from Mohamed et al.  (2005). The
transitions falling within the QUIJOTE line survey have large $J$ numbers and
energies above 20 K. This molecule was reported as detected through stacking
techniques by Cordiner et al.  (2017). These authors assume a rotational
temperature of 6 K and obtain a column density of (7.8$\pm$0.9)$\times$1011
cm-2. Using these values, we predict an antenna temperature of 0.5 mK for the
$J$=57/2-55/2 $e$ and $f$ $\Lambda$-doubling components (31.275 GHz). No other
lines could be detected within the sensitivity of the QUIJOTE line survey for
this rotational temperature. However, we have explored all the lines up to 40
GHz and found several lines of HC7O up to the $J$=69/2-67/2. They are shown in
Fig. 7. Although some of the lines are blended with other lines or with
negative features resulting from the folding of the frequency switching data,
the detection of HC7O is now secure and based on the detection of several
individual lines. The observed velocities range from between 5.5 and 6 km s-1,
which is probably due to the limited signal-to-noise ratio for the weak
emission of this species, and to the large extrapolation in quantum numbers
compared with the laboratory data ($\nu_{max}$=19.2 GHz, $J_{max}$=35/2).

A rotational diagram indicates that the rotational temperature of HC7O could
be close to 10 K, rather than the 6 K previously assumed (Cordiner et al. ,
2017). Using a model fitting to the observed line profiles (Cernicharo et al.,
2021d) with a rotational temperature of 10 K, we derived a column density of
(6.5$\pm$0.5)$\times$1011 cm-2, which is in reasonable agreement with the
value of Cordiner et al.  (2017). Although our data cannot support a
rotational temperature as low as the value adopted by Cordiner et al.  (2017),
the effect on the derived column density is moderate. This effect has been
discussed in detail in the analysis of the column density of HCS+ and HC3S+
(Cernicharo et al., 2021f). The HC5O/HC7O abundance ratio is 2.2$\pm$0.3.

We searched for HC4O and HC6O without success. The derived upper limits are
given in Table 1.

Figure 6: Observed lines of HC5O in TMC-1. The abscissa corresponds to the
rest frequency of the lines. The vLSR, linewidth, and integrated intensity of
each line are given in Table LABEL:obs_line_parameters. The ordinate is the
antenna temperature, corrected for atmospheric and telescope losses, in milli
Kelvin. The quantum numbers for each transition are indicated. The red line
shows the computed synthetic spectrum for this species (see Appendix B.3).
Blank channels correspond to negative features produced in the folding of the
frequency switching data.  Figure 7: Observed lines of HC7O in TMC-1. The
abscissa corresponds to the rest frequency of the lines. The vLSR, linewidth,
and integrated intensity of each line are given in Table
LABEL:obs_line_parameters. The ordinate is the antenna temperature, corrected
for atmospheric and telescope losses, in milli Kelvin. The quantum numbers for
each transition are indicated. The red line shows the computed synthetic
spectrum for this species (see Appendix B.3). Blank channels correspond to
negative features produced in the folding of the frequency switching data.
Figure 8: Observed transitions of CCO in our line survey towards TMC-1 . The
abscissa corresponds to the rest frequency of the lines. The vLSR, linewidth,
and integrated intensity of each line are given in Table
LABEL:obs_line_parameters. The ordinate is the antenna temperature, corrected
for atmospheric and telescope losses, in milli Kelvin. The quantum numbers for
each transition are indicated in the upper right corner of the corresponding
panel. The red line shows the computed synthetic spectrum for this species
(see Appendix C.1). Blank channels correspond to frequencies affected by
negative features produced in the folding of the frequency switching data.

##  Appendix C CnO species

###  C.1 Dicarbon Monoxide, CCO

This molecule has a ${}^{3}\Sigma^{-}$ ground electronic state and its
microwave and millimetre wave spectra are well-known (Yamada et al. , 1985;
Ohshima et al. , 1995). We note that CCO was detected in TMC-1 by Ohishi et
al.  (1991). The laboratory data cover frequencies up to 184.8 GHz and
$J_{max}$=9, which guarantees excellent frequency predictions in the frequency
domain of our line survey. A dipole moment of 1.305 D was derived through ab
initio calculations by Thomson & Wishart  (1973). The lines observed with the
Yebes 40m and IRAM 30m radio telescopes are shown in Fig. 8, and the derived
line parameters for all lines are given in Table LABEL:obs_line_parameters. A
rotational diagram assuming a source of uniform brightness temperature and
80′′ in diameter (Fossé et al., 2001) provides a rotational temperature of
7.0$\pm$1.5 K and a column density of (7.5$\pm$0.3)$\times$1011 cm-2. This
value is in good agreement with that obtained by Ohishi et al.  (1991), and it
is slightly smaller than the one derived by Cernicharo et al. (2020a) who
assumed an uniform rotational temperature of 10 K and did not consider the 3mm
lines. The effect of the adopted rotational temperature on the column density
is much more critical for this ${}^{3}\Sigma^{-}$ molecule than for a linear
species of a similar rotational constant. This is due to the large energy
difference between the levels $J=N-1,N$, and $N+1$, and the possible
propensity rules for collisions between these levels. The comparison of the
observed intensities at 3 and 7mm suggest that at 3mm ($N=4$) the excitation
temperature is below 6 K, while at low frequency it could be close to the
kinetic temperature of 10 K.

In order to evaluate the possible excitation effects, we used the MADEX code
to perform a large velocity gradient calculation (LVG; MADEX follows the
formalism described by Goldreich & Kwan  (1974)). For this purpose, we adopted
the collisional rates of OCS (Green & Chapman , 1978) and the infinite-order-
sudden approximation for a ${}^{3}\Sigma$ molecule (Alexander & Dagdigian ,
1983; Corey & McCourt , 1983; Fuente et al. , 1990). For a volume density of
4$\times$104 cm-3, the excitation temperatures obtained using this approach
are $\sim$10 K for the lines observed at low frequency (QUIJOTE data).
However, the excitation temperature decreases very fast to 6.5 K for $N$=4 and
to $\sim$5.5 K for $N$=5. We have explored the effect of n(H2) on the
excitation temperatures of CCO. For n(H2)$\geq$9$\times$103 cm-3, the $N$=1
and 2 lines have an excitation temperature close to TK . However, the lines
associated with the levels with $N$=4 show a sharp dependence of Texc with
density. The best fit to the observations is obtained for n(H2)=104 cm-3 and
N(CCO)=7.5$\times$1011 cm-2, that is, the value derived from the rotational
diagram. For this density, the excitation temperature of the $N$=5 lines
decreases to 4-5 K.

The synthetic spectrum computed with the parameters resulting from the best
fit is shown in Fig. 8. The match between the observations and the model is
rather good despite the uncertainties on the adopted collisional rates, which
also affect the derived volume density. Collisional rates for CCO are needed
in order to derive more accurate physical conditions from its rotational
transitions.

###  C.2 Oxopropadienylidene, CCCO

There is an extensive literature on laboratory experiments concerning the
centimetre, millimetre, and sub-millimetre spectroscopy of oxopropadienylidene
(Brown et al. , 1983; Tang et al. , 1985; Klebsch et al., 1985; Bogey et al. ,
1986; Bizzocchi et al. , 2008). The data cover frequencies up to 740 GHz and
$J_{max}$=77. The molecule has a dipole moment of 2.39 D (Brown et al. ,
1983). This molecule has been detected in the interstellar and circumstellar
media (Matthews et al., 1984; Brown et al. , 1985; Tenenbaum et al., 2006).
The protonated form of this species, HCCCO+, has been recently detected in
TMC-1 (Cernicharo et al., 2020a). A study of CCO and CCCO in low-mass star-
forming regions has been recently made by Urso et al.  (2019).

The observed lines with the Yebes and IRAM radio telescopes are shown in Fig.
9. The derived line parameters are given in Table LABEL:obs_line_parameters.
Assuming a source of 80′′ in diameter with a uniform brightness temperature,
we obtain a rotational temperature of 7$\pm$1 K and a column density of
(1.2$\pm$0.2)$\times$1012 cm-2. This value is in good agreement with that
reported by Matthews et al. (1984), Brown et al.  (1985), and Cernicharo et
al. (2020a). The synthetic spectrum computed for these values of the
rotational temperature and the column density is shown by the red lines in
Fig. 9. The agreement between the model and observations is excellent. In
order to check the assumption of a uniform rotational temperature for all
rotational levels of CCCO, which could be the main source error in the
determination of the column density (Cernicharo et al., 2021b), we performed
large velocity gradient calculations adopting the collisional rates of HC3N
(Faure et al. , 2016) for CCCO. Adopting a density of 4$\times$104 cm-3
(Cernicharo & Guélin, 1987; Fossé et al., 2001), we obtain an excitation
temperature of $\sim$10 K for all levels below $J\leq$5, and a continuous
decrease in $T_{exc}$ with increasing $J$, reaching a value of $\sim$6 K for
$J=11$. Hence, the derived rotational temperature is strongly determined by
the transitions with high-$J$ levels observed with the IRAM 30m radio
telescope.

Figure 9: Observed transitions of C3O in TMC-1. The abscissa corresponds to
the rest frequency of the lines. Their vLSR, linewidth, and integrated
intensity of each line are given in Table LABEL:obs_line_parameters. The
ordinate is the antenna temperature, corrected for atmospheric and telescope
losses, in milli Kelvin. The quantum numbers for each transition are indicated
in the upper right corner of the corresponding panel. The red line shows the
computed synthetic spectrum for this species (see Appendix C.2).

###  C.3 C4O and C6O

We note that C4O and C6O were observed in the laboratory by Ohshima et al.
(1995). Both have a ${}^{3}\Sigma^{-}$ ground electronic state. These
molecules have been implemented in MADEX and we searched for their strongest
transitions within the QUIJOTE line survey. None of their lines are detected.
Assuming a rotational temperature identical to that of C5O (Trot=10 K), we
derived 3$\sigma$ upper limits to their column density of 9.0$\times$1010 and
1.1$\times$1011 cm-2, respectively.