# Spectroscopic Survey of Faint Planetary-Nebula Nuclei. IV. The Abell 35-Type
Central Star of Pa 27111Based on observations obtained with the Hobby-Eberly
Telescope (HET), which is a joint project of the University of Texas at
Austin, the Pennsylvania State University, Ludwig-Maximillians-Universität
München, and Georg-August Universität Göttingen. The HET is named in honor of
its principal benefactors, William P. Hobby and Robert E. Eberly.

Howard E. Bond Department of Astronomy & Astrophysics, Pennsylvania State
University, University Park, PA 16802, USA  Space Telescope Science Institute,
3700 San Martin Dr., Baltimore, MD 21218, USA  Gregory R. Zeimann Hobby-Eberly
Telescope, University of Texas at Austin, Austin, TX 78712, USA Howard E. Bond
<EMAIL_ADDRESS>
###### Abstract

We present optical spectroscopy of the 12th-mag central star of the planetary
nebula (PN) Patchick 27 (Pa 27), obtained during a survey of faint PN nuclei
(PNNi) with the Low-Resolution Spectrograph (LRS2) of the Hobby-Eberly
Telescope. The optical spectrum of Pa 27 is that of a K0 III red giant with
rotationally broadened lines. However, the star is detected in the near-
ultraviolet (near-UV) with GALEX, showing that a hot binary component is also
present. The spectral-energy distribution from the near-UV to the mid-infrared
can be fitted with a combination of the K0 III giant and a hot PNN with an
effective temperature of about 50,000 K. Photometric observations of Pa 27,
both ground-based and from TESS, show a low-amplitude sinusoidal variation
with a period of 7.36 days, probably due to starspots on a rotating and
magnetically active cool giant. Pa 27 is a new member of the rare class of
“Abell 35-type central stars,” which are binary PNNi consisting of a spotted
late-type star and a hot pre-white dwarf. They are likely the result of a
situation where an AGB star ejects its outer layers in a dense wind, part of
which is captured by a distant companion, spinning up its rotation by
accretion of material and angular momentum. We suggest several useful follow-
up observations.

##  1 Introduction

The central stars of planetary nebulae (PNe)—planetary-nebula nuclei
(PNNi)—exhibit a diverse range of spectroscopic and photometric phenomena,
including unusual chemical compositions, binarity, pulsations, strong stellar
winds, and rapid evolution (see review articles by Weidmann et al. 2020 and
Kwitter & Henry 2022). This is the fourth in a series of papers presenting
results from a spectroscopic survey of central stars of faint Galactic PNe.
The survey is carried out with the second-generation Low-Resolution
Spectrograph (LRS2; Chonis et al. 2016) of the 10-m Hobby-Eberly Telescope
(HET; Ramsey et al. 1998; Hill et al. 2021), located at McDonald Observatory
in west Texas, USA.

An overview of the survey, a description of the instrumentation and data-
reduction procedures, target selection, and some initial results, were
presented in our first paper (Bond et al., 2023a, hereafter Paper I). Paper II
(Bond et al., 2023b) discussed the central star of the “PN mimic” Fr 2-30, and
Paper III (Werner et al., 2024) presents discoveries of three new extremely
hot hydrogen-deficient PNNi. About 50 central stars have been observed to
date. Future papers will discuss several more individual objects of special
interest, and another publication will present results on a group of nuclei
with fairly normal hydrogen-rich spectra. In this fourth paper we describe our
discovery of a remarkable central star, that of the PN Patchick 27 (Pa 27). It
is a new member of a small group of binary nuclei composed of a rotating and
spotted late-type star and a hot companion. We present some initial findings,
and we encourage follow-up observations.

##  2 The Faint Planetary Nebula Pa 27

As explained in our previous papers, we assembled a lengthy target list of
central stars for our survey, most of them belonging to faint PNe discovered
in recent years by amateur astronomers, and this list was submitted to the HET
observing queue (see Shetrone et al. 2007). Exposures were chosen for
execution by the telescope schedulers, essentially at random, depending on sky
conditions and lack of observable higher-ranked targets.

Pa 27 (PN G075.0$-$07.2) is a very low-surface-brightness PN discovered by
amateur Dana Patchick,222See https://www.astrobin.com/users/DanaPatchick a
member of the Deep Sky Hunters (DSH) collaboration. It was included in a list
of PNe found by the DSH team that was presented in a conference
poster333Available at
http://www.astroscu.unam.mx/apn6/PROCEEDINGS/B3-Kronberger.pdf by Kronberger
et al. (2014). The poster shows a deep narrow-band image of Pa 27, obtained
with the Kitt Peak 2.1-m telescope in H$\alpha$ and [O III] 5007 Å. Pa 27 lies
just off the northern edge of the Veil Nebula supernova remnant in
Cygnus,444Deep, wide-angle images of the field are available at
https://www.astrobin.com/full/g3myxl/0/ (Andrzej Polkowski) and
http://cosmicneighbors.net/pa27-big.jpg (Mike Keith). but is an unrelated
background object. The PN is roughly circular, with a diameter of about
$72^{\prime\prime}$. The Kitt Peak image shows bright filaments on its
northern rim, possibly due to an interaction with the interstellar
medium.555Alternatively, the filaments on the northern edge of Pa 27 could be
superposed foreground features belonging to the Veil Nebula. The fact that the
proper motion of the PN central star is not in a northerly direction may
support this possibility. Further information about Pa 27 is given in the
online Hong-Kong/AAO/Strasbourg/H$\alpha$ Planetary Nebulae (HASH)
database666http://hashpn.space/ (Parker et al., 2016; Bojičić et al., 2017),
including a spectrum of the PN and direct images at several wavelengths from
the ultraviolet (UV) to the mid-infrared.

##  3 Central Star

A conspicuous 12th-mag star lies near the center of Pa 27. An image obtained
by the Galaxy Evolution Explorer (GALEX), which is shown at the HASH website,
reveals that this star is bright in the near-UV, making it likely to be the
ionizing source and central star of the PN. Table 1 gives information on the
star, taken from Gaia Data Release 3777https://vizier.cds.unistra.fr/viz-
bin/VizieR-3?-source=I/355/gaiadr3 (DR3; Gaia Collaboration et al. 2016,
2023), including its celestial and Galactic coordinates, parallax and proper
motion, magnitude and color, and radial velocity and line broadening.

Table 1: Gaia DR3 Data for Central Star of Pa 27  Parameter | Value
---|---
RA (J2000) | 20 48 58.358
Dec (J2000) | +32 18 14.73
$l$ [deg] | 75.00
$b$ [deg] | $-7.20$
Parallax [mas] | $0.686\pm 0.011$
$\mu_{\alpha}$ [mas yr-1] | $+7.586\pm 0.008$
$\mu_{\delta}$ [mas yr-1] | $+2.653\pm 0.012$
$G$ [mag] | 12.27
$G_{\rm BP}-G_{\rm RP}$ [mag] | $1.24$
Radial velocity [${\>\rm km\>s^{-1}}$] | $-47.36\pm 2.31$
Line broadening [${\>\rm km\>s^{-1}}$] | $30.8\pm 22.3$

A large majority of PNNi have spectra indicating very high surface
temperatures, which is true of all of the objects from our survey described in
our first three papers. However, the central star of Pa 27 is unusually red at
optical wavelengths, with a Gaia color index of $G_{\rm BP}-G_{\rm RP}=1.24$
(see Table 1). Since the nucleus is also a GALEX source, the system is likely
to be a binary consisting of the optical star and a UV-bright hot companion.

The reciprocal of the Gaia parallax gives a distance to the star (and to the
nebula) of $1458\pm 23$ pc. Using a Bayesian analysis, Bailer-Jones et al.
(2021) refine the estimated geometric distance to $1437_{-24}^{+23}$ pc. At
this distance, the foreground interstellar extinction, determined using the
online Stilism tool888https://stilism.obspm.fr/ (Capitanio et al., 2017), is
$E(B-V)=0.225\pm 0.04$. Based on these values, and an apparent magnitude of
$V=12.68$ from the APASS catalog,999https://www.aavso.org/download-apass-data
we find an absolute magnitude for the central star of $M_{V}=+1.2$. Thus the
optical star appears to be a late-type giant or bright subgiant.

##  4 HET/LRS2 Observations and Data Reduction

Our previous papers give full details of the LRS2 instrumentation used for our
survey. In summary, LRS2 is composed of two integral-field-unit (IFU)
spectrographs: blue (LRS2-B) and red (LRS2-R), with fields of view of
$6^{\prime\prime}\times 12^{\prime\prime}$. The observations discussed in this
paper were made with LRS2-B, which employs a dichroic beamsplitter to send
light simultaneously into two units: the “UV” channel (covering 3640–4645 Å at
a resolving power of 1910), and the “Orange” channel (covering 4635–6950 Å at
a resolving power of 1140). The data are initially processed using
Panacea,101010https://github.com/grzeimann/Panacea which performs bias and
flat-field correction, fiber extraction, and wavelength calibration. An
absolute-flux calibration comes from default response curves and measures of
the telescope mirror illumination, as well as the exposure throughput from
guider images. We then apply
LRS2Multi111111https://github.com/grzeimann/LRS2Multi to the un-sky-
subtracted, flux-calibrated fiber spectra, to perform source extraction using
a 2$\arcsec$ radius aperture, and subtraction of the spectrum of the sky and
nebula based on an annular aperture surrounding the target. Spectra from
multiple exposures are then combined. The final spectrum, from both channels,
is resampled to a common linear grid with a 0.7 Å spacing. An observation log
for our LRS2-B exposures on Pa 27 is presented in Table 2.

Table 2: Log of HET LRS2-B Observations of Pa 27  Date | Exposure
---|---
[YYYY-MM-DD] | [s]
2020-08-06 | 90
2022-07-15 | 180
2022-08-16 | 180

##  5 Optical Spectrum

We carried out spectral classification for the central star by comparing its
spectrum with those of bright stars with known spectral types. We downloaded
their spectra from the MILES website121212The acronym stands for Medium-
resolution Isaac Newton Telescope Library of Empirical Spectra. See
http://miles.iac.es (Falcón-Barroso et al., 2011), which provides a library of
observed spectra with a resolution similar to that of our HET data. Guided by
the discussion in the next section, we focused on red giants with spectral
types of late G to early K. A good match to the spectrum of Pa 27 was obtained
with the K0 III star $\delta$ Cancri. The MILES site gives this star
parameters of $T_{\rm eff}=4657$ K, $\log g=2.51$, and $\rm[Fe/H]=-0.06$. Its
absolute magnitude, based on an apparent magnitude of $V=3.94$ (Argue, 1963)
and a Gaia parallax of 23.83 mas, is $M_{V}=+0.8$, very close to the value for
Pa 27 (+1.2; Section 3).

In Figure 1 we plot our HET LRS2-B spectrum of the central star of Pa 27,
split into two segments: 5200–6600 Å (blue line in top panel) and 3800–5200 Å
(blue line in bottom panel). Also plotted (red lines in both panels) is the
spectrum of $\delta$ Cnc; it has been scaled to the absolute flux of Pa 27,
and to match the line profiles of the HET spectrum we applied a boxcar
smoothing of 3 pixels (2.7 Å).

Figure 1:  Spectra from 5200 to 6600 Å (top panel) and 3800 to 5200 Å (bottom
panel). The spectrum of the central star of Pa 27 (from the HET LRS2-B
spectrograph) is plotted as a blue line in both panels. For comparison an
archival spectrum of the K0 III star $\delta$ Cnc, scaled to the flux level of
Pa 27, is plotted as a red line. Several conspicuous spectral features are
marked. See text for discussion.

The top panel in Figure 1 shows the close similarity of the spectra at longer
wavelengths, except that the Na I D lines are stronger in Pa 27, due to a
component of interstellar absorption.131313One difference is that a line at
6517 Å is stronger in Pa 27 than in $\delta$ Cnc. We suspect that this feature
is affected by an instrumental artifact, since a similar artifact is present
in several of our spectra of hot central stars. See especially Figure 3 in
Paper I.

However, in the bottom panel of Figure 1, differences in the spectra become
more apparent as we move to shorter wavelengths. The atomic lines and CH
become weaker in the central star than in $\delta$ Cnc, while the Balmer lines
become stronger. The Ca II K absorption line of the cool star is partially
filled in (but note a sharp interstellar component at its core). And the
energy distribution of the nucleus becomes shallower than in the K0 III
standard star. All of these spectral features indicate the presence of a hot
companion of the central star, contributing an increasing amount of flux at
shorter wavelengths, and diluting the contribution from the K0 component.

Our IFU frames of the central star show a stellar profile, as do images in
publicly available sky surveys, so the binary pair is unresolved in ground-
based data. This does not provide a tight constraint on the orbital
separation; for example, a separation of $0\farcs 5$ would correspond to a
projected physical distance as large as $\sim$700 AU.

##  6 Spectral-Energy Distribution

In this section we construct the spectral-energy distribution (SED) of the Pa
27 nucleus. Table 3 presents stellar magnitudes for the central star in a
range of bandpasses, and the corresponding absolute fluxes. We gathered these
data from the following sources: (1) A near-UV magnitude from the GALEX source
catalog141414https://galex.stsci.edu/gr6/?page=mastform (unfortunately there
was no GALEX observation in the far-UV). (2) Magnitudes in the $g$, $r$, $i$,
$z$, and $y$ bands from the photometric
catalog151515https://catalogs.mast.stsci.edu/panstarrs/ of the Panoramic
Survey Telescope and Rapid Response System (Pan-STARRS or PS1; Kaiser et al.
2010). (3) Near-infrared magnitudes from the Two Micron All Sky Survey (2MASS;
Skrutskie et al. 2006). (4) Mid-infrared magnitudes from the Wide-field
Infrared Survey Explorer (WISE; Wright et al., 2010). The 2MASS and WISE data
were obtained from the NASA/IPAC Infrared Science
Archive.161616http://irsa.ipac.caltech.edu/frontpage

Table 3: Spectral-Energy Distribution of Pa 27 Central Star

Bandpass | MagnitudeaaSources for magnitudes in column 2 are given in column 3 and described in the text. Uncertainties are about $\pm$0.01 mag for GALEX, $\pm$0.01–0.02 mag for PS1, $\pm$0.02 mag for 2MASS, and $\pm$0.02–0.03 mag for WISE $W1$ to $W3$. The $W4$ magnitude is heavily contaminated by the surrounding PN.  | SourceaaSources for magnitudes in column 2 are given in column 3 and described in the text. Uncertainties are about $\pm$0.01 mag for GALEX, $\pm$0.01–0.02 mag for PS1, $\pm$0.02 mag for 2MASS, and $\pm$0.02–0.03 mag for WISE $W1$ to $W3$. The $W4$ magnitude is heavily contaminated by the surrounding PN.  |  $\lambda_{\rm eff}$bbEffective wavelengths are from the following sources. GALEX NUV: https://asd.gsfc.nasa.gov/archive/galex/; PS1: Tonry et al. (2012); 2MASS: http://coolwiki.ipac.caltech.edu/index.php/Central_wavelengths_and_zero_points; WISE: Wright et al. (2010). |  $F_{\lambda}$ccAbsolute fluxes were determined using photometric zero-points for 2MASS and WISE from the Caltech compilation cited in Footnote b. GALEX and PS1 magnitudes are on the AB scale.
---|---|---|---|---
|  |  | [$\mu$m] | [$\rm erg\,cm^{-2}\,s^{-1}\,$Å-1]
NUV | 14.467 |  GALEX | 0.2267 | $3.62\times 10^{-14}$
$g$ | 13.163 | PS1 | 0.481 | $2.56\times 10^{-14}$
$r$ | 12.295 | PS1 | 0.617 | $3.45\times 10^{-14}$
$i$ | 11.951 | PS1 | 0.752 | $3.19\times 10^{-14}$
$z$ | 11.762 | PS1 | 0.866 | $2.86\times 10^{-14}$
$y$ | 12.637 | PS1 | 0.962 | $2.60\times 10^{-14}$
$J$ | 10.543 | 2MASS | 1.235 | $1.90\times 10^{-14}$
$H$ | 9.918 | 2MASS | 1.662 | $1.22\times 10^{-14}$
$K_{s}$ | 9.756 | 2MASS | 2.159 | $5.36\times 10^{-15}$
$W$1 | 9.633 |  WISE | 3.35 | $1.16\times 10^{-15}$
$W$2 | 9.668 |  WISE | 4.60 | $3.30\times 10^{-16}$
$W$3 | 9.339 |  WISE | 11.56 | $1.31\times 10^{-17}$
$W$4 | 5.785 |  WISE | 22.09 | $2.49\times 10^{-17}$

.

The final column in Table 3 gives absolute fluxes for the central star,
converted from the magnitudes in column 2 using the zero-points referenced in
the table footnotes. These fluxes are plotted against wavelength as filled
circles in Figure 2, color-coded as indicated in the figure legend. We also
plot our calibrated HET spectrum as an orange line.

Figure 2:  Spectral-energy distribution for the nucleus of Pa 27. Photometric
fluxes from GALEX, Pan-STARRS, 2MASS, and WISE are plotted as filled circles,
color-coded as indicated in the legend. The orange line is our calibrated HET
spectrum. The SED indicates that two stars are present. The blue curve
represents a model atmosphere for a star with $T_{\rm eff}=4750$ K, $\log
g=3$, and solar metallicity, with its flux scaled to match the observed
spectrum longward of $\sim$7000 Å. The green curve is for a pure-hydrogen
atmosphere with $T_{\rm eff}=50,000$ K and $\log g=4$, scaled to match the
GALEX near-UV flux. The dotted magenta curve is the sum of the two models. A
reddening of $E(B-V)=0.225$ has been applied to both model spectra. See text
for further discussion.

The SED clearly shows an excess in the near-UV, confirming the presence of two
stars in the system. In order to estimate the parameters of the cool
component, we compared the SED with synthetic model-atmosphere spectra
computed using ATLAS9 (Castelli & Kurucz, 2003), which are conveniently
available online at the Spanish Virtual Observatory (SVO)
website.171717http://svo2.cab.inta-csic.es/theory/newov2 Based on the known
absolute magnitude, we assumed $\log g=3$. We applied a reddening of
$E(B-V)=0.225$ (see Section 3) to each theoretical spectrum, using the
formulation of Cardelli et al. (1989), and then scaled it to the observed flux
level. The best fit to the SED points longward of about 7000 Å was obtained
for a model with parameters $T_{\rm eff}=4750$ K, $\log g=3$, and
$\rm[Fe/H]=0$. These are consistent with the K0 III spectral type derived in
the previous section. The SED of this reddened theoretical model is plotted as
a blue line in Figure 2.

The $\sim$4750 K cool component provides a very good match to the SED points
at the $i$ band and longward, but the hot companion contributes increasing
amounts of flux at shorter wavelengths. Unfortunately, the parameters of the
hot star can only be loosely constrained, since we have just the single GALEX
point in the near-UV, along with a small blue excess seen at optical
wavelengths. We selected several theoretical SEDs for hot stars computed with
the Tübingen Model-Atmosphere Package (TMAP; Werner et al., 2003), which are
available at the SVO website cited above. Given the strong Balmer lines in the
optical spectrum, we considered pure-hydrogen models, and we assumed $\log
g=4$ based on typical post-asymptotic-giant-branch (post-AGB) evolutionary
tracks (see, e.g., Figure 7 in our Paper II). We then applied a reddening of
$E(B-V)=0.225$, and scaled each TMAP SED to match the GALEX near-UV point. We
obtained a reasonable match with a $T_{\rm eff}=50,000$ K model, shown as the
green line in Figure 2. The magenta dotted line shows the sum of the two
theoretical SEDs. This sum provides a good match to the HET spectrum and the
photometric points shortward of the $i$ band, and explains the dilution of the
K0 spectrum in the bottom panel of Figure 1. However, our adopted nominal
temperature for the hot component must be considered as only approximate.

##  7 Photometric Variability

The nucleus of Pa 27 was found to be a periodic variable star by the Asteroid
Terrestrial-impact Last Alert System (ATLAS; Heinze et al., 2018), designated
ATO J312.2432+32.3041. It is described181818See the ATLAS variable-star
catalog at https://vizier.cds.unistra.fr/viz-bin/VizieR?-source=J/AJ/156/241
as having sine-wave variability with a period of 7.365430 days and a peak-to-
peak range of 0.09 mag in a “cyan” filter and 0.10 mag in an “orange” filter.
Very similar results are given by the All-Sky Automated Survey for Supernovae
(ASAS-SN; Shappee et al., 2014; Kochanek et al., 2017), whose
catalog191919https://asas-sn.osu.edu designates the object as ASASSN-V
J204858.33+321814.7. ASAS-SN classifies the star as a “rotational” variable,
i.e., a rotating spotted star. The period was found to be 7.3677381 days, with
a $V$-band peak-to-peak amplitude of 0.10 mag.

We obtained aperture photometry of the central star from the Transiting
Exoplanet Survey Satellite (TESS) mission, using the online TESSExtractor
tool202020https://www.tessextractor.app (Serna et al., 2021). TESS monitors
sky “Sectors” for continuous durations of $\sim$27 days. The location of Pa 27
has been observed by TESS during the Sector 15 run (30-minute cadence), and
the Sectors 41 and 55 runs (10-minute cadences). We edited the TESSExtractor
downloads to remove data affected by bright background and other instrumental
effects. We determined eight times of maximum light, over an interval of 1097
days, by fitting parabolas to the light-curve peaks, and calculated a linear
fit to these times. The derived ephemeris is

$\displaystyle T_{\rm max}\,({\rm BJD})\,-2457000=(1717.300\pm 0.111)$
$\displaystyle+\,(7.3638\pm 0.0011)\,E\,.$

The TESS light curves, phased to this ephemeris, are plotted in Figure 3. The
curves are nearly perfect sine waves, with peak-to-peak amplitudes averaging
0.045 mag in the broadband TESS photometric system.

Figure 3:  Light curves of Pa 27 from three TESS runs, phased with the
ephemeris given in the text. For clarity, the curves from Sectors 41 and 55
are shifted fainter by 0.1 and 0.2 mag, respectively, relative to the Sector
15 curve.

We attribute the variability of Pa 27 to dark starspots on its rotating K-type
red giant. In principle, a sinusoidal light curve of the cool component could
alternatively be due to a close binary with a heated hemisphere producing one
light maximum per orbital period, or to ellipsoidal variations in a system
with twice the period given above. However, at least two arguments support the
rotational interpretation. (1) A well-established class of rotationally
variable late-type PNNi, with UV-bright hot companions, exists (see next
section). (2) The phases and amplitudes of variation in Pa 27 change with
time. First, the times of maximum light in TESS Sector 41 are slightly earlier
in phase than in the other two TESS runs. Second, the peak-to-peak amplitudes
for the three individual high-precision TESS runs are slightly variable, at
0.056, 0.040, and 0.044 mag, respectively. More dramatically, the amplitudes
seen at the epochs of the TESS observations are less than half that seen at
earlier epochs in the orange filter of ATLAS. The orange bandpass has a
similar effective wavelength to that of TESS. These time-variable behaviors
would not occur due to thermal or geometric effects in a close binary, but are
consistent with starspots that vary in both location on the star, and in the
amount of coverage of the stellar surface.

As a check on this interpretation, we estimated the radius of the star, using
an absolute magnitude of $M_{V}=+1.2$ (Section 3) and an ATLAS9 bolometric
correction of 0.48 mag, yielding $R=9.5\,R_{\odot}$. For a rotation period of
7.3638 days, we find an equatorial rotational velocity of $65\,{\>\rm
km\>s^{-1}}$. This is consistent with the Gaia line broadening measurement of
$v\sin i\simeq 30.8\pm 22.3\,{\>\rm km\>s^{-1}}$ from Table 1, given that the
inclination angle is unknown.

##  8 Pa 27 as an Abell 35-type Planetary Nucleus

To summarize the above results, the central star of the PN Pa 27 is a binary
system composed of a K0 III red giant, accompanied by an unresolved UV-bright
hot companion with an effective temperature of roughly 50,000 K. The
starspotted K star has rotationally broadened spectral lines, and is a low-
amplitude variable with a photometric period of 7.36 days.

These findings show that Pa 27 belongs to a small class of PNNi for which the
name “Abell 35-type central stars” was proposed by Bond et al. (1993). These
objects are binary nuclei of PNe consisting of a rotating and spotted late-
type star and a hot companion. Bond et al. (1993) listed the three members of
the class known at that time: Abell 35, LoTr 1, and LoTr 5, along with a field
star with similar properties, HD 128220.

A physical explanation for Abell 35-type nuclei was proposed by Jeffries &
Stevens (1996, hereafter JS96): they arise in a situation where an AGB star
ejects a dense wind, part of which is captured by a moderately distant
companion star. The companion accretes material and angular momentum from the
wind, spinning up its rotation. The remnant core of the AGB star is now an UV-
bright hot white dwarf or pre-white dwarf. JS96 describe the spun-up cool
companions as “wind-accretion induced, rapidly rotating stars (or WIRRing
stars).” Their calculations suggest that the spin-ups can occur in binaries
with separations as large as $\sim$100 AU.

In this picture, rotation drives magnetic surface activity on the late-type
star, creating extensive starspots and hence photometric variations at the
spin period. The periods of the light variations are much shorter than the
orbital periods of the binaries.

The prototypical system, Abell 35 itself, was found to have a
nucleus212121Several authors (e.g., Frew & Parker, 2010; Ziegler et al., 2012)
have argued that the Abell 35 nebula is actually not a true PN, but is instead
a “PN mimic,” created when a hot star passes through and photoionizes an
overdense region of the interstellar medium. with a late-type (G8 III-IV)
spectrum by Jacoby (1981). Periodic light variations due to rotation of the
star were discovered by Jasniewicz & Acker (1988). UV observations reveal a
hot companion (Herald & Bianchi, 2002, and references therein). To our
knowledge, the orbital period of the binary remains unknown, but it is longer
than at least several decades (e.g., Gatti et al., 1998).

JS96 predicted that accretion of processed material from the AGB wind could
create overabundances of carbon and $s$-process elements on the WIRRing
companion. This prediction was borne out by the discovery by Bond et al.
(2003) of a barium star—a cool red giant with overabundances of carbon and
$s$-process elements—at the center of the PN WeBo 1. The nucleus of WeBo 1, at
optical wavelengths, is a late-type star with photometric variability at a
period of 4.7 days. Its hot companion is detected in the UV (Siegel et al.,
2012). Another barium-star PNN, that of Abell 70, was discovered by Miszalski
et al. (2012); it too is a photometric variable, at a period of 2.06 days,
with a changing amplitude (Bond & Ciardullo, 2018; Jones et al., 2022).

The system of LoTr 1 is remarkably similar to Pa 27, with a nearly identical
absolute magnitude of $M_{V}=+1.3$. It was studied in detail by Tyndall et al.
(2013), who show that its central star is a binary containing a K1 III giant
and a hot companion, showing photometric variations with a period of 6.4 days.
Its orbital period is as yet unknown. The central star of the PN LoTr 5 is
still another case of a rapidly rotating G-type star and a UV-bright
companion. It has a photometric period of 5.95 days, but in this case the
orbital period of the binary has been established to be about 2700 days (Aller
et al., 2018, and references therein). Lastly, the central star of the PN Hen
2-39 is a late-type barium star, again with a hot companion (Löbling et al.,
2019). It is a photometric variable with a period of 5.46 days (Miszalski et
al., 2013).

We used TESSExtractor to obtain TESS light curves of the relatively bright
central stars of Abell 35, LoTr 1, LoTr 5, and WeBo 1.222222Unfortunately, the
central stars of Abell 70 and Hen 2-39 are too faint and/or crowded for useful
TESS photometry. We downloaded single sector runs for each of these targets,
and they are compared with a single run on Pa 27 in Figure 4. Remarkably, all
five objects show almost pure sinusoidal variations, but with a range of
timescales from 0.77 days for Abell 35 to 7.36 days for Pa 27. The photometric
amplitudes are similar as well.

Figure 4:  TESS light curves for central stars of Abell 35 (Sector 64), LoTr 1
(Sector 32), LoTr 5 (Sector 49), and WeBo 1 (Sector 49), compared with the
Sector 41 run on Pa 27.

##  9 Future Studies

The main purpose of this paper is to encourage further studies of the central
star of Pa 27, a new member of the Abell 35 class of binary systems. Radial-
velocity (RV) monitoring could establish the orbital period of the binary,
which is likely to be much longer than the 7.36-day spin period of the cool
component. Our three HET spectra do not provide useful information on this
question. However, Gaia DR3 indicates a peak-to-peak range of $47.5\,{\>\rm
km\>s^{-1}}$ in its RV, so binary motion has been detected. Unfortunately, at
this writing, the individual Gaia RVs are not publicly available.

UV spectra could provide constraints on the nature of the hot pre-white-dwarf
companion.

An abundance-analysis study of this relatively bright star would be of
interest. We see no gross evidence for enhancements of carbon and $s$-process
elements like strontium and barium in our spectra, but a study at higher
resolution and signal-to-noise would be more informative.

We thank the HET queue schedulers and nighttime observers at McDonald
Observatory for obtaining the data discussed here. The Low-Resolution
Spectrograph 2 (LRS2) was developed and funded by The University of Texas at
Austin McDonald Observatory and Department of Astronomy, and by The
Pennsylvania State University. We thank the Leibniz-Institut für Astrophysik
Potsdam (AIP) and the Institut für Astrophysik Göttingen (IAG) for their
contributions to the construction of the integral-field units. We acknowledge
the Texas Advanced Computing Center (TACC) at The University of Texas at
Austin for providing high-performance computing, visualization, and storage
resources that have contributed to the results reported within this paper.
Based on data from the MILES library service developed by the Spanish Virtual
Observatory in the framework of the IAU Commission G5 Working Group: Spectral
Stellar Libraries. This work has made use of data from the European Space
Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the
Gaia Data Processing and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement. This work is based in part
on observations made with the NASA Galaxy Evolution Explorer. GALEX was
operated for NASA by the California Institute of Technology under NASA
contract NAS5-98034. The Pan-STARRS1 Surveys (PS1) have been made possible
through contributions of the Institute for Astronomy, the University of
Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its
participating institutes, the Max Planck Institute for Astronomy, Heidelberg
and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns
Hopkins University, Durham University, the University of Edinburgh, Queen’s
University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las
Cumbres Observatory Global Telescope Network Incorporated, the National
Central University of Taiwan, the Space Telescope Science Institute, the
National Aeronautics and Space Administration under Grant No. NNX08AR22G
issued through the Planetary Science Division of the NASA Science Mission
Directorate, the National Science Foundation under Grant No. AST-1238877, the
University of Maryland, and Eotvos Lorand University (ELTE). This research was
made possible through the use of the AAVSO Photometric All-Sky Survey (APASS),
funded by the Robert Martin Ayers Sciences Fund and NSF AST-1412587. This
publication makes use of data products from the Two Micron All Sky Survey,
which is a joint project of the University of Massachusetts and the Infrared
Processing and Analysis Center/California Institute of Technology, funded by
the National Aeronautics and Space Administration and the National Science
Foundation. This publication makes use of data products from the Wide-field
Infrared Survey Explorer, which is a joint project of the University of
California, Los Angeles, and the Jet Propulsion Laboratory/California
Institute of Technology, funded by the National Aeronautics and Space
Administration. Funding for the TESS mission is provided by NASA’s Science
Mission directorate. This research has made use of the SIMBAD and Vizier
databases, operated at CDS, Strasbourg, France. This research has made use of
the Spanish Virtual Observatory (https://svo.cab.inta-csic.es) project funded
by MCIN/AEI/10.13039/501100011033/ through grant PID2020-112949GB-I00.

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