Spectroscopic Survey of Faint Planetary-nebula Nuclei. IV. The A35-type Central Star of Pa 27

We present optical spectroscopy of the 12th-magnitude central star of the planetary nebula (PN) Patchick 27 (Pa 27), obtained during a survey of faint PN nuclei (PNNs) with the Low-Resolution Spectrograph 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 “A35-type central stars,” which are binary PNNs consisting of a spotted late-type star and a hot pre–white dwarf. They are likely the result of a situation where an asymptotic-giant-branch (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.


INTRODUCTION
The central stars of planetary nebulae (PNe)-planetarynebula 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 andKwitter &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.

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, 1 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 poster 2 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 Hα and [O III] 5007 Å. Pa 27 lies just off the northern edge of the Veil Nebula supernova remnant in Cygnus, 3 but is an unrelated background object.The PN is roughly circular, with a diameter of about 72 ′′ .The Kitt Peak image shows bright filaments on its northern rim, possibly due to an interaction with the interstellar medium. 4Further information about Pa 27 is given in the online Hong-Kong/AAO/Strasbourg/Hα Planetary Nebulae (HASH) database 5 (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.

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 3 6 (DR3; Gaia Collaboration et al. 2016Collaboration et al. , 2023)), including its ce- 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 BP − G 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 UVbright hot companion.
The reciprocal of the Gaia parallax gives a distance to the star (and to the nebula) of 1458 ± 23 pc.Using a Bayesian analysis, Bailer- Jones et al. (2021) refine the estimated geometric distance to 1437 +23 −24 pc.At this distance, the foreground interstellar extinction, determined using the online Stilism tool7 (Capitanio et al. 2017), is E(B−V ) = 0.225± 0.04.Based on these values, and an apparent magnitude of V = 12.68 from the APASS catalog,8 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.

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 ′′ × 12 ′′ .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,9 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 LRS2Multi10 to the un-sky-subtracted, flux-calibrated fiber spectra, to perform source extraction using a 2 ′′ 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.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 website11 (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 δ Cancri.The MILES site gives this star parameters of T eff = 4657 K, log g = 2.51, and [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 δ 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).
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. 12owever, 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 δ 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. ′′ 5 would correspond to a projected physical distance as large as ∼700 AU.

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 catalog13 (unfortunately there was no GALEX observation in the far-UV).( 2) Magnitudes in the g, r, i, z, and y bands from the photometric catalog14 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. 15he 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.
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. 16Based 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 eff = 4750 K, log g = 3, and [Fe/H] = 0.These are consistent with the K0 III spectral type derived in 5200 5400 5600 5800 6000 6200 6400 6600 the previous section.The SED of this reddened theoretical model is plotted as a blue line in Figure 2.
The ∼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 purehydrogen 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 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.

PHOTOMETRIC VARIABILITY
The nucleus of Pa 27 was found to be a periodic variable star by the Asteroid Terrestrial-impact Last Alert  (2010).c Absolute 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. .System (ATLAS; Heinze et al. 2018), designated ATO J312.2432+32.3041.It is described17 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 catalog18 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 tool19 (Serna et al. 2021).TESS monitors sky "Sectors" for continuous durations of ∼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 T max (BJD) − 2457000 = (1717.300± 0.111) 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.
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 ⊙ .For a rotation period of 7.3638 days, we find an equatorial rotational velocity of 65 km s −1 .This is consistent with the Gaia line broadening measurement of v sin i ≃ 30.8 ± 22.3 km s −1 from Table 1, given that the inclination angle is unknown.

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 prewhite 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 ∼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 nucleus20 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 sprocess 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. 21 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.

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 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.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.

WavelengthFigure 1 .
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 δ 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.

Figure 2 .
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 Teff = 4750 K, log g = 3, and solar metallicity, with its flux scaled to match the observed spectrum longward of ∼7000 Å.The green curve is for a pure-hydrogen atmosphere with Teff = 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.

Figure 3 .
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.

Table 1 .
Gaia DR3 Data for Central Star of Pa 27 Galactic coordinates, parallax and proper motion, magnitude and color, and radial velocity and line broadening.

Table 2 .
Log of HET LRS2-B Observations of Pa 27