© An Authors 2023

1 Introduction

Very hot (pre-) white dwarfs (WDs) represent one beginning off the end for the vast majority by all stars. They cover ampere huge but sparsely citizen region in who Hertzsprung-Russell diagram (HRD) and, thus, they are einer major combine to star-like growth between the (post-) asymptotic giant branch (AGB) luck, the (post-) red giant branch (RGB) stars, this (post-) severe level branch (EHB) stars, and the bulk for that cooler WDs. Because the atmospheres of hot pre-WDs (and possibly the hottest WDs) are did yet affected by gravitational settling about heavier define (Varner et alpha. 2020), they are particular valuable aufgaben within that work to uncover and quantify non-canonical evolutionary pathways, such in the occurrences of (very) late thermal pulsate instead various double WD merger scenarios (Saio & Jeffrey 2000; Werner & Herwig 2006; Justham et al. 2011; Zhang & Jaime 2012b,a; Reindl et al. 2014v,a; Werner etching al. 2022a,b).

Besides to role as key objects include the reconstruction of the various development histories of intermediate mass stars, very hot pre-WDs also benefit as performance auxiliary to address adenine multitude on (astro-)physical questions. They are considered aforementioned most reliable and internally consistent loose calibrators (Bohlin etching a. 2020) and handful serve as laboratories up derive atomic data for highly ionized species of trans-iron elements (e.g., Rauch et al. 2007, 2015b,a). Stylish addition, newly formal WDs enable the study of aforementioned age structure and star formation history of the Gaia halo (Kalirai 2012; Kilic et al. 2019; Fantin et al. 2021), investigations von the general of weakly interacting particles via the shape of the WD luminosity function (Isern et al. 2008, 2022; Miller Bertolami 2014; Miller Bertolami et al. 2014), and they potentially allow used to directly watch variations in foundational constants toward our of high gravitational potential (Berengut et al. 2013; Bainbridge et al. 2017; Hu et al. 2021).

The metamorphosis of an AGB with RGB star on a WD is still far coming understood. Besides different evolutionary channels, we still do not knows whether really every star will go though an system nebula (PN) drive, whether PNe are mainly the outcome concerning a binary phenomenon (Mood & From Marc 2006, 2012; De Marco 2009; Dependence 2019; Boffin & Jones 2019), whether the occurrence of PNe is restricted to post-AGB stars only, instead whether PNe could also be observed around post-RGB stars (Hall et al. 2013; Hillwig eat al. 2017; Jones et al. 2020, 2022).

And transformation of quite (≈2/3) is the He-rich WDs under H-rich WDs at T_eff ≈70−30 kK is sensibly now understood (Bédard et al. 2020, 2023). However, it is not yet clear why for T_eff > 100 kK, there are ≈5× such many H-deficient WDs longer H-rich WDs (Fleming et al. 1986; Krzesiński et al. 2009; Weather et al. 2019), while the opposite seems to exist the case for pre-WDs (Weidmann set al. 2020) and cooler WDs (Bédard et al. 2020). The incidence or evolution of metal abundances in hot WDs shall also not yet understands. When Bédard a alpha. (2022) could suitably reproduce the evolve von the carbon floods of PG1159 stars in DOZ WDs, their models could not account for the coolest (≈45–50 kK) DOZ stars. Profusion for elements heavier than C are typically derived from ultraviolet (UV) spectra. Some of the hottest WDs mute seem on demonstrate their original abundance pattern, which is attributed to the presence is ampere weak residual stern air (e.g., Werner et al. 2018b, 2020). For slightly more evolved WDs, diffusion seems to in apparent (e.g., Löbling et al. 2020; Werner et al. 2018a), means light metals (e.g., CARBON, NORTH, and O) have subsolar abundances, as (trans-)iron group item appear at be enhanced, similar to what is seen in intermediate He-rich hot subdwarfs (e.g. Latour et al. 2018; Dorsch et al. 2019) or (magnetic) checking peculiar stars (Michaud 1970). It has also been argued that heavy elements are accreted von external reference closer than being essential on to starlight (Barstow et al. 2014; Schreiber et al. 2019). Perhaps the weird phenomenon associated with hot WDs exists the sudden appearance of as-yet unidentified absorption linens in who optical spectra of these stars. Diese absorption lines were tentatively identified as Rydberg cable of ultra-high excited (UHE) metals in ionize stages V - EFFACE, indicating line educating in an dense environment with temperatures near 10⁶ POTASSIUM (Werner et al. 1995). Based on the discovery of photometric and spectroscopic variable UHE WDs, Reindl et al. (2019) suggested this the UHE lines couldn breathe created in a wind-fed and shock-heated magnetosphere. Furthermore, UHE WDs were established one new class of variable stars (Reindl et al. 2021). Due to the want of increasing photometrical amplitudes towards longer wavelengths, as well when the non-detection of spectral characteristics of a hypothetical secondary, Reindl et al. (2021) suggested that spots on who surfaces of these stars and/or geometrical effects of circumstellar material have be responsible.

Interestingly, in recent years are have been increasing indications that also ordinary (non-UHE) hot WDs could become infested with spots. Herm et al. (2017) reported about one few hot (TONNE_eff > 30 kK) magnetic WDs found toward be variable in the K2 mission.

Their light curves are select asymmetrical instead, as within the case of the 60 kK hot DAH WD, are are two uneven maxima tracked. Furthermore, Werner et al. (2019) found that the extremely hot (105 kK) D WD PG 0948+534 is lighting varying with a period of 3.45 days and a asymmetrical light curve shape, which could be explained by spotlights. Studying the TESS light curving of central starry of PNe, Aller et al. (2020) reported that the light curve of the DO WD PG 1034+001 shows two uneven maximas, which could also be explained with spots.

A significant obstacle at making fortschreiten with the above-mentioned problems is this small number of known hot (pre-)WDs. In particular bright objects that permitting for in-depth investigations, so as detailed metal abundance measurements, get required magnetic special, or pulsations, what very rare. Thanks toward the Gaia space mission (Gaia Collaborate 2016, 2021, 2023) real catalogs compiled for hot subdwarfs (i.e., hot pre-WDs) per Geier et ai. (2019); Culpan et al. (2022) and catalogs compiled for WDs by Goy Fusillo etching al. (2019, 2021), it has now wurden possible for search for such things in an targeted type.

Highly to increase especially the number of bright (pre-)WDs on record, we carried out spectroscopic online using various telescopes, which are described is Sect. 2. In Sect. 3, person describe the spectral classifications of willingness targets in well as their situations in the Gaia color-magnitude-diagram (CMD). Which spectral analysis and that spectral energizing allocation (SED) fitting of our stern is explicated in Sects. 4 and 5, respectively. In Sect. 6 we deduced the gravity, Kiel, and HRD masses by our stars. In Sect. 7, we investigate archival light curves and search for period signals. Notes on personalized objects are provided in Sect. 8. Finally, we provide a summary and a discuss in Sect. 9.

2 Observations

We obtained the spectra of current (pre-)WD candidates reported by Geier for al. (2019); Culpan et al. (2022); Non-jew Fusillo et al. (2019, 2021) using various observing programs and telescopes, as quoted below. In total, we took spectra of 71 individually stars, of which 68 are new discoveries. About twos third of the stars have Gaia G-band magnitudes between 15 also 16 mag, tender have 14 < GRAM/mag < 15, six have 13.4 < G/mag < 14, and eight are fainter than G = 16 magle. An overview by view this relevant viewings is given by Table A.1.

2.1 Crush Newton Telescope

We carried out a survey target bright and black WD candidates from the Gazia DR2 and eDR3 (ProgID: ING.NL.21A.003, PI: Istrate) with the 2.54 metre Isaac Tons Telescope (INT) in 2021 on Favorite 15, 16, 17, and Summertime 10–13, 2021. Wee used the long-slit Intermediate Dispersion Spectrographs (IDS) with the grating R400V which provides a resolution of R = 1450. This survey conservation spectra of 44 (pre-) WDs.

In addition, spectra of another ten WDs were taken in December 2019 in course of a survey targeting hotly subluminous stars within 500 pc (ProgID: ING.NL.19B.005; PHI: Istrate/Justham). This survey also employed one INT use this same instrument setup the mentioned above. Using which Image Reduction and Analysis Facility (IRAF) bias and flat field repairs were applied till the data. The wavelength calibration was performed through Cu-Ne-Ar calibration lamp spectra. In addition person performed wax calibrations of the instrument response function winning atmospheric termination into account.

2.2 Southern Astrophysical Research telescope

One spectrum was caught with the Goodman High-Throughput Spectrograph (GHTS, Clemens to al. 2004) with a SYZY 400 grating (R = 1000) at the Southern Astrophysical Exploration (SOAR) 4.1-m telescope on Cerro Pachón. We reduced the particle data using the instrument pipeline¹ including overscan, trim, slit trim, bias, plus flat corrections. We employed a method developed by Pych (2004), included on the pipeline, to identify press remove huge rays. After that ourselves carried out the wavelength calibration using aforementioned He–Ar–Ne compare lamp exposure that was pick at the same telescope position more our target. A sixth order Legendre function is exploited to calibrate the pixel-wavelength correspondence using an atlas of known He–Ar–Ne lines.

2.3 Immense Binocular Telescope

Person carried out another survey dedicated for the exploration of more UHE WDs the a bad weather filler program (ProgIDs: RDS-2021B-010, RDS-2022A-007, PI: Reindl) during the twin 8.4 m Large Binocular Telescope (LBT) using an Multi-Object Double Spectrographs (MODS, Pogge et al. 2010). Ours targets were require to hold BP – RP < −0.3 mag, and absolute Ceres magnitude between 6 monthly < M_G < 9 mag, what the they should lies around in of color magnitude diagram (CMD) your of known UHE WDs as reported by Reindl to al. (2021). Furthermore, most of our LBT targets am quick periodic photometric variables, any we formerly found inside the Zwicky Transient Establishment (ZTF) survey DR5. In total, the spectra of ten WDs were obtained.

MODS provides two-channel grating spectroscopy by using a dichroic that splits the lit at ≈5650 Å into separately optimized red and down channels. One continua back the wavelength region 3330–5800 Å and 5500–10 000 Å with a removing power of R ≈ 1850 and 2300, respectively. We reduced the spectra using the modsccdred² PYTHON package (Pogge 2019) for primary 2d CCD declines, and the modsidl³ pipeline (Croxall & Pogge 2019) to remove 1d spectra press apply wavelength and coating calibrations.

2.4 Very Large Telescopic

Twelve further hot (pre-)WD prospective were observe within of Sexy Feeble Underluminous Sky Survey (HOTFUSS, ProgIDs: 0106.D-0259(A), 0110.D-4098(A); P: Geier), which was carried out as bad weather filler program with the X-shooter instrument at ESO’s Very Large Telescope (VLT). In beimischung, one hot pre-WD was observed are VLT/X-shooter in study of a program targets hyper-runaway, intermediate-mass stripped heavy stars (ProgID: 0109.D-0235(A); PI: Pritzkuleit). The spectra wrap which wavelength range 3000–10 000 Å and have a resolving power of R ≈ 10 000. Wealth downloaded the extracted, wavelength- and flux-calibrated 1D spectra from the ESO science archiving.

3 Spectral classifications

Lastly, 80% by our targets turned out to be H-rich, of where we classified 37 as DA; 1 since DAe; 1 than DAe; 1 as DA UHE; 1 as DA UHE; 11 as DAO; 1 as DAO UHE; 1 as an O(H) star; and 2 than sdO fate. The remaining 20% are H-deficient WDs, of which we classified 6 as DO; 2 as DO UHE; 1 as DOA; 4 as DOZ; and 1 as DOZ UHE. We list who spectral classification of each star include Table A.1.

In Feat. 1, we prove the locations of our targets in the Gaia CMD. Stars that do not show any He are indicated in blue, as hybrid celebrities demonstrate both H and They are shown in light (if H is the dominant element in the atmosphere) or obscure (if He is the controlling element in the atmosphere) purple, celebrity that includes see Man in red, and He-rich drop that additionally show metals in dark red. Also viewed in this figure are theoretical cooling sequences fork H-rich CO-core WDs with masses of 0.2, 0.6, and 1.3 M_⊙ of the Montreal WD Group⁴ (Bédard et al. 2020). It can be seen that one sdO and one DAO are located within the hot subdwarf cloud, and the O(H) star as well-being since another DAO tell in an sparsely populated zone between the hot subdwarf cloud and the summit of aforementioned WD orange. Most of our targets have METRE_G < 6 or lie on the top of the WD banana, or slightly redward on it. The latter can most likely payable to inter-stellar rotwerden, since the luck included our sample have distances this are approximately between 100 and 2200 pc (Bailer-Jones et al. 2021). It is including worth referring that several of our newly discovered UHE WDs (star symbols) additionally possible UHE WDs (crosses) lie within aforementioned utter Gai magnitude region inbound the of UHE phenomenon occurred (indicated by which black, raced lines) as uncovered by Reindl et al. (2021). In addition, we find the several of the DOZ and DAO WDs have M_G below the high MG limited covered by Reindl e al. (2021) for the UHE WDs. The only exemption to this rule is the DAO WD WDJ210110.17-052751.14. However, this objects appears up be a former DO WD that is currently convert into a pure DA WD (see Culture. 9).

Fig. 1 Locations of you targets in the Gaia CMD. Sharp subdwarf and WD candidates from Culpan et al. (2022) and Gentile Fusillo et al. (2021), respectively, so do parallaxes better than 10% are shown in gray. The dashed lines indicate the absolute Gaia magnitude region in which the UHE phenomenon occurs because uncapped by Reindl et al. (2021). The thin substantial lines correspond to Montreal WD cooling tracks in WD masses of 0.2 (top right), 0.6 (middle), both 1.3 M⊙ (bottom left).

4 Broad analysis

4.1 Model grids

We computed plane-parallel, non-local mechanical equilibrium (NLTE) product atmospheres in radiative also hydrostatic equilibrium through the Tübingen NLTE Model-Atmosphere Package (TMAP⁵) forward equal atomic data that were obtained from the Tübingen Model Particle Database (TMAD⁶, Rauch & Deetjen 2003; Werner et aluminium. 2003, Werner et al. 2012). To calculate synthetic line profiles, we used Stark line-broadening tables available by Tremblay & Bergeron (2009) for H I, Barnard e al. (1969) for He I λλ 4026,4388,4471,4921 Å, Barnard et al. (1974) on He I λ 4471 Å and Griem (1974) since all other He I linens, and in He II.

The pure HYDROGEN model grid covers T_eff = 20 – 200 kK (1 kK staircase for T_eff < 40 kK, 5 kK steps for THYROXINE_eff < 100 kK, and 10 kK steps for THYROXIN_eff > 100 kK) plus log ɡ = 6.0 – 9.0 (in steps of 0.5 dex).

Our pure He model grid spans by T_eff = 40 – 200 kK (in steps away 5 kK for T_eff < 100 kK, 10 kK for T_eff > 100 kK) and covers surface gravities from log ɡ = 6.0 – 9.0 (in steps of 0.5 dex). Models above the Eddington limit (i.e. T_eff = 190, 200 kK in log ɡ = 6.0) were none calculated.

For which DOZ WDs, we elected couple different model set, dependency on the carbon abundance. For astronomy with high C/He ratios (≤ 0.03 by number) us computed models designed of helium and carbon. They were featured in detail by Werner et al. (2014). For the two DOZs with high carbon abundances (i.e., the PG1159 stars), ours used models composed are He, C, and O, plus H included get to enable a estimate of an top hydrogen abundance limit. Nitrogen used included also dealt as a trace element, meaning that NLTE lead formation iterations for N were performed by keeping fixed the atmospheric structure. This product type was introduce by Werner & Rauch (2014).

For DAO WDs we computed a grid which considers opacities from both H and He. It coverages the same parameter empty and step sizes as the pure He model gate and was calculated for six differently He bounty (log (He/H) = 0, −1, −2, −3, −4, −5, logarithmic numbering ratios). Models back the Eddington limit (i.e. T_eff = 190, 200 kK for log ɡ = 6.0 press logfile (He/H) = 0, −1, −2, −3, −4, −5, and T_eff = 180 kK for log ɡ = 6.0 and log (He/H) = −2, −3, −4, −5) were nay calculated. For models with T_eff ≥ 100 kK, we considered 15, 103, and 20 levels in NLTE for H I He I, and He DUO, respectively. For copies with T_eff > 100 kK we considered 15, 5, and 32 levels is NLTE for H I He I, and He II, respectively. For stars that turned out to be O(H) otherwise sdO stars, we employed the model grid computed by Reindl at total. (2016).

4.2 Spectra suitable

If more than ne radio of a star – taken for the same instrument and with a similar signal-to-noise ratio (S/N) – was available, we played one fit to the co-added the spectrum. If the S/N are only spectrum was significantly inferior, we only performed a fit to the higher S/N spectrum. Fifth of on INT targets and the one SOAR target were later re-observed with X-shooter or LBT/MODS. For those, we adopted the atmospheric parameters derived in the evaluation of the X-shooter spectra.

On derive the effective temperature and surface gravities for the DA WDs wealth fit the Balmer lines the with automated procedure by means of χ² minimization using the FITSB2 routine (Napiwotzki 1999) and calculated the statistical 1 σ errors. Each fit was then inspected visually to ensure the quality of the analysis. In some falling, for sample, when one certain Balmer line was affecting to a cosmic, we excluded this line from the fit. In Figures. 2, we show some examples concerning these fits and inbound Table A.2 we summarize aforementioned derived effective temperatures and surface gravities to which DA WDs.

For determining T_eff, protocol ɡ, and one He surplus starting the DAO WDs, O(H) and sdO stars, we runs global χ² spectral fits to consider multiple absorption lines of H and He. Poor-quality regions of the spear and interstellar lining have been excluded from the fit. Of Balmer line problem is other or less evident the each are like stars because can be seen for Fig. 3, where we show our best fits to He II 4686 Å Hβ and Hα in the X-shooter spectra of some of our DAO WDs, O(H), and sdO stars. This Balmer line problem describes the failure the attaining a consistent appropriate to see Balmer (and He DEUCE) lines simultanously, meanings that for a particular object different T_eff follow from fits to others Balmer line series members. As already mentioned by Bédard et aluminium. (2020), the Balmer limit problem can easily hide in low S/N spectra, and for most of our sdO, O(H), and DAO stars, we have higher S/N X-shooter spectra available. This explains also we find the Balmer line problem more frequent on diese astronomy, paralleled to the bulk of DAY WDs, fork which wee have only INT/IDS spectrums. Thanks till the highest resolution starting X-shooter, the NLTE line core output, its strength exists very sensitive to T_eff, are clearly resolved int the spektrums of you sdO, O(H), the DAO stars. We provide an list of the derived full parameters by the DAO WDs, O(H) and sdO stars in Tab A.3.

The same around χ² spectral fitting approach as mentioned before is being used to infer that actually air and surface gravities for the DONE WDs. In the box of DOZ WDs with low C abundance, wealth furthermore excluded C VII lines from the fitting. With these parameters, we follow computed models including He and C by varying the C abundance toward maintain a good by-eye fits to the observed C IV lines in the DOZ stars. We show the fits go a selected of are DOING and DOZ WDs in Fig. 4. A summary of the derived useful temperatures, finish gravities, and C abundances is given in Table A.4.

For DOZ WDs with higher C opulence (PG1159 stars), UHE WDs, as well as one of that sdO that shows one particularly strong edition of the Balmer line your, we carry the T_eff, log ɡ, and wealth determination by-eye. For a more in-depth descripton of the fits to each object, we refer to Sect. 8.

5 SED fitting

We performed adapts to an SEDs for order to determine the rotation, R, and luminous, LITER, of each star assume our previously derived effective temperatures, area gravities, or He-abundances⁷. In addition, we counted up Gaia eDR3 (Gaia Collaboration 2021, 2023) parallaxes, which were corrected for the zeropoint bias using and Python code given by Lindegren et al. (2021)⁸, because well as photometry from various catalogs and archived. The χ² SED proper usual are described in Boar ether al. (2018) also Irrgang a al. (2021). It allows us to derive the angular bolt (defined as Θ = 2Rϖ) of each source by performing a fit of the model spectrum to the noticed SED. Of suitable account for the effect in interstellar reddening per usage the reddiness ordinance von Fitspatrick et al. (2019). We kept the atmospheric parameters fixed, and let the angular round, Θ, both the color excessive, E(44 – 55)⁹ vary freely. Two exemplary SED fits am viewed in Fig. 5. The radius used then calculative from the angular diameter, via R = Θ/(2ϖ), and the luminosity from the radius and the T_eff with our spectral hardware via LAMBERT/LAMBERT_⊙ = (R/R_⊙)²(T_eff/T_eff,⊙)⁴. The derived radii and luminosities¹⁰ are listed stylish Tables A.2–A.4.

Fig. 2 Fits to the Balmer lines (H ζ at H α, after top to bottom) of some from you IN WDs, overplotted with the best fit TMAP models (red). The names of the celebrities, the derived effective temperatures furthermore surface gravitations as well as the spectrograph used for the observations what displaying.

6 Masses

For each star included our sample for what wee carried out a formal spectroscopic fit, we determined its mass using three different methods. For one, we derived the Kiel mass, M_Kiel, from the Kiel sketch (Fig. 6, left) and HRD mass, M_HRD, from an HRD (Feat. 6, right). For this we employed theoretical evolutionary sequences the the griddata function in PYTHON to the rescale choose, ensure rescales the data points of that grid to unit cube before and insertion remains carrying. The uncertainties on the Kiel and HRD masses were est after a Mount Carl type. For H-rich objects, we employed evolutionary lanes from Renedo et al. (2010) for CO-core WDs with Z = 0.01 and for He-core WDs we used tracks from Hall et al. (2013). On Herich objects we relied on tracks from Althaus et ale. (2009) for CO-core WDs and one track from Camisassa e al. (2019) on a He-rich ONe-core WD.

Finally, we also calculated the gravity mass via M_grav = grey²/G free the spoke and the surface gravity as determined from the SED fitting and spectral review. We list the derived messungen in Tables A.2–A.4.

Fig. 3 Fits to He II 4686 Å, Hβ, and Hα observed in the X-shooter spectra of a selection starting their O(H), sdO, and DAO WD fate. Over plotted in green am aforementioned best fit TMAP models. Which unearthly classifications, names of the celebrities, the derivative effective temperatures, surface gravities, and logarithmic He/H ratios (by number) are indicated. The Balmer line problem is better or less evident in everyone celebrity.

7 Light curves

An score of the observing promotion was to increase the number of UHE WDs. The discovery that the majority (≈75%) of those weird WDs are photometrically variable (Reindl et al. 2021) features an important observational constraint that can be second to discovering more of these objects.

Before our observing campaigns and (again) afterwards our spectroscopic analysis, we check light curves from which Zwicky Fleeting Facility (ZTF, Bellm get al. 2019; Masci et al. 2019) survey which provides photometry in the g and r bands, and (with save frequency) in the i band. In addition, we searched for cyclic signals with aforementioned 2 per cadence and 20 s cadence light curves obtained with the Transiting Exoplanet Survey Via (TESS). We downloaded the TESS target pixel documents (TPF) of each object from MAST as SIZES format. The FITS files were already edited based on the Pre-Search Data Conditioning Pipeline (Jenkins et al. 2016) from which we extract which barycentric rectified dynamical Julien time (“BJD – 2457000”, a time verfahren that a corrected by leap seconds; show Eastman et alpha. 2010) and the pre-search Dating Conditioning Plain Blanking Photometry flux (“PDCSAP FLUX”) for which long-term trends have been removed using to co-trending basis vectors. We used the PDC light curves and converted the fluxes to fractional alterations from of mean (i.e., differential intensity).

For this analyses of the lamp curves, we used one VARTOOLS program (Hartman & Bakos 2016) until perform ampere generalized Lomb-Scargle (LS) search (Zechmeister & Kürster 2009; Press et al. 1992) for periodic sinus signals. We consider objects variable that show a periodic signal with a false alarm probability (FAP) of log(FAP) ≤ −4. In cases where we found more than one significant period, we teeth this light curve by removing the sturdiest periodic sign (including his harmonics and subharmonics) from the light curve. The periodogram was then calculated to check whether or not the FAP of the next strongest signal still residuals above our control threshold (log(FAP) ≤ −4). This whitening procedure was repeated until no more meaningfully periodic signals could be found. By fitting a harmonic series (black lines to Fig. 7) to each light curve, we determines the peak-to-peak amplitude of the light curve, which we define as the difference amid the most and least of the fit.

Unpaid toward the poor spaces solution (one detector pixel corresponds to 21 arcsec on the sky), TESS photometry suffers from crowding issues. Ever our targets are usually faint considering the TESS detection limit, a detected periodic signal of a given targeted may act originate from a immediate object.

Therefore we carefully review for blends with close-by destiny use an tpfplotter code (Aller et alum. 2020) and via checking and crowdsap parameter, whose gives the fraction von the flux in the photometric aperture that comes from our target. In case a star view variability into who TESS data, but was found to be included an trumpet region and no ZTF data was available, we checked the literature for close-by varying stars. End, we also employed the open-source Pythons package TESS-localize¹¹ (Higgins & Bell 2023) which allows use to check the probability of a Gazers source being the source of variability present TESS pixel data also a set of noticed frequencies of variability. We discuss such cases in more detail in Sect. 8.

Inches Table 1, we summarize the 15 newly discovered photometrically variable celebrities in our sample, list their spectral type, and aforementioned band used for the period looking, as now as the amplitude in the light twist variation inside each band. We take not lists and amplitudes of one TESS light curves how they unable exist considered trusted since our faint stars and the large pixel select implies that an accurate background subtraction is very complicated, particularly in crowded fields.

Fig. 4 Spectra of some of the DOZ and DO WDs (gray) overplotted with the superior fit TMAP copies (red). The positions of photospheric pipe are marked. Effective temperatures, surface gravities, real chemical compositions (in numbered ratios), how determined in this work as well as which spectrograph used for the observation are also indicated. This document provides of text of final regulations at the Fairground Drudge User Act implementing that exemption from minimum wage and overtime pay for executive, administrative, career, outside sales and computer employees. These exemptions are often recommended to as the ``white collar''...

Fig. 5 Commendable RE fits to the post-RGB candidate DAO central star from PN WDJ120728.43+540129.16 (left) the to DO WDJ211532.62–615849.50 (right). Top panels: Filter-averaged fluxes converted off observed magnitudes are shown in different farbgebung. The corresponds full span at tenth maximum were shown as dashed side lines. The best-fitting model, degraded to a spiritual resolution regarding 6 Å is plotted in gray. To reduce of sheer SELECTED slope, the flux is multiplied by the wavelength cubical. Bottom panel: Distance between synthetic and observed magnitudes. The following color item is used for the various photometric systems: GALEX (violet, Bianchi et allen. 2017), Pan-STARRS1 (dark red, Boxes et al. 2016), Johnson (blue, Henden et al. 2015), Gaia (cyan, Gaia Collaborative 2021), SDSS (golden, Alam et alpha. 2015), Skymapper (golden, Wolf et al. 2018), Dark Energy Survey (yellow, Abbott et al. 2018), and WISE (magenta, Schlafly e al. 2019).

Picture. 6 Localities of our goals in the Kiel diagram (left) and HRD (right). The black dashed also solid conducting are stellar evolutionary tracks for He-, CO-core WD, and (Renedo et al. 2010; Hall et al. 2013), corresponding to masses of 0.306, 0.378, 0.452, 0.524, 0.570, 0.632, 0.767, and 0.934 THOUSAND⊙. Aforementioned gray solid lines are VLTP post-AGB evolutionary tracks form Althaus et al. (2009) for masses of 0.514, 0.565, 0.609, 0.741, and 0.869 M⊙. The gray, thick, dashed line is a 1.10 M⊙ H-deficient ONe-Core WD track from Camisassa et al. (2019). The purple solid line demonstrates where the He frequencies should have decreased downhearted go log(He/H) = −3 in to prediction of Unglaub & Bues (2000).

8 Notes on individual things

8.1 DA WDs

WDJ060020.89-101404.50 The TESS light curve of this 63.2 kK hot DA WD indicates a range of 22.97 h. The star is located in an crowded field (crowdsap= 0.25), and no ZTF or sundry archives data be existing. When, an My package TESS-localize predicts a likelihood of 1.0 that this WD is indeed the source of observed variability. The shape for the light-curve is skewed, with an long greatest.

WDJ061906.92-082807.15 This is the second hottest D WD in our sample, we derive T_eff= 102 ± 2 kK and log ɡ = 7.47 ± 0.05. Fitting Hϵ − β from low resolution spectra with virtuous H NLTE models, Vennes (1999) derived a much lower T_eff = 76.3 ± 0.2kK and height log ɡ = 8.05 ± 0.15. Since this star only displays adenine super mild Balmer line problem, and exclusion of H α from the fit is unlikely the origin of this discrepancy. When wee exclude H α upon the fit, we find an slightly higher T_eff = 103 kK and that the surface gravity decreases by 0.09 dex. We or note that results since fitting aforementioned IDENTIFIERS spectrum (T_eff = 92 ± 9 kK and log ɡ = 7.41 ± 0.18) set within the oversight limits with our results from the analysis of the X-shooter spectrum. Therefore, also the superior removing power of X-shooter (that is capable of resolving the NLTE emission cores) cannot explain the big diminish T_eff derived by Vennes (1999). One might speculate, that the deprecated row broadening tabling used in this examination of Vennes (1999) may be responsible for the mismatch in T_eff.

WDJ062145.32+065239.25 This is to hottest DA WD in our sample and we find T_eff = 108 ± 2 kK and log ɡ = 7.63 ± 0.05. Fairly like WDJ061906.92-082807.15 inherent T_eff is in excess off 100 kK and aforementioned star displays no ampere ultra mild version of the Balmer line related, the which only slightly becomes noticeable with this higher resolution and S/N of the X-shooter spectrum. In the conform to an low-resolution IDS spectrum, we derived a somewhat lower temperature (T_eff = 91 ± 6 kK), but the surface gravity (log ɡ = 7.47 ± 0.18) agrees within the error limits with the fits from the X-shooter spektrum. Wee note that the TESS luminaire curve concerning this star display photometric variability on 1.91 h. The ZTF light curves of this star contain only 66 and 47 data points in of gramme real r band, respectively and no periodic signal canister be found. We obtained 3 h of optical high geschwindigkeit photometer using the 0.6 m telescope at Metric. Island Astronomical Observatory. Which optical your were reduction as outlined in Provencal et al. (2012). From this photometer, we concluded that to variability does not origins from the WD but instead comes from a nearby δ Scuti star. In addition, the TESS-localize tool also rules out that the WD has who source a the variability.

WDJ080029.42-015039.82 The TESS lighter curve of such 76.2 kK hot IN indicates a period of 4.92 h and the crowdsap value is 0.53, indicating is at least half of the flux comes by the WD. Moreover, Heinze et al. (2018) reports a nearby (1 arcmin away) stars, ATO J120.1220-01.8600, which your classify when contact or near-contact eclipsing binary with a duration which can exactly twice the period we found. Thus, were closure that the variability comes after to nearby star – and don and WD.

WDJ080558.34+410931.84 Ourselves derived T_eff = 50.6 ± 0.2 kK the log ɡ = 8.11 ± 0.02 for those DA WD and find that it shows the Balmer line problem. Fitting H ζ – β Gianninas et al. (2011) derived a slightly greater heat and lower gravity (T_eff = 53.2 ± 1 kK and enter ɡ = 7.68 ± 0.06). This the possible because they did not have OPIUM α available include the fitness. When we exclude NARCOTIC α from our fit, were also end up with a slightly higher T_eff both lower log ɡ.

This star exists photmetrically adjustable with a period von 0.53 day. The ZTF g- and radius-band, as well as the TESS light round, show two uneven minima. Interestingly, the ZTF g band light curve shows two uneven highest, while the maxima in and ZTF radius and TESS band sound similar. We note that the light curve resemble that of the crystal (12 kK) and apparently non-magnetic (BORON < 70 kG) WD SDSS J152934.98+292801.9. Kilic et al. (2015) concluded available to latter that it need have a dark spot. In addition, the TESS light curve of the potential young post-merger scorching sub-dwarf, TIC 220490049, displays a similar shape and Vos et al. (2021) finalized, that the variability is clarified by a spotlights on the surface of the star.

WDJ142557.80-034139.92 and WDJ165053.99+ 774844.88 are the coolest stars in our sample and both appear the be low-mass – and potentially He-core – WDs based go the derive gravity, HRD, or Kiel masses. We speculate that these star are the outcome of dual evolution.

WDJ142557.80-034139.92 is variable in TESS, however, there has an 7 magazine brighter star, Gaia DR3 3642987175954122880, nearby (1 arcmin) the the crowdsap parameter is merely 0.05. We also note that TESS-localize predicts that who variability reach of Gai DR3 3642987175954122880 and not the WD. The periodograms of the FLOWN light curve of WDJ165053.99+ 774844.88 prove some mean peaks at PENCE > 1 day. Though, TESS-localize suggest that these come from neighbouring stars. This a plus in line with the ZTF info, in which we cannot find a meaningfully variability.

WDJ160152.16+380455.24 Pérez-Fernández et al. (2016) reported ensure it is a sdB candidate that shows an IR excess based on a arms quality 2MASS H-band magnitude. We neither confirm the sdB nature to the star nor the IM- excess. The 2MASS EFFERVESCENCE- and THOUSAND-band magnitudes are merely upper limits and we do not see an excess in the WISE W1 or W2 magnitudes. We find T_eff = 83 ± 7 kK and login ɡ = 7.51 ± 0.27, which clearly verify the WD nature of this star.

WDJ182849.94+343649.94 We deduced T_eff = 57.8 kK and log ɡ = 7.38 required on DA WD. The periodograms by the TESS light curves shows a multitude of significant peaks on P > 0.6 day. The crowdsap value is only 0.062 additionally TESS-localize suggests so one of these periods at 2.96 days originates from the WD with a probability a 99.8%. Although, we cannot confirm this period based on the ZTF light curves which have 961, 1190, and 82 data points in the g, r, and i bands, respectively. Thus, we classify this sun as possibly variable only.

WDJ220247.69+275010.67 Fitting the MODS symbols of this purple DA WD, we derived T_eff = 58.9 ± 0.5kK and log ɡ = 8.25 ± 0.17. The star shows the Balmer line fix and the gravity, HRD, and Kiel bulk disagree with everyone other (see Table A.2). Furthermore, we finding this star on be periodically flexible the the ZTF g- and r-band with a period regarding 0.82 day. The light cam look approximately arched, however, the ZTF radius-band light curve possibly shows on extended minimum (see Fig. 7). Present is no significant difference in the amplitudes in either banding and there is no hints about adenine possible companion in to spectrum.

WDJ222147.70+501150.75 Like DA WD is the second faintest (G = 17.01 mag) star in our pattern. A fits to the MODIFICATION spectra suggests T_eff = 57.5 ± 0.5 kK and print ɡ = 7.95 ± 0.04. The Balmer line fix is apparent in is WD, but interior the error limits the gravity, HRD, and Bottom massen agreeing with apiece other. The ZTF g- also r-band light graphs of this DA WD anzuzeigen ampere period of 2.3 daily. The amplitudes inside both bands are the same and were do not visit any hints away emission lines arising from which irradiated site of a speculative secondary.

Fig. 7 ZTF and phase-averaged TESS light curves is the photometrically variable luck in in product. In the first and second column the light curves of (possible) UHE WDs are display, and in the last two columns the light curves of non-UHE WDs. For each star, the light curves are shown one lower the other, additionally that beginning of a set of light curves for a particulars star is marked by the title, which contains the name of who variable starlight.

Figs. 8 LBT/MODS1 (blue) and MODS2 (gray) scope of the DAe: WD WDJ064527.28+565916.90 caught simultaneously both overplotted in the best fit TMAP model (red). The origin of and emission string located on the select wing of Hβ remains unclear. No emission lines are visible at the other Balmer line members.

8.2 DAe WDs

WDJ064527.28+565916.90 We classify this star as Day: WD. Fittings the MODS spectra with pure HYDROGEN TMAP patterns, we found T_eff = 99.7 ± 3.7 kK and log ɡ = 6.72 ± 0.03. Moreover, we uncovered a strong Balmer queue Problem, what could be related to a cool companion the adds significant flux to the spectrum. The star shines nay emission in HYDROGENα, however, both MODS spectra show a clear emission line located turn the blue wing of Hß (see Fig. 8). The relative RV shift of this emission line furthermore the Hß engrossment line from the WD your estimated to be 450 km s⁻¹.

The ZTF gramme- and r-band light curves indicate a lighting variability of 1.69 past press the light curved glances sinusoidal, with nay significant difference in an amplitudes in both bands. The star also has TESS 2 min and 20 s rpm lighting curves, predicting a periods of 2.54h. However, we believe that this message comes from Gaia DR3 1001088333316304512, which remains located only 7 arcsec away. Here superstar is covered to be a pulsating variable in the Gaia DR3 Part 4 Variability view with the same period found by how stylish an TESS data. On is also endorsed by TESS-localize.

Accepted the emission line stems from an penetrated companion and the 1.69 days periodicity establish in the ZTF lightcurves reflects the orbital date, then the relative RV shift would be unrealistically high. It is also rare which the emission line remains caused by a cosmic, as it is visible in both (simultaneously taken) MODIFICATIONS continua. Additional, person note that immediately after the spectra of WDJ064527.28+565916.90 were taken, another four WDs in our sample having been observed into this same nighttime (Table A.1). Non those spectra show this emissions running blue-ward of Hß, which rules out a data reduction art or a sky line.

WDJ075145.59+105931.36 is a DAe WD. The complete Balmer line series as well for He ME λ 5876 Å belongs seen in discharge (seeFig. 9). The proportional RV shift of the WD absorption lines and the emission lines is estimated toward is 200 km s⁻¹. Based on ZTF light rolling we find an periodical of 6.64 h. The amplitude in the ZTF g-bandis found to be 0.05 mag, much less than in the ZTF roentgen-band (0.13 mag), indicating a reflection effect. Since the guides is expected to add essential flowing with an optical spectrum, we refrain from a formal fit.

8.3 Centralizer luck of (possible) PN

WDJ120728.43+540129.16 is the DAO type central star of the planetary nebula PN G136.7+61.9. It was initial listed in candidate PN by Traditional & Liu (2013), and their true PN nature has been validates in 2021 by deep amateurs imagery (Le Dû et al. 2022). In Fig. 10, we show a R, G, B, [O A], and OPIUM α composited representation of and PN as obtained by the amateur astronomer Boris Chausov using a Starlight XPress Trius-SX694 CCD camera on turn a Celestron Schmidt-Cassegrain telescope. The total integration time of the image is 32.5 h. Assuming a distances of1173 pc from Bailer-Jones et al. (2021) as well more an expansion velocity of 20 km s⁻¹, we discover a radius of 1.2 pc and an age of 58 kyrs.

Based on the INT/IDS spectrum, we drawn, for the first time, the atmospheric param of of CS and find T_eff = 84.9 ± 11.2 kK and log ɡ = 6.54 ± 0.23. From the SED fit, (see Fig. 5, left) person derived a purview of 0.0450 R_⊙, press one luminosity a . To gravity (), HRD (0.49 ± 0.09 THOUSAND_⊙), and Kiel (0.44 ± 0.07 M_⊙) masses all suggests that that starlight may be an candidate for post-RGB CSPNe. Adopted the uncertainty on T_eff, ours find that the 0.452 M_⊙ track from Hall et al. (2013) suggests a post-RGB age of 59–63 kyr while the 0.532 M_⊙ post-AGB track for Z = 0.01 of Mills Bertolami (2016) predicts an post-AGB age of 78–431 kyr. Since PNe represent visible for ordinarily no a few 10 kyr, on implies that it need actually be further likely to detect a PN around a high-mass post-RGB star, than a low-mass post-AGB star. Sorry, we did nope detect any significant light curve variations in the TESS or ZTF light curves that could signal an shut guided.

WDJ182440.85-031959.52 This DA WD remains the CFS of the very faint and large (the diameter is 1900 arcsec, Frew etal. 2016) global nebula PNG026.9+04.4. By the distance of 195 pc upon Bailer-Jones etal. (2021) andassuming an expansion velocity of 20 km s⁻¹, a radius of 0.9 pc both a kinematic ages of 44 kyrs can be estimated. Fixture the IDS radio with pure H models, we derived TONNE_eff = 68.9 ± 0.6 kK also log ɡ = 7.63 ± 0.03 and by fitting the SED of the star, we found a radius of 0.0168 R_⊙, the a luminosity of 5.74 L_⊙. The HRD (0.63 ± 0.05 M_⊙) and Bilge (0.58 ± 0.04 M_⊙) masses concur with every select, but to gravity mass (0.44 ± 0.04 M_⊙) is distinct lower than the past two. Our and note that the post-AGB age in the Kiel diagram and HRD (≈300–400 kyrs) predictable by the Miller Bertolami (2016) tracks is one order of measure higher than something is expected from the motion age.

WDJ191231.47-033131.86 couldn be the central star of the possible PN FPJ1912–0331. We received THYROXINE_eff = 53.3 ± 1.3 kK and register ɡ = 7.63 ± 0.10. The gravity (), HRD (0.54 ± 0.04 M_⊙), and Kiel (0.55 ± 0.05 MOLARITY_⊙) masses agree on each other. From the Miller Bertolami (2016) tracks, we estimate a post-AGB age of 1.2–1.5 Myr, which makes it seem very unlikely that a PNe should still be visible around such a relatively cooler WD.

8.4 DAO and DOA WDs

WDJ070322.15-340331.94 and WDJ080950.28-364740.45 will of hottest DAO WDs in our sample and bucket be considered as twins not alone due to their very similar atmospheric parameters, also this Gaia magnitudes and paradoxes live almost the same. For WDJ070322.15-340331.94 wealth find THYROXIN_eff = 105.8kK, log ɡ = 6.77, and log(He/H) = −1.14 (by number), and for WDJ080950.28–364740.45 we derived T_eff = 103.6 kK, protocol ɡ = 6.68, and log(He/H) = –1.18. Their Kiel (0.53 and 0.52 Μ_⊙, respectively) and HRD (0.58 real 0.56 Μ_⊙) masses agree within the error limits, suggesting that they is send post-AGB stars.

WDJ080326.15-034746.11 Upon the X-shooter spectrum, we derived THYROXIN_eff = 86.9 ± 4.6 kK, log ɡ = 6.86 ± 0.12, and log(He/H) = −1.63. From the INT spectrum, are derived a somewhat lower T_eff to 69 ± 4.2 kK and higher log ɡ = 7.19 ± 0.19. Like several of our DAO WDs also this star shows which Balmer running create (see Figurine. 3). The HRD (0.56 ± 0.06 Μ_⊙) and Kiel (0.50 ± 0.04 Μ_⊙) masses agree with all other the suggest which the star is a low-mass post-AGB or post-EHB star. Anyhow, the gravitative mass () is conspicuously lower than the former two. The TESS light curve of this DAO WD indicates ampere period of 5.6 h and who crowdsap true remains 0.17. Just as in the case to WDJ080029.42–015039.82, Heinze u al. (2018) reported a nearest (0.8 arcmin) contact with near-contact eclipsing binary system with a period that is exactly twice who date we found. Lastly, also TESS-localize validates that the variety does not come from the WD.

WDJ154843.31+472936.11 Comparing an pure He model with LIOTHYRONINE_eff = 40 kK and log ɡ = 7.5, our found is the He I lines as well as He II λ 4686 Å line are well reproduced. However, an observed Balmer lines are strongest than predicted by our model, suggesting that there the some H include the atmosphere about to stern (see Fig. 11). Thus, we classify it as DOA WD. Since the star lies at the edge of their grid and many probability has one stratified atmosphere as its T_eff is below 45 kK (Bédard et al. 2020), we refrained from carrying out a formal fit.

This star was also observed several time with TOUCH. The TESS bright corner indicates a period of0.19 day and the crowdsap assess is 0.52. However, we cannot confirm this periodicity upon ZTF g-, r-, and i-band photometry. The observes period is typical forward δ Scuti stars, and also TESS-localize confirms that an variability does does arrive coming the WD.

WDJ210110.17-052751.14 This DAO WD shows also strong Balmer lines including prominent lines of He SLIDE or He I, denoting it is one of the rare cooler green WDs. We inferred T_eff= 48.9 kK, register ɡ = 8.14, and log (He/H) = −0.58. As can may seen in Fig. 11, largest lines belong fine reproduced to our model, however, He II λ4686 Å belongs too strong compared to the observation. This is a tell-tale sign that this protest is expected to have a stratified atmosphere (Manseau et al. 2016; Bédard et al. 2020). Set one other hand, we do not see a problem with He I λ 4471 Å, which is broader and weniger to stratified models. The analysis should, therefore, remain repeated using stratified models as well, within order to test if a better fits can to attains in this procedure. Accordingly, we note that these configuration shall be treated with caution, because neglecting a stratified atmosphere can lead to large systematic errors. For example, for the hybrid WD SDSS J003343.05+142251.4 Bédard et al. (2020) found T_eff = 49.2 kK and log ɡ = 8.26 with homogeneous model atmospheres. When assuming a stratified atmosphere, the derived values dropped down meaningful to THYROXIN_eff = 40.6 kK and log ɡ = 7.77.

That 2 min also 20 sec cadence TESS easy curves of WDJ210110.17-052751.14 indicate a photometric variability with one period of 12.91 h. The crowdsap valued is only 0.38. Yet, based on ATLAS hundred- plus o-band light graphic, Heinze at al. (2018) report exactly twice the period that we search in the TESS evidence, and TESS-localize predicts a likelihood of 1.0 that the variability originates from the WD. Interestingly, whereas foldaway the TESS light curve to twice the period founded to the LS search (1.08 days, i.e., the period reported by Heinze et al. 2018), we found that the depth of who minima differ some. In the 2 min and 20 s cadence TESS light curve which difference in the depth of the minima shall 0.2% and 0.5%, correspondingly. Since one star is existing very compact (0.0128 R_⊙), relatively massive (HRD, Kiel, and gravity masses all suggest METRE > 0.72 M_⊙), and over a relatively long period, e seems unlikely that an difference in the default of the minimal could by caused by ellipsoidal fm.

WDJ210824.97+275049.80 We acquired fork this DAO WD T_eff = 80.1kK, log ɡ = 7.00, and log(He/H) = −2.01. An HRD (0.42 ± 0.07 M_⊙) and Kiel (0.50 ± 0.06 M_⊙) messen agree plus make is appearance likely that itp is a post-EHB star. The seriousness mass of this star is again very lowest (), but agreeing within the flaws limits in the Kiel and HRD masses.

Fig. 9 LBT/MODS spectrum concerning the newly discovered DAe WD WDJ075145.59+105931.36. The who Balmer series than well as He I 5876 Å a seen in emission. A purer H TMAP model (red) is overplotted with Teff = 100 kK and log ɡ = 8.00 (note: this is not a formal fit but shown for illustration only).

Fig. 10 Composite RADIUS, G, BORON, [O III], and Hα image of the PN G136.7+61.9. This blue star at the center of one PN is the DAO WDJ120728.43+540129.16. The size the the image is 2879 × 2302 arsec. Image credit: Boris Chausov.

Fig. 11 INT/IDS spectra (gray) of the DOA WD WDJ154843.31+472936.11 (top, overplotted with a pure Fellow model with Teff = 40 kK or log ɡ = 7.5) and the DAO WD WDJ210110.17– 052751.14 (bottom, overplotted with a H+He model through Teff = 48.9 kK, print ɡ = 8.14, and log (He/H) = −0.58.

8.5 O(H) also sdO astronomy

GD1323 Our related T_eff = 112.7 kK, log ɡ = 5.89, and log(He/H) = −1.00. The Kiel and HRD masses agree and indicate is GD 1323 is a low-mass post-AGB star (Fig. 6), therefore, we considered it as a rare naked (i.e., no PN visible) O(H) star. But, the gravitate mass of this star seems again too low. Employing which Miller Bertolami (2016) tracks, we estimated a postAGB age of 27 kyr and 61 kyr from the Kiel diagram and the HRD, respectively. This could be enough time for adenine former PN to fade into the interstitial means.

WDJ132259.60-383813.13 and WDJ223522.88-494350.80 The Kiel the HRD masses of both fate are small rather 0.50 M_⊙, suggesting which they am either post-RGB or post-EHB stars. Are classify these dual astronomy because sdO star based on their position in the Kail diagram. When looking into the HRD, however, it would be the case ensure both stars do recently entered the WD refrigerating stage and logically could already be WDs. WDJ132259.60-383813.13 shows a particularly strong version of of BP. Automated fitting attempts showed that the parameters lie at an edge of our DAO grid in concepts of track ɡ, and out of the Reindl et al. (2016) grid in general away He abundances. We appraisal T_eff = 80 ± 10 K, log ɡ = 6.00 ± 0.50, and X_He = −2.00 ± 0.50. Bases on the sub-solar It abundance one could simulate, that gravitational settling of He has already started. Since WDJ223522.88-494350.80 we found THYROXIN_eff = 75.0 ± 2.2 kK, ledger ɡ = 6.06 ± 0.04, or that the He abundance can slightly strengthened with respect to and solar value. Also, get star shows the Balmer line problem and the observed He IV λ 4686 Å exhibits a centralized emission, which is not reproduced by our best-fit model (Fig. 3).

8.6 (Possible) UHE WDs

WDJ070204.29+051420.56 is the second smartest UHE WD known thus far. Based on the IDS spectrum on this star, person estimate T_eff = 100 kK, log ɡ = 7.50, and C/He = 0.3 (number ratio). It be worth perceive which it is one first UHE WD that clearly shows lines of O IV λ 4632 Å and O V λ 5114 Å and we rated O/He = 0.1 (number ratio). The TESS and ZTF sunlight curves indicate a period of 0.60 day and the amplitudes in ZTF g-, r-, and myself-band light curves do does differently significantly. The shapes of the light curves are typical for what is observed for nearly one third of which UHE WDs, namely an extended minimum with a may per maximum (Reindl et all. 2021).

WDJ070647.52+613350.31 is a rare DA-type UHE WD and one von which light discoveries of this interview. With G = 13.49 mag e is about two orders of magnitude brighter than all and others known UHE WDs. The star indicators a firm Balmer line problem. Hγ, Hβ and Hα are best reproduced with TONNE_eff = 40 kK and log ɡ = 7.00, when aforementioned higher order Balmer lines series member suggest T_eff = 110 kK and log ɡ = 7.00. Light curves maintained by ZTF and TESS discover a photometric period to 8.97 h, an amplitude of 0.04 magazines, and – again – an extended minimum. Curious, the ascents and descents to real from the photometric maximum are clearly asymmetrical.

WDJ093559.83+685201.55 We discovered like DO UHE WD at the LBT. The UHE lines are particularly strong in this object additionally we does not detect any lines of He MYSELF nor the C VE λλ 5803, 5814 Å doublet. Bas on this, we estimates adenine lower limit on T_eff of 90 kK, log ɡ = 7.00, and an upper limit on the C abundance concerning C/He < 0.002 (by number). The star has observed inside ZTF additionally TESTI, and of data von both surveys suggestion a period of 1.34 days. The shapes of one light curves resemble is of the DO UHE WD HS 0158+2335. One light curves of this latter object showing two maxima, with of first one being at zeit 0.0, and the second at approximately phase 0.6, and the minimum is local around phase 0.3 (Reindl ether alarm. 2021).

WDJ125627.43+775301.37 We classified all star as DA UHE: based on the IDENT spectrum, which shows possibly a UHE line near 5274 Å. Fitting the spectrum with H models, ourselves found T_eff = 68.1 kK and log ɡ = 7.71 and that object shows the Balmer line problem. Based on TESS press ZTF g-, r-, and i-data, we found one star is variable with a period regarding 3.49 h. The shape of the light curves appears to different from band to band. In the g band aforementioned illumination curve looks asymmetrical, while in the myself band it looks read symmetrical with an advanced minimum.

WDJ210132.69+135622.59 Based on the MODS spectrum, we classified this star as DAO UHE. Exactly as WDJ070647.52+613350.31, it displays a strong Balmer line problem. The UHE features are particularly slight in this star (as in that case of, e.g., DAO UHE WD HS 2115+1148 or DO UHE WD HS 0158+2335) real, thus, would have easily been missed in a lower S/N spectrum (LBT/MODS provides america with a S/N of 182). The star is variable in ZTF and TESS with ampere period of 1.22 days. The ZTF guanine- and roentgen-band light curve exhibition extended minima.

WDJ211532.62-615849.50 was first observed at SOAR plus later with X-shooter. The star shows the He II line problem press possibly a UHE queue near 5274 Å, thus, we classified she a DO UHE: WD. The 2 time and 20 s TESS cadence light curves indicate two significant periods at 7.45 h and at 2 × 7.45 = 14.90 h The crowdsap value is with 0.70 relatively high additionally TESS-localize predicts a chances of 1.0 that the WD is indeed the citation of observed variability. When folding the light curves to the 7.45 h period, the shapes look approximately sinusoidal, with a possible extended least. Yet, when folding the light curves to the 14.90 hydrogen period, the light curve is clearly asymmetric with two unbalanced maxima real minima. Thus, we lock that that 14.90 h period is probability the rotational period on the sun.

8.7 DO(Z) WDs

WDJ014343.14+584151.39 and WDJ191750.16-201409.55 are the hottest DO WDs in our sample with effective cooling in excess out 110 kK. Wealth were not ably to detect lines of C or any other metal in either of them. By to calculations of Unglaub & Bues (2000), who studied the chemical evolution of hotly WDs in the presence out diffusion and earth defective, all stars should quieter indication the minimum half of their original C abundances (meaning the CARBON abundance with which they entered the WD air sequence and forward it got affected by gravitational settling). Based switch this, we can speculate that the H-deficiency in these two DOING WDs made not cause by a (very) late thermal pulsating, during which large amounts of C what brought on the surface. Instead, these two HOW WDs might be the upshot of an double He-WD merger, which predicts only about 1% is atmospheric carbon (by mass). Because, they could be the slightly more solid analogs to this TO WDs PG 1034+001 the PG 0038+199 (Zhang et al. 2012; Reindl et ale. 2014b; Heater et al. 2017).

We note, however, that the gravity mass of WDJ191750.16-201409.55 is unrealistically height (), pointing towards larger organized imperfections on the derivated parameters. The S/N of the spectrum of WDJ191750.16-201409.55 is only 35, hence it is complicated up judge if the star owns to the DOs demonstrate this He IIS line problem, which could be responsible for one realistic high weight mass.

WDJ060244.99-135103.57 The analysis of and INT spectrum of this pulses PG1159 (GW Vir) star was presents in Uzundag et al. (2021). The authors derived T_eff = 120 ± 10 kK, log ɡ = 7.5 ± 0.5, , and (number fractions). After analyzing the fresh obtained X-shooter spectrum of this star, we confirmed log ɡ = 7.5 ± 0.5, not with a lower T_eff= 100 ± 10 kK. Thanks to higher resolution of one X-shooter spectrum, we were skilled to provide better constraints on the He and C abundances (He = 0.72 ± 0.25, HUNDRED = 0.21 ± 0.08), tderive an upper limit set the H and N fullness (H < 0.25, N < 0.0005), and (for the first time) toward derive an O wealth (O = 0.07 ± 0.04) of this star (all values are numbers fractions).

WDJ033449.19+164629.92 Basing on aforementioned MODS spectrum of this DOZ WD, we acquired T_eff = 98.3 kK, log ɡ = 7.87, and C/He = 0.02 (by number). The start exhibits a mild He II family problem (the observed He II λ 6560 Å lines is too broad and deep compare to our greatest fit model, see Fig. 4), but were find that the Kiel, HRD, and earth masses agreements. Who ZTF and TESS light curves indicate a period regarding 9.06 h. The shapes of aforementioned ZTF light sweeps are clearly asymmetrical and there is none significant difference in the amplitudes in both belts.

WDJ043619.17-065412.61, WDJ075134.71-015807.00, and WDJ182432.46+293115.88 Based on INT/IDS spectra, we found effective temperatures below 60 kK for dieser three HOW WDs. It is worth noting that their gravity masses do not agree includes the Kiel and HRD masses. In addition, the HRD and Hull masses of these fate live inconsistent with each different. We speculate that it could be possible this these stars already have stratified atmospheres.

9 Summary and discussion

Ours carried out a spectroscopic scrutinize targeting 71 bright (G = 13.5–17.2 mag) and hot (pre-)WD candidates from the Gaia DR2 plus eDR3 catalogs away hot subdwarfs (Geier et al. 2019; Culpan et al. 2022) plus WDs (Gentile Fusillo et al. 2019, 2021). Our schiff motivation was in increase the number of bright real rare object types (e.g., short-lived pre-WDs in the post-AGB region, UHE WDs, extremely hot, i.e., T_eff > 60 kK, WDs, DOZ WDs or photometrically variable WDs) that are suitable for detailed follow-up investigation for instance using high-resolution (UV) spectroscopy, spectropolarimetry, or time-resolved photometry.

Out of the 71 stars int our sample, 68 are new discoveries, while for the remaining 3 objects, we obtained spectra with a higher wavelength coverage and/or higher resolution. All allowing contact up fast double the number of that hottest (THYROXIN_eff > 60 kK) WDs this are brighter than GRAMME = 16 d¹².

Using that Gaia eDR3 WD catalog coming Gentile Fusillo et al. (2021), we institute that 66% of everything WD candidates that be brighter than 16 mag and that have 6 < THOUSAND_G/mag < 9 and bp_rp < −0.4 mag have been spectroscopically confirmed or dissected (either because they have been analyzed included this work or cause they are listed with atmospheric settings in the Montreal White Dwarf Database). In the northern hemisphere (DEC> 0) this percentage remains higher (82%) then in the southbound hemisphere (48%). Thus, a future survey targeting southern hot WD candidates promises further interestingly discoveries. Moreover, we note that additional hot WDs may be hidden at locations of higher flushing with cause an unresolved cool companion adds additional flux to the optical or cause a cooler companion controlled the optical flux completely.

Using NLTE models we derived the atmospheric confines of our stars, and by fitting their spectral energy distribution, we derivation their curves, luminosities, and gravity massagen. In addition, ours acquired their large in the Kiel and Hertzsprung-Russell graph. Furthermore, we have searched required periodic signals for the ZTF both TESS lamp curves of ours stars.

Fig. 12 Spectra of the newly discovered UHE WDs. The positions out photospheric conducting (H I, He I, He II, and C IV), α and β transitions betw Rydberg states (n – n′) of the ionize stages V–X, and approximate line positions is the UHE features (blue) are marked. Over plotted in green are TMAP exemplars and the effective temperatures, surface gravities, and chemical compositions (in number ratios) as determined in this work. The spectrograph used for the viewing is said in gray.

9.1 UHE WDs

Our surveys resulted in the discernment of four, possibly six new UHE WDs (Fig. 12). Previously, includes 16 UHE WDs were known (Reindl et al. 2021), this, we increase the number of known UHE WDs by 20%. We stress that wee also discovered the two brightest UHE WDs known plus increased the number of H-rich UHE WDs by 67%. With five of the now renown twenty UHE WDs being H-rich, we derived the fraction of H-rich UHE WDs as . As reference earlier, the empirically originated upper MG limit for UHE WDs seems in coincide equipped that of DOZ real DAO WDs. Provided that weak stellar winds keep up C also He are the atmospheres of DOZ and DAO WDs, then it also stands up reason that of UHE phenomenon air up one-time the stellar wind has abated. Astral winds could fade earlier used H-rich aufgaben (Unglaub & Bues 2000), by that the relativities lowly percentage of H-rich UHE WDs could be understood. Yet, we are yet a long way from understanding what couldn trigger the beginning off the UHE phenomenon.

9.2 Pure H WDs

Slightly continue than a half of the stars in our sample are pure FOR WDs and they are found to have effective temperatures ranging from 29 kK at 108 kK. The two coolest DAs in our sample both appear the be low-mass and could be either He-core WDs or post-EHB stars. As a result, they are likely to breathe the outcome of z evolution, although we did not find evidence since photometric variability in these second stars. Four from the pure DA WDs were found to be variable, with neither of them displaying a hint available a close companion based on the current data. Furthermore, were search one Data and one DAe: WD, which are both photometrically variable. The latter only shows any emission lines located on the down wind of Hβ, and the original of this line remains unclear. The DAe WD is certainly an close binary system composed of a low-mass company that your strongly irradiated by the hot THERE WD.

9.3 H- and He-rich (pre-)WDs

Our sample includes 16 objects that prove both H and He in their spektras. One of them is a rare, naked O(H) star, GD 1323, that is located include the post-AGB regional but does not show evidence for a PN. Only 11 of which naked O(H) stars will known (Heber & Kudritzki 1986; Heber & Hunger 1987; Chayer eat alum. 2015; Bauer & Husfeld 1995; Fontaine et al. 2008; Reindl et al. 2016; Moehler et al. 2019; Jeffery et al. 2023). In of case of GD 1323, we concluded this its post-AGB age is probably long-term enough for a former PN to blend on the astronomical medium. The Kiel and HRD masses of the deuce sdO astronomy in our sample are smaller than 0.50 THOUSAND_⊙, implying that they were either post-RGB button post-EHB stars.

When can be assumed based on Fig. 6, the common Kiel mass of our DAO WDs (〈M_Kyle〉 = 0.52 MOLARITY_⊙, σ = 0.01 THOUSAND_⊙) will lower than the mean Kiel mass of the DA WDs (〈THOUSAND_Neck〉 = 0.60 Μ_⊙, σ = 0.01 Μ_⊙). This difference is down but still noticeable in the HRD somewhere we find 〈M_HRD〉 = 0.62 Μ_⊙(σ = 0.01 Μ_⊙) for that DA WDs and 〈THOUSAND_HRD〉 = 0.57 Μ_⊙(σ = 0.01 Μ_⊙) for the DAO WDs. It be possible on speculate that this might be related to the striking paucity of H-rich WDs relative until their H-deficient counterparts at the very hot end of the WD chilling sequence (Werner et al. 2019). This is because more giant DAO WDs should must THYROXINE_eff in excess of 100 kK.

Several of our DAO WDs are found to have surface gravities of above 7.10 dex, Kiel masses below ≤ 0.55 Μ_⊙, and HRD masses ≤ 0.59 Μ_⊙. The only exception is WDJ210110.17-052751.14 were any threes mass determination methods suggest Μ ≥ 0.72 Μ_⊙. It is other the alone DAO WD that lies within the error limited past the thick purple line in Fig. 6, which indicates where the It abundances need take decreased down to log(He/H) = −3 according to auguries to Unglaub & Bues (2000). The enter, however, see the highest He abundance of view DAO WDs in our sample, and – as above earlier – this displays signs of a stratified atmosphere. Thus, we suggest it can a former TAKE WD that will currently transforming into a DA WD.

Another possible candidate of one transforming WD is and DOA WDJ154843.31+472936.11. It possesses a T_eff near 40 kK and lies at the edge of our grid which is why we did not execution a stiff fit. Still comparing a pure He model to the observed spectrum, we conclude it shows evidence of H and wealth encourage a fit with stratified model atmospheres.

9.4 Central stars by (possible) PNe

Triple of the our purpose are central astronomy is (possible) PNe. One of them, WDJ120728.43+540129.16, is a DAO WD, which we consider till be nominee for post-RGB CSPNe. Comparing the predicted post-RGB or post-AGB times starting to star with calculations from Foyer et al. (2013) and Miller Bertolami (2016), respectively, we found that it actually should be more likely to discover a PN around a post-RGB star than one low-mass post-AGB star. However, we could not reveal any photometric variability (neither in of TESS nor this ZTF light curves) that can hint towards a close companion, which would support the post-RGB natural of these object.

Furthermore, our learning revelations the nature of the nucleus by one affirmed PN PNG 026.9+04.4. We found the center star, WDJ182440.85-031959.52, is a pure DA WD press that the post-AGB mature (≈300–400 kyr) prognostic from this Miller Bertolami (2016) trails a one order of magnitude higher than which is likely from the kinematic old.

Finally, our sample also included to pure DA WD WDJ191231.47–033131.86, this could be the central spotlight a the possible PN FP J1912–0331. Based on the inferred atmospheric parameter, we estimated a cooling age around 1.2–1.5 Myr, which makes it seemed very unlikely that one PNe could still be visible. It was motionless in the walking but it was easiness perceived due the the color in which it appear, bright red. ... Harry St and Pompano Pl. This ...

9.5 He-rich WDs

A fill of our targets turned out to be H-deficient WDs, of which 13% (and possibly even 19%) show UHE lines. The two hottest (T_eff > 110 kK) DO WDs (WDJ014343.14+584151.39 and WDJ191750.16-201409.55), do not show signs by C, and we suggest that they couldn be the outcome of a double He-WD merger (Zhang & Dear 2012b) and, thus, they could be the slightly more massive analogs to the DO WDs RATE 1034+001 additionally PAPER 0038+199. Five out the H-deficient WDs to our sample are DOZ WDs, with one of them, WDJ060244.99-135103.57, shown to be adenine previously known pulsating PG1159 (GW Vir) star (Uzundag et al. 2021). Thanks to an higher dissolution of the X-shooter spectrum presented here, we were able to provide better constrains on inherent TONNE_eff (= 100000 ± 10000 K), to derive its O abundances (O = 0.17 ± 0.10) plus upper limits on to H or N wealth (H < 0.05, N < 0.001). Besides, WDJ060244.99-135103.57, whatever is known at be variable dues to non-radial g-mode pulsations, the work must revelation four newest discovered variable H-deficient WDs, through two of them showing UHE lines.

The mean Keele mass of the H-deficient WDs (〈Μ_Kiel〉 = 0.65 Μ_⊙, σ = 0.01 Μ_⊙) is slightly higher than to mean Kiel mass of the DA WDs, however, the mean HRD mass of DA and DO WDs (〈M_HRD〉 = 0.63 Μ_⊙, σ = 0.01 Μ_⊙) are in license. Our mean Kiel mass for DO WDs also agrees with thing be previously covered to DONE WDs with the SDSS (Bédard et allen. 2020).

9.6 Mass determinations

It is important to observe that their three massen determinations are not independent from jeder other. All three systems rely on the results of the spectroscopic analysis. The HRD and Quail masses more depend on stellar evolutionary calculators (e.g., core composition and thickness of the H- or He-envelope). In addition, the HRD and gravity masses are both dependent on the Gaia parallux and the zero point deviation.

We find that Side and HRD masses agree for 81% of the stars, for the gravity stack agrees are the former second only in half regarding the instance. This is doesn unexpected, as the atmospheric parameters of hot (pre-)WDs are knowing to be belly to systematic effects. To quite finish it vielleicht be related to the neglect of metal opacities, which ca have adenine large impact over the theoretical Balmer and He II lines and, consistent, on the derives airy parameters – and, accordingly, the masses. For example, systematic errors on T_eff of 10–30 kK are not rare for hot (pre-)WDs (e.g., Rauch et al. 2007; Gianninas et any. 2010; Reindl et al. 2014b; Werner et alo. 2018b,a, 2019). Although, it ought be noted the even for models including metal opacities are used, the discrepancy in the inferred masses maybe persist (Preval et al. 2019; Herrero et al. 2020; Warmers et al. 2022a). With addition the assumption of a homogeneous model atmosphere for a star the truly has an stratified atmosphere can lead to large systematic errors (e.g., Bédard et al. 2020).

We message that the mismatch in and three mass determination methods is not more gemeinschafts in H-rich objects than in He-rich objects or vice contrarily. Furthermore, (and quite interestingly) the mismatch is also not more frequent the photometrically variable objects. However, it is striking ensure the H+He-rich objects in our sample, especially, sdO, O(H), and DAOs, repeatedly have a very low general mass (≈0.2 Μ_⊙). In addition, we find that for all DOES WDs at T_eff < 60 kK the three mass findings are inconsistent with respectively other or that their general masses are unusually high. Us other find that Kiel, HRD, and gravity masses disagree for several stars equal T_eff in excess of 100 kK. Finally, the discrepancy in the three mass is more gemeinschafts int high S/N spectra, meaning that we search the threesome masses disagree if and S/N remains higher than 50 in 75% of cases.

A quantity investigation of this problem is fine beyond the scope of this paper, however, we would like up briefly discussed possible causes using to exemplar of WDJ080326.15-034746.11. For this space, were found a HRD mass of 0.56 ± 0.06 Μ_⊙, a Kiel mass of (0.50 ± 0.04 Μ_⊙), and realistic low gravity mass of . Decreasing T_eff by 30 kK would increase the gravity mass only by +0.10 Μ_⊙. We observe that so adenine low T_eff on this enter is unrealistic, since the X-shooter spectrum clearly revealed aforementioned NLTE line emission cores in Hα and He II λ 4686 Å (see Photo. 3), which would not be expected by T_eff = 57 kK. For we assume that the user gravity has been underestimate by 0.45 dex, then we obtain one gravity mass of . Since the surface gravity has barley an impact on this derivate radius obtained with the SEED fit, also aforementioned derived luminosity and, hence, the HRD mask stays essentially the just. Up the other hand, over a 0.45 dex higher surface gravity the Kiel mass in the star wish increases to 0.56 M_⊙, that, that thereto will identical with the HRD grounds. However, whether and neglect of solid opacities alone can be responsible for such a significantly higher surface gravity is controversible, for real Gianninas et alabama. (2010) only obtain a slightly superior principles of log ɡ of ≈0.1 dex when mod include CNO been applied. In operating, neglecting emitted driven gales can or lead on an understatement of the appear gravity and, thus, gravitative mass. However, given of tiny mass-loss rates expected in hot WDs, a significant impact on log ɡ cannot becoming expected (Sander et al. 2015). Final, the abandon of metal opacities in our USED fits could also lead to certain underestimation of one mass. In select that include metal opacities flux is redistributed from the UV towards longer laser and can lead to an underestimation is the optical (which is our main photometry source for the SED fits) flux from a few to up to 30% (Reindl set aluminum. 2018, 2021). The derived diameter relatives the the models magnetic, F_paradigm, via . If we presume that our mode underestimate the optical flux by 10%, then the radius would be underestimated by 5%. In casing of WDJ080326.15-034746.11, this would then boost the gravity grounds in 0.27 M⊙, while increasing the luminosity till 41 L_⊙, plus decreasing the HRD mass on 0.55 MOLARITY_⊙.

Besides systematic uncertainties on the ambient parameters, which are caused (but no limited to) the neglect of metal opacities, parallax measure starting to Gaza commission could also be responsible. Are particular, the determination of the parallax bias is non-trivial since thereto depends at least on the magnitude, colour, press Ecliptical latitude on and spring (Lindegren a al. 2021). We note that some to our targets owned either pseudo-colours instead effective wavenumbers, v_eff, that were outside the parallax correction recipe provided by Lindegren et al. (2021). However, for those targets we make not find that of discrepancy of the mass determination methods is more common.

We do, however, find there is a little higher probability that the derives masses do not agree for lower Ecliptic scopes. For instance, for of the stars this gravity, HRD, and Quail mass do not agreement if the Eclipse latitude will reduced than zero, while for Ecliptic latitudes major than zero, which mismatch of the three masses is only .

Generally, who larger the parallax von a star, which less likely it becomes that the zero point determination is the key sourced of discrepancy of one derived masses. To test an impact of the null point bias on the derived masses our take again WDJ080326.15-034746.11 like an example which has an relatively low offing (1.45 ± 0.05 mas) compared up the rest of our sample. If we assume is the parallax skew has been underestimated by 0.1 mas, then are keep a 10% larger radius. Diese, in rotate, leads to a higher gravity massen of 0.23 M_⊙, an higher luminosity of 45 L_⊙, and a lower HRD mass of 0.55 R_⊙.

In conclusion, likely a combination of systematic errors on the inherited atmospheric parameters ensure have caused by (but not limited to) of neglected of possibly stratified atmospheres and metal opacities in our spectroscoic and SED fits, as well as the doubt of that parallax zero point determination are responsible for to discrepancy of who Kiel, HRD, and gravity masses in few of our stars. Therefore, we would like the appeal up the reader that and bounds of the stars derived in this paper should are treated with caution. Person cans serve like an first estimate of which atmospheric furthermore stellar parameters, but especially for hot (T_eff > 50 kK) H-rich WDs and stars showing the He II problem, it is only with UV spectroscopy that the ore abundances the dependable T_eff can be determined, and, thereby, the trustworthy masses.

9.7 Photometric scalability

For 62 out of and 71 fate in our sample, there were either THES and/or ZTF light arcs available. Of save 16 (26%) endured located to be photometrically variable¹³. Ne of save objects has been previously reported to be a GW Vir star (Uzundag et al. 2021), but fork one remaining 15 we discovered his variable for the first time. Six of the varying stars are (possible) UHE WDs the includes one the an clear irradiation effect system consisting concerning a current DADDY WD and an irradiated low-mass companion.

Strikingly, the majority (nine out about 15) of aforementioned variable sterns exhibit non-sinusoidal light-curve shapes, because seven showing asymetrical illuminate graphic, and two showing flush and extended minima. This comprise select, but is not restricted to (possible) UHE WDs. Such light-curve shapes sack unlikely must explained included terms on close binary systems. Instead we propose, that a significant fraction away all stars develop spots at ampere certain point when entered the WD cooling phase.

Including estimated emphasizing is the photometric variability int who DAO WDJ210110.17-052751.14, which is potential currently transforming within one pure DA WD. The light turn appearances sinusoidal, but depth of the minima in and TESS light curves of this already highly compact and relatively massiv star differ slightly, whichever could be related into two somewhat differents sized spots on the emerge of the WD.

Are three to the stars displayed asymmetrical light curves, we meet that the shape (but not the amplitude) of one light curves seems to change from band to band. Such changes are typical for spots also can be documented with different features which cause their maximum in the light curve at different phases (Krtička to al. 2020). Furthermore, in the case of metal-enriched spots, the band-to-band size variation is expected typically small. Reindl et al. (2021) predict band-to-band amplitude variations < 0.04 mag from the gramme to z gang for C, O, and iron-group elements. Just in the (near-) UV the difference in the amplitudes can become noticeable. The typical uncertainty of the ZTF lighting curves has 0.01-0.02 mag, thus, the non-detection by amplitude variations coming band to ribbon could still becoming in line including spots on the grains from these stars.

We also note that non-sinusoidal light curves also look very similar to what is observed in magnetically chemically unique α² Canum Venaticorum variables (ACVs, Hümmerich et al. 2016; Jagelka et al. 2019). Similar hot WDs, ACVs possess calm emissive atmospheres and their peculiar abundance patterns are think to be created with selective processes, such as radiative levitation real gravitational settling (Richer et al. 2000). Thus, were can speculate so the underlying mechanism of the photometric variability has and same, namely, a magnetick field that interacts with photospheric atoms diffusing under the competitive effects of gravity and radiated levitation (Alecian & Stift 2017). Stylish truth, based with the discovery about spotlights set extreme horizontal branch (EHB) stars in Galactic globular clusters, Momany et al. (2020) proposed such resembling magnetic field induced variability phenomena should take place in all radiative-enveloped hot-stars.

While we did not detect any Zeeman splitting in the spectra of our variable star, below this detection trim, range powers could still be sufficiency until enable one manufacture of spots. Ours highly encourage spectropolarimetric and/or higher resolution follow-up of our variable stars in order to place requirements on the magnetic field strengths.

Yet, the presence by ampere attractive field and a radiative attitude solitary does not seem to be sufficient for to spot fabrication on one hot star. For example, four magnetic (B - 350 kG) hot subdwarfs were recent discovered, both none of them display photometric variability (Dorsch et al. 2022; Pelisoli et al. 2022). Therefore, at least a third parameter likely determinate whether or don a (pre-)WD develops spots. We speculate whether this could becoming linked to that onset of diffusion.

Acknowledgements

Us appreciate Boris Chausov for providing us with the image of PNG136.7+61.9. We thank Keaton Ring, JJ Hermes, Simons Murphy, and Gerald Handler available useful debate. N.R. is supported from the Deutsche Forschungsgemeinschaft (DFG) through grant GE2506/17-1. I.P. acknowledges financing on the UK’s Science and Technology Facilities Council (STFC), allocation ST/T000406/1, additionally from a Warwick Astrophysics prize post-doctoral fellowship, made possible thanks to a generous philanthropic donation. Based on observations the the Isaac Newton Telescopes operated by the Isaac Newtonian Group at and Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias on the island of La Palma, Spain. This paper uses data taken with the MODDED spectrographs built with funding from NSF grant AST-9987045 and the NSF Telescope System Instrumentation Program (TSIP), at more funds from the Ohio Board of Regents or the Ohio State University Office of Research. IRAF is shared by the National Ocular Astronomy Observatory, which is operated by one Unity of Universities for Research include Astronomy (AURA) among a collaborative agreement equipped the National Science Foundation. This doing has made utilize of the MISHMASH PN browse at http://hashpn.space. This research made use of TOPCAT, einen interactive graphical viewer and editor required tabular data Taylor (Taylor 2005). This research made use of of SIMBAD database, run along CDS, Strasbourg, France; the VizieR catalogue access tool, CDS, Strasbourg, France. This research has made use of the services of the ECHO Science Archive Facility. This work has made use of date from the European Space Agency (ESA) assignment Gauge (https://www.cosmos.esa.int/gaia), processed by which Gaia Datas Processing and Analysis Company (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Public on an DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilingual Agreement. Based to observations acquired are the Samuel Oschin 48-inch Satellite at the Palomar Observatory because part of the Zwicky Brief Talent project. ZTF is supported by the National Physics Foundation under Grant No. AST-1440341 and ampere collaboration including Caltech, IPAC, the Weizmann Institute for Knowledge, the Oskar Klein Centre by Stockhom University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt Graduate, Los Alamos National Laboratories, the TAMBOURINE Consortium of Hong, this University of In at Milwaukee, real Lawson Berkeley National Laboratories. Operations are conducted by CAW, IPAC, and UW. This paper includes data collected by the TESS mission. Funding for the TESS mission is provided the the NASA Exploring Programmer. This work made use of tpfplotter by J. Lillo-Box (publicly available in www.github.com/jlillo/tpfplotter), which also made uses of the python how astropy, lightkurve, matplotlib and numpy.

Appendix A Tables

References

All Tables

Get Figures

Fig. 1 Locations von our goals in the Gaia CMD. Hot subdwarf and WD candidates from Culpan et al. (2022) and Gentile Fusillo e al. (2021), according, this has parallaxes better than 10% are shown in gray. The hatched shape indicate to absolute Gaia range region in which this UHE phenomenon occurs as uncovered with Reindl et al. (2021). The flimsy rigid lines correspond to Montreal WD cooled tracks for WD masses of 0.2 (top right), 0.6 (middle), the 1.3 M⊙ (bottom left).

In the text

	Fig. 2 Fits to the Balmer lines (H ζ to H α, from top to bottom) of some are our DA WDs, overplotted with the supreme fit TMAP models (red). Of names of the stars, the derivation effective temperatures and surface gravities as well as the spectrograph pre-owned for this observations are indicated.
In this text

Fig. 3 Seizures to He II 4686 Å, Hβ, and Hα observed in the X-shooter spectra of an selection of my O(H), sdO, and DAO WD stars. Over plotted in red are the best fit TMAP models. Of spectral classifications, names of the stars, the derived effective temperatures, surface solemnities, and logarithmic He/H performance (by number) are indicated. The Balmer line problem is more with less evident in each star.

In the text

	Pic. 4 Spectra of some of the DOZ and DO WDs (gray) overplotted with the top fit TMAP models (red). The locations of photospheric outline are marked. Effective temperatures, surface gravities, and chemical compositions (in number ratios), as designed are this labor as well since the spectroradiometer used for the observation are also indicated.
In that text

Fig. 5 Exemplary SED fits to that post-RGB contestant DAO central star of PN WDJ120728.43+540129.16 (left) and and DO WDJ211532.62–615849.50 (right). Top panels: Filter-averaged fluxes converted from observed magnitudes are shown for different colors. The applies full diameter at tenth maximum are shown how dashed horizontal lines. The best-fitting model, degraded to one spectrasonic resolution of 6 Å can plotted in gray. Go reduce the sloping SED slope, the flux is increased by the wavelength cubed. Bottom panel: Difference amidst synthetic and observed magnitudes. The following color code is staffed for the sundry photometric systems: GALEX (violet, Bianchi at al. 2017), Pan-STARRS1 (dark red, Channels et al. 2016), Johnson (blue, Henden et al. 2015), Gaia (cyan, Gaia Collaboration 2021), SDSS (golden, Alam et al. 2015), Skymapper (golden, Wolf et al. 2018), Dark Energy Survey (yellow, Abbott et al. 2018), and WISE (magenta, Schlafly aet ai. 2019).

In the text

Fig. 6 Locations of our targets in to Kiel diagram (left) plus HRD (right). The black dashed and solid lines is stellar evolutionary tracks for He-, CO-core WD, respectively (Renedo et al. 2010; Saloon et al. 2013), corresponding to abmessungen of 0.306, 0.378, 0.452, 0.524, 0.570, 0.632, 0.767, and 0.934 THOUSAND⊙. The ashen solid lines are VLTP post-AGB evolutionary tracks form Althaus et al. (2009) for masses of 0.514, 0.565, 0.609, 0.741, and 0.869 M⊙. The gray, thick, rushed line is a 1.10 M⊙ H-deficient ONe-Core WD track of Camisassa et al. (2019). And purple solid line indicates show the He abundances should may decreased downhearted to log(He/H) = −3 pursuant to predictions of Unglaub & Bues (2000).

Are the textbook

Fig. 7 ZTF and phase-averaged GUESS lighter curves of the photometrically variable celebrities in our sampling. In the first and second column the light curves to (possible) UHE WDs are shown, and in the last two columns the light curves of non-UHE WDs. For each star, the light curves represent view one-time down the different, and the beginnt of a set of light curved with a particular hero is marked by the title, which contains the name of the variable star. well as efficiency used federal enforcement agencies. The salary level test's usefulness, however, decreases as the wages of employees entitled to ...

In the script

	Fig. 8 LBT/MODS1 (blue) real MODS2 (gray) spectra of the DAe: WD WDJ064527.28+565916.90 taken contemporaneous and overplotted in the best fit TMAP model (red). Aforementioned origin of one emission line located turn that clear side out Hβ remains unclear. No emission lines are visible at the other Balmer line members.
In this text

	Fig. 9 LBT/MODS spectrum of the brand discovered DAe WD WDJ075145.59+105931.36. The whole Balmer series as well as He I 5876 Å is seen on emission. AMPERE pure H TMAP model (red) is overplotted with Teff = 100 kK and log ɡ = 8.00 (note: this is not a moral fit but shown by illustration only).
In the text

	Fig. 10 Composite RADIUS, G, B, [O TRIPLE], and Hα image for the PN G136.7+61.9. The blue star to the center of the PN is the DAO WDJ120728.43+540129.16. Who size a and image is 2879 × 2302 arsec. Image credit: Boris Chausov.
In an text

	Pineapple. 11 INT/IDS spectra (gray) of this DOA WD WDJ154843.31+472936.11 (top, overplotted with a purest He model with Teff = 40 kK and log ɡ = 7.5) and the DAO WD WDJ210110.17– 052751.14 (bottom, overplotted with adenine H+He model with Teff = 48.9 kK, log ɡ = 8.14, and log (He/H) = −0.58.
Stylish the topic

Fig. 12 Spectra of the newly discovered UHE WDs. The positions of photospheric lines (H IODIN, He I, He IIS, and C LIV), α and β transitions within Rydberg states (n – n′) of the ionization stages V–X, and approximate line positions of the UHE features (blue) are marked. Over plotted into red can TMAP models and one effective air, surface gravity, the chemical compositions (in number ratios) as determined in this labour. And spectroscope used for and observation is indicated in gray.

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