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1. A Detailed Model Atmosphere Analysis of Hot White Dwarfs in DESI DR1

   https://doi.org/10.3847/1538-4357/ae43ee  

   Kilic, Mukremin; Bergeron, Pierre; Moss, Adam; Blouin, Simon; Green, Matthew J; Jewett, Gracyn; Barrientos, Manuel; Albright, Alexander L; Brown, Warren R (March 2026, The Astrophysical Journal)

   Abstract We present a detailed model atmosphere analysis of hot white dwarfs in the Dark Energy Spectroscopic Instrument (DESI) Data Release 1. Our sample includes 19,321 unique targets withG_BP − G_RP≤ 0. We use the DESI spectra along with Gaia parallaxes and Sloan Digital Sky Survey (SDSS), Pan-STARRS, and SkyMapper photometry to perform spectroscopic and photometric fits. We find a significant discrepancy between the photometric and spectroscopic masses for DA white dwarfs (a systematic offset of 0.05–0.06M_⊙), indicating problems with the broad hydrogen line profiles in DESI spectroscopy data. Our photometric fits are consistent with a peak at the canonical mass of 0.6M_⊙. A remarkable feature of the mass distribution is the prevalence of magnetic white dwarfs among the ultramassive DA population and that of warm DQs in the non-DA distribution. We identify 70 DQs in the DESI hot white dwarf sample, including nine DAQs with carbon and hydrogen atmospheres. We constrain the ratio of non-DA to DA white dwarfs as a function of temperature, and discuss the implications for the spectral evolution of white dwarfs in the temperature range of 10⁵–10⁴K. We also discuss unusual objects in the sample, including metal-rich white dwarfs and extremely low-mass white dwarfs. This analysis provides the first look at the large sample of Gaia-selected white dwarf candidates that will be observed with multiplexed spectroscopic surveys such as DESI, SDSS-V, the 4 m Multi-Object Spectroscopic Telescope, and the William Herschel Telescope Enhanced Area Velocity Explorer over the next several years. 

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2. The Emerging Class of Double-faced White Dwarfs

   https://doi.org/10.3847/1538-4357/adbd3a  

   Moss, Adam; Kilic, Mukremin; Bergeron, Pierre; Jewett, Gracyn; Brown, Warren R (April 2025, The Astrophysical Journal)

   Abstract We report our findings on a spectroscopic survey of seven unresolved DA+DB binary white dwarf candidates. We have discovered extreme spectroscopic variations in one of these candidates, SDSS J084716.21+484220.40. Previous analysis failed to reproduce the optical spectrum using a single object with a homogeneous atmosphere. Our time-resolved spectroscopy reveals a double-faced white dwarf that switches between a DBA and DA spectral type over 6.5 or 8.9 hr due to varying surface abundances. We also provide time-series spectroscopy of the magnetic DBA, SDSS J085618.94+161103.6 (LB 8915), and confirm an inhomogeneous atmosphere. We employ an atmosphere model with hydrogen caps and a helium belt that yields excellent fits to our time-resolved spectra. We use the oblique rotator model to derive the system geometry for both targets. With the addition of these two objects, the emerging class of double-faced white dwarfs now consists of seven members. We summarize the properties of this new class of objects, and discuss how magnetism impacts the convective processes and leads to the formation of double-faced white dwarfs. We identify cooler versions of white dwarfs with inhomogeneous atmospheres among the cool magnetic DA white dwarf sample, where the Hαline is shallower than expected based on pure hydrogen atmosphere models. 

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3. Discovery of 16 New Members of the Solar Neighborhood Using Proper Motions from CatWISE2020

   https://doi.org/10.3847/1538-3881/ac4713  

   Kota, Tarun; Kirkpatrick, J. Davy; Caselden, Dan; Marocco, Federico; Schneider, Adam C.; Gagné, Jonathan; Faherty, Jacqueline K.; Meisner, Aaron M.; Kuchner, Marc J.; Casewell, Sarah; et al (February 2022, The Astronomical Journal)

   Abstract In an effort to identify nearby and unusual cold objects in the solar neighborhood, we searched for previously unidentified moving objects using CatWISE2020 proper motion data combined with machine learning methods. We paired the motion candidates with their counterparts in 2MASS, UHS, and VHS. Then we searched for white dwarf, brown dwarf, and subdwarf outliers on the resulting color–color diagrams. This resulted in the discovery of 16 new dwarfs, including 2 nearby M dwarfs (<30 pc), a possible young L dwarf, a high-motion early-T dwarf, and 3 later-T dwarfs. This research represents a step forward in completing the census of the Sun’s neighbors. 

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4. The merger fraction of ultramassive white dwarfs

   https://doi.org/10.1093/mnras/stac3182  

   Kilic, Mukremin; Moss, Adam G; Kosakowski, Alekzander; Bergeron, P; Conly, Annamarie A; Brown, Warren R; Toonen, Silvia; Williams, Kurtis A; Dufour, P (November 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We search for merger products among the 25 most massive white dwarfs in the Montreal White Dwarf Database 100 pc sample through follow-up spectroscopy and high-cadence photometry. We find an unusually high fraction, 40 per cent, of magnetic white dwarfs among this population. In addition, we identify four outliers in transverse velocity and detect rapid rotation in five objects. Our results show that $$56^{+9}_{-10}$$ per cent of the $$M\approx 1.3\, {\rm M}_{\odot }$$ ultramassive white dwarfs form through mergers. This fraction is significantly higher than expected from the default binary population synthesis calculations using the α prescription (with αλ = 2), and provides further support for efficient orbital shrinkage, such as with low values of the common-envelope efficiency. 

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5. Discovery of 74 new bright ZZ Ceti stars in the first three years of TESS

   https://doi.org/10.1093/mnras/stac093  

   Romero, Alejandra D; Kepler, S O; Hermes, J J; Amaral, Larissa Antunes; Uzundag, Murat; Bognár, Zsófia; Bell, Keaton J; VanWyngarden, Madison; Baran, Andy; Pelisoli, Ingrid; et al (February 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We report the discovery of 74 new pulsating DA white dwarf stars, or ZZ Cetis, from the data obtained by the Transiting Exoplanet Survey Satellite mission, from Sectors 1 to 39, corresponding to the first 3 cycles. This includes objects from the Southern hemisphere (Sectors 1–13 and 27–39) and the Northern hemisphere (Sectors 14–26), observed with 120 s- and 20 s-cadence. Our sample likely includes 13 low-mass and one extremely low-mass white dwarf candidate, considering the mass determinations from fitting Gaia magnitudes and parallax. In addition, we present follow-up time series photometry from ground-based telescopes for 11 objects, which allowed us to detect a larger number of periods. For each object, we analysed the period spectra and performed an asteroseismological analysis, and we estimate the structure parameters of the sample, i.e. stellar mass, effective temperature, and hydrogen envelope mass. We estimate a mean asteroseismological mass of 〈Msis〉 = 0.635 ± 0.015 M⊙, excluding the candidate low or extremely low-mass objects. This value is in agreement with the mean mass using estimates from Gaia data, which is 〈Mphot〉 = 0.631 ± 0.040 M⊙, and with the mean mass of previously known ZZ Cetis of 〈M*〉 = 0.644 ± 0.034 M⊙. Our sample of 74 new bright ZZ Cetis increases the number of known ZZ Cetis by ∼20 per cent. 

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