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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. 

Abstract We report on second-epoch imaging of two candidate planet-hosting white dwarf (WD) stars, WD 2105−82 and WD 1202−232. Both stars showed evidence of resolved, planet-mass candidate companions in observations using the MIRI mid-infrared imager on JWST. WD 2105 also showed evidence of an infrared excess consistent with an unresolved 1.4M_Jcompanion with an orbital separation of <4 au. Our second epoch observations confirm that the source of the excess shares common proper motion with the star. The excess is almost certainly due to a companion planet or debris disk. However, neither of the two resolved sources with projected separations of >1″ in the first epoch of JWST observations show measurable proper motion and are thus likely faint, unresolved background galaxies. We also search for common proper-motion companions out to hundreds of astronomical unit, but find no evidence of widely separated companions. 

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. 

Abstract A population of anomalous ultramassive white dwarfs discovered with Gaia, often referred to as the Q branch, show high (multi-Gyr) cooling delays produced by exotic physical mechanisms. They are believed to be the products of stellar mergers, but the exact origin and formation channel remain unclear. We obtained a spectroscopically complete, volume-limited sample of the Q branch region within 100 pc and found significant differences in atmospheric composition and rotation rates as a function of tangential velocity. In particular, we discover that stellar remnants with the longest cooling delays do not show strong magnetism nor detectable short-period rotational variability, as opposed to what is generally believed for double-degenerate mergers. This indicates that either these white dwarfs arise from a formation channel with no strong magnetism induced, or that the magnetism produced from the merger dissipates over the cooling delay timescales. Our follow-up photometry has also discovered pulsations in the second and third hydrogen-dominated DAQ white dwarfs, one hotter than 15,500 K, possibly extending the boundaries of the DAV instability strip for white dwarfs with thin hydrogen layers. 

Abstract After carbon and oxygen,²²Ne is the most abundant element in white dwarf (WD) interiors. As C/O WDs crystallize, they are predicted to go through a distillation process in the central layers if they have a sufficiently high²²Ne mass fraction of ≳2.5%. Observational evidence for distillation comes from an overdensity of WDs on the Q-branch in Gaia color–magnitude diagrams, which indicates that ∼6% of massive WDs are delayed in their cooling by as much as ∼10 Gyr. However, it is unclear how these stars end up with such a high concentration of²²Ne and if a significant fraction of the more common average-mass WDs go through distillation. We argue that a significant metal-rich stellar population in the solar neighborhood should lead to distilled WDs, without requiring a binary merger. We use MESA along with the carbon, nitrogen, and oxygen abundances derived from high-resolution spectroscopy of stars included in the Hypatia catalog to predict the²²Ne mass fraction in their descendant WDs. We find that 0.6%–2.5% of the WDs in the solar neighborhood have sufficient²²Ne in their interiors to go through multi-Gyr cooling delays, which could significantly inflate their numbers in the observed samples. Hence,²²Ne distillation and long-lived habitable zones around WDs should be relatively common in the solar neighborhood. We also use a Galactic model to predict the fraction of WDs that go through distillation as a function of Galactocentric distance. The fraction of distilled WDs is ∼2%–8% near the Galactic center and declines steadily toward the outer disk. 

Abstract We present the discovery and analysis of a nearby eclipsing ultracompact accreting binary SMSS J1138−5139, the first well-constrained LISA-detectable Type Ia supernova progenitor. Our time-series optical spectroscopy identifies its orbital period through radial velocity monitoring at $P_{\mathrm{orb,RV}}=27.682\mathrm{minutes}$ , twice the photometric period seen in 2 minute cadence data from TESS Sector 37. We model our optical spectroscopy together with new simultaneous multiband time-series photometry from Gemini to place constraints on the binary parameters. Our light-curve modeling finds that SMSS J1138−5139 contains anM₂= 0.25 ± 0.01M_⊙pre-white-dwarf donor with a massiveM₁= 1.02 ± 0.05M_⊙white dwarf accretor at orbital inclination ​​​​​​ $i=88\stackrel{\circ }{.}6\pm 0\stackrel{\circ }{.}1$ . Based on our photometrically derived system parameters, we expect that gravitational-wave radiation will drive SMSS J1138−5139 to a merger withinτ= 5.7 ± 0.3 Myr and result in a Type Ia supernova. Even without a direct merger event, the component masses of SMSS J1138−5139 and active hydrogen accretion suggest that eventual helium accretion will likely also trigger a Type Ia supernova explosion through the dynamically driven double-degenerate double-detonation (D6) channel. We expect LISA to detect the gravitational-wave emission from SMSS J1138−5139 with a signal-to-noise ratio of 7–10 after a 48 month mission. 

Abstract We present precise parameters for two compact double white dwarf (DWD) binaries, SDSS J232230.20+050942.0 (J2322+0509) and SDSS J063449.92+380352.2 (J0634+3803), with orbital periods of 20 and 26.5 minutes, respectively. These systems will serve as verification sources for the Laser Interferometer Space Antenna (LISA). To significantly improve the electromagnetic (EM) constraints on these two systems and the LISA detectability predictions, we conducted spectroscopic follow-up observations using Hubble Space Telescope/Space Telescope Imaging Spectrograph, Keck I/LRIS, and Keck II/Echellette Spectrograph and Imager. Our analysis significantly improves the temperature, surface gravity, and mass constraints for both primary and secondary components in J2322+0509, as well as dynamical properties such as radial velocities and orbital periods in both systems. For J2322+0509, we derive an updated inclination ofi= 25 ${}_{-3.0}^{+4.5}$ deg, while for J0634+3803, we obtaini= 43 ${}_{-5.6}^{+7.0}$ deg. We assess the detectability of these sources using LDASOFT. Incorporating EM priors on inclination significantly enhances the gravitational-wave signal recovery, reducing uncertainties in amplitude by a factor of 2–4 and shortening the detection time by up to a few months. Our results underscore the importance of multimessenger observations in characterizing DWD binaries and maximizing LISA’s early scientific capabilities. 

Abstract 2MASS J05513444+4135297 (hereafter J0551+4135) is the only pulsating DAQ white dwarf (WD) known with a carbon and hydrogen atmosphere. Its unusual atmospheric composition and kinematics indicate a WD merger origin. We present time-series photometry of J0551+4135 obtained using the Apache Point Observatory 3.5 m, Gemini North 8 m, and Gran Telescopio Canarias 10 m telescopes. J0551+4135 exhibits variations in pulsation amplitude and frequency over time. We detect 10 significant recurring peaks across different subsets of observations, with frequencies ranging from 987 to 1180μHz, consistent with nonradial gravity (g)-mode oscillations. We present new evolutionary models suitable for spectroscopic characterization of DAQ WDs, and derive a mass of 1.13 ± 0.01M_⊙and a cooling age of 1.7 ± 0.1 Gyr for a CO core, and 1.12 ± 0.01M_⊙and 1.6 ± 0.1 Gyr for an ONe-core WD, respectively. However, detailed asteroseismology of this unique pulsator has to wait until fully consistent DAQ evolutionary models are available. Further observations, including multisite campaigns to reduce daily aliasing and to improve the signal-to-noise ratio, would be helpful for the identification of additional modes and constraining the internal structure of this unique pulsator. 

Abstract The majority of merging white dwarfs leave behind a white dwarf remnant. Hot/warm DQ white dwarfs with carbon-rich atmospheres have high masses and unusual kinematics. All evidence points to a merger origin. Here, we demonstrate that far-UV (FUV) and optical photometry provides an efficient way to identify these merger remnants. We take advantage of this photometric selection to identify 167 candidates in the Galaxy Evolution Explorer All-Sky Imaging Survey footprint, and provide follow-up spectroscopy. Out of the 140 with spectral classifications, we identify 75 warm DQ white dwarfs withT_eff > 10,000 K, nearly tripling the number of such objects known. Our sample includes 13 DAQ white dwarfs with spectra dominated by hydrogen and (weaker) carbon lines. Ten of these are new discoveries, including the hottest DAQ known to date, withT_eff ≈ 23,000 K andM = 1.31M_⊙. We provide a model atmosphere analysis of all warm DQ white dwarfs found, and present their temperature and mass distributions. The sample mean and standard deviation areT_eff = 14,560 ± 1970 K andM = 1.11 ± 0.09M_⊙. Warm DQs are roughly twice as massive as the classical DQs found at cooler temperatures. All warm DQs are found on or near the crystallization sequence. Even though their estimated cooling ages are of order 1 Gyr, their kinematics indicate an origin in the thick disk or halo. Hence, they are likely stuck on the crystallization sequence for ∼10 Gyr due to significant cooling delays from distillation of neutron-rich impurities. Future all-sky FUV surveys like Ultraviolet Explorer have the potential to significantly expand this sample. 

Abstract Infrared-faint white dwarfs are cool white dwarfs exhibiting significant infrared flux deficits, most often attributed to collision-induced absorption (CIA) from H₂–He in mixed hydrogen–helium atmospheres. We present James Webb Space Telescope (JWST) near- and mid-infrared spectra of three such objects using Near-Infrared Spectrograph (0.6–5.3μm) and Mid-Infrared Instrument (5–14μm): LHS 3250, WD J1922+0233, and LHS 1126. Surprisingly, for LHS 3250, we detect no H₂–He CIA absorption at 2.4μm, instead observing an unexpected small flux bump at this wavelength. WD J1922+0233 exhibits the anticipated strong absorption feature centered at 2.4μm, but with an unexpected narrow emission-like feature inside this absorption band. LHS 1126 shows no CIA features and follows aλ⁻²power law in the mid-infrared. LHS 1126's lack of CIA features suggests a very low hydrogen abundance, with its infrared flux depletion likely caused by He–He–He CIA. For LHS 3250 and WD J1922+0233, the absence of a 1.2μm CIA feature in both stars argues against ultracool temperatures, supporting recent suggestions that infrared-faint (IR-faint) white dwarfs are warmer and more massive than previously thought. This conclusion is further solidified by Keck near-infrared spectroscopy of seven additional objects. We explore possible explanations for the unexpected emission-like features in both stars, and temperature inversions above the photosphere emerge as a promising hypothesis. Such inversions may be common among the IR-faint population, and since they significantly affect the infrared spectral energy distribution, this would impact their photometric fits. Further JWST observations are needed to confirm the prevalence of this phenomenon and guide the development of improved atmospheric models.