We present a substantial update to the MESA Isochrones and Stellar Tracks (MIST) library, extending the MIST model grids and isochrones down the white dwarf (WD) cooling sequence with realistic physics for WD cooling timescales. This work provides a large grid of MESA models for carbon-oxygen core WDs with hydrogen atmospheres (spectral type DA/DC), descended from full prior stellar evolution calculations. The model tracks, isochrones, and WD cooling timescale contours are available on the MIST project website and at doi:10.5281/zenodo.15242047. Our WD models provide a very large, publicly available grid with detailed physics for WD cooling timescales: realistic interior and envelope compositions, with element diffusion and heavy-element sedimentation, nuclear burning at the base of the WD hydrogen envelope, core crystallization, and C/O phase separation. As a large grid of open-source stellar evolution models, these WD models provide both out-of-the-box model tracks for comparison with observations and a framework for building further WD models to investigate variations in WD physics.

Many white dwarfs are polluted by metals, which are generally understood to be the accreted remnants of a planetary system. Modeling these systems typically assumes that the metal concentration is homogeneous throughout the white dwarf's atmosphere. However, the magnetic fields of a white dwarf may affect the accretion geometry of the white dwarf via magnetospheric accretion. Convection in the white dwarf's photosphere will then transport the metals across the surface, with a structure set by the relative sinking versus spreading timescales. In this work, we construct models for the accretion geometry, subsequent spreading, and observed pollution of magnetic white dwarfs. We show that the magnetic fields will initially concentrate the pollution into a narrow region of the white dwarf's surface. The relative spreading and sinking timescales determine whether the metals become uniformly distributed or remain confined to localized patches. If the magnetic field and spin poles are misaligned, then patchy white dwarfs exhibit periodically variable pollution signatures, which enable constraints on the patch area. We explore this model as a possible explanation for the recent detection of periodically variable pollution signatures in magnetic white dwarfs. Finally, we also demonstrate that the concentration of material due to the magnetic field may lead to systematic underestimates of the mass accretion rate onto these objects.

We present a comprehensive multi-wavelength spectroscopic and photometric analysis of the 44 confirmed white dwarfs within 13 pc of the Sun. Combining flux-calibrated ultraviolet spectroscopy from the Hubble Space Telescope (STIS and COS) with ground-based optical spectroscopy, as well as photometry from Gaia, 2MASS, and WISE, we employ a combined fitting method to calculate atmospheric parameters. Each white dwarf is fitted with a bespoke model depending on its detailed atmospheric composition. Two strongly magnetic stars could not be fitted due to the complex splitting of their spectral lines. We find a systematic discrepancy in hydrogen-atmosphere white dwarfs with effective temperatures below 10,000K, where fits incorporating ultraviolet spectra result in effective temperatures that are 1 - 5 per cent higher than those derived from optical and infrared photometry alone. We re-classify three helium-atmosphere white dwarfs as metal enriched following a magnesium detection in their near-ultraviolet spectra: WD 0435-088, WD 1132-325 and WD 1917+386. In total, we identify five stars in the sample for which metals are only detected in the ultraviolet. Overall, we find that 30 per cent of the 13 pc white dwarfs show spectroscopic evidence of evolved planetary systems. Our analysis reveals no measurable difference between the hydrogen content of DQ and DC white dwarfs, although the upper limits of carbon in DCs are significantly below that of the DQ population. We find a multiplicity fraction of 33 per cent for the 13 pc white dwarfs.

The small DAHe and DAe spectral classes comprise isolated, hydrogen-dominated atmosphere white dwarfs that exhibit variable photometric flux and Balmer line emission. These mysterious systems offer unique insight into the complex interplay between magnetic fields, stellar rotation and atmospheric activity in single white dwarfs. DAHe stars have detectable magnetic fields through Zeeman-split spectral lines, whereas DAe stars lack such splitting. We report the first discovery and characterization of magnetism in the DAe white dwarf WD J165335.21-100116.33 with new time-resolved spectropolarimetry from FORS2. We detect a weak but variable longitudinal magnetic field with values (B-z) > -9.2 +/- 2 . 4 kG and (B-z) < -2.2 +/- 1 . 0 kG. Independent ZTF and ATLAS photometry reveal a consistent period of P = 80 . 3070 +/- 0 . 0007 h. Time-resolved optical spectroscopy obtained with six ground-based instruments demonstrates strong modulation in the strength of the H alpha and H 16 Balmer line emission with P = 80 . 2922 +/- 0 . 0108 h. The photometric flux and Balmer emission strength vary in antiphase, with the strongest magnetic detections coinciding with phases of low photometric flux and strong line emission. These characteristics support the theory that a magnetically active, temperature-inverted spot/region is producing an optically thin chromospheric emission region. Comparison with other DAe and DAHe white dwarfs reveals all systems have a strikingly similar antiphase phenomenology, reinforcing the theory that they are subject to a unified physical mechanism. With the detection of a weak magnetic field, we reclassify WD J165335.21-100116.33 as a low-field DAHe white dwarf.

The DECam Rogue Earths and Mars Survey (DREAMS), a NOIRLab survey program, has been conducting a three-year survey covering a 5 deg^2 area in the Galactic bulge since 2025 June. Its primary science goal is to detect low-mass free-floating planets through microlensing, while its minute-level cadence also enables the detection and characterization of rapid phenomena on timescales of minutes to hours such as stellar flares and pulsating stars. Here, we present the data reduction and calibration of the DREAMS observations obtained in 2025 and introduce the first DREAMS data release (DR1). DR1 includes 1,856 z-band observations and 325 r-band observations for 59,372,789 stars. The DREAMS DR1 catalog contains at least twice as many stars as any previous catalog covering the same 5 deg^2 area. We present DREAMS light curves for a known blue large-amplitude pulsator and a known transiting system to demonstrate the survey's capabilities. We also perform a pilot search for short-duration variables over about 0.4

White dwarf stars, the endpoint of stellar evolution for 97

Binaries of two white dwarfs (WDs) are an important class of astrophysical objects that help us reconstruct the complex processes involved in stellar evolution and are theorized to lead to Type Ia supernovae. We report the discovery of SDSS J090618.44+022311.6, a rare post-common-envelope binary of a hydrogen atmospheric DA WD and a DQ WD that shows carbon absorption features and is only the fourth such binary known. We combine the available spectroscopic, photometric, and radial velocity data to provide a self-consistent model for the binary and discuss its history as a binary DA+DQ. The system has a period of 31.17 hr with masses of 0.39 M circle dot for the DA WD and 0.49 M circle dot for the DQ WD. The corresponding cooling ages point to a scenario where the lower-mass DA WD forms first when the more massive of the two progenitor stars transfers mass on to the companion after the main sequence. The companion later evolves to form the more massive DQ WD. The system has a merger timescale of 450 Gyr and will lead to the formation of a massive WD. We find that the stellar properties of all four known post-common-envelope DA+DQ systems are in a narrow range. The dynamical mass measurement of the DQ WD in this work supports the existing hypothesis that DQ WDs preferentially evolve from low-mass DB WDs.

The disruption and accretion of planetary material onto white dwarfs is expected to be inherently dynamic and stochastic, potentially driving variability in the accretion rate and therefore the shape and depth of the photospheric metal absorption lines. This paper presents an 18-year optical spectroscopic monitoring campaign of five warm (11,000-23,000K) polluted white dwarfs with sinking timescales of days-months, observed using Magellan/MIKE and SALT/HRS to directly test this prediction. At four of the five systems, no statistically significant variability is detected over baselines of 15-18 years corresponding to hundreds to thousands of diffusion timescales, with inferred accretion rates stable to within 15-30

The fifth-generation Sloan Digital Sky Survey (SDSS-V) includes the first large-scale spectroscopic survey of white dwarfs (WDs) in the era of Gaia parallaxes. SDSS-V collects multiple exposures per target, making it ideal for binary detection. We present a search for hydrogen atmosphere (DA) double WD (DWD) binaries in this rich dataset. We quantify radial velocity variations between subexposures to identify binary candidates, and we measure the orbital period for a subset of DWD binary candidates. We find 60 DWD binary candidates, of which 43 are new discoveries, and report tentative periods for 9 of these binaries. From these binary candidates, we derive a Galactic WD binary fraction f _bin,0.4 = 9% for binary separations <0.4 au and the power-law index of the initial separation distribution α = −0.62. Using the simulated binary population, we find that approximately two to five super-Chandrasekhar-mass binaries that merge within a Hubble time are expected in our sample at a 95% confidence interval. We predict that approximately two systems in our sample should be detectable via gravitational waves by the Laser Interferometer Space Antenna (LISA), one of which has already been identified as a LISA verification source. We also estimate a total of about 10,000–20,000 LISA-detectable DWD binaries in the Galaxy. Our catalog of WD+WD binary candidates in SDSS-V is now public and promises to uncover a large number of exciting DWD systems.

Many white dwarfs are observed in compact double white dwarf binaries, and through the emission of gravitational waves, a large fraction are destined to merge. The merger remnants that do not explode in a Type Ia supernova are expected to initially be rapidly rotating and highly magnetized. In this work, we present our discovery of the variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF J2008+4449, as a likely merger remnant showing signs of circumstellar material without a stellar or substellar companion. The nature of ZTF J2008+4449 as a merger remnant is supported by its physical properties: it is hot (35 500 +/- 300 K) and massive (1.12 +/- 0.03 M-circle dot), rapidly rotating with a period of approximate to 6.6 minutes, and likely possesses exceptionally strong magnetic fields (similar to 400-600 MG) at its surface. Remarkably, we detect a significant period derivative of (1.80 +/- 0.09)x10(-12) s/s, indicating that the white dwarf is spinning down, and a soft X-ray emission that is inconsistent with photospheric emission. As the presence of a mass-transferring stellar or brown dwarf companion is excluded by infrared photometry, the detected spin-down and X-ray emission could be tell-tale signs of a magnetically driven wind or of interaction with circumstellar material, possibly originating from the fallback of gravitationally bound merger ejecta or from the tidal disruption of a planetary object. We also detect Balmer emission, which requires the presence of ionized hydrogen in the vicinity of the white dwarf, showing Doppler shifts as high as approximate to 2000 km s(-1). The unusual variability of the Balmer emission on the spin period of the white dwarf is consistent with the trapping of a half ring of ionized gas in the magnetosphere of the white dwarf.

About 20% of white dwarfs are observed to host large-scale magnetic fields, but the origin of white-dwarf magnetism remains uncertain. Small-scale turbulent dynamos (SSDs), which efficiently generate magnetic fields in solar- and stellar-convection simulations, have only been studied in white dwarfs through equipartition arguments in one-dimensional models so far. We therefore investigated whether turbulent convection in white-dwarf surface layers can sustain SSD action through local, three-dimensional, radiation–magnetohydrodynamics simulations of a white dwarf with a convective pure-hydrogen (DA) atmosphere, including the full convection zone together with the underlying overshoot and stably stratified layers. Starting from a weak seed field of 1 mG, the magnetic energy undergoes exponential amplification before saturating at a magnetic-to-kinetic energy-density ratio of about 5.5% at the visible surface, demonstrating that SSD action naturally generates kilogauss-strength magnetic fields in convective white-dwarf atmospheres. The resulting magnetic field is characterised by a mixed-polarity small-scale structure, with kilogauss field concentrations contributing about 14% of the total unsigned magnetic flux at the visible surface. Despite a rate of magnetic-energy generation amounting to roughly one-seventh of the bolometric flux, no significant modification of the mean stratification is found. Although these fields remain spatially unresolved for observations, they may contribute to spectral line broadening, suggesting that small-scale magnetism in white dwarfs could be more widespread than currently inferred from observations.

Many white dwarfs are observed in compact double white dwarf binaries, and through the emission of gravitational waves, a large fraction are destined to merge. The merger remnants that do not explode in a Type Ia supernova are expected to initially be rapidly rotating and highly magnetized. In this work, we present our discovery of the variable white dwarf ZTF J200832.79+444939.67, hereafter ZTF J2008+4449, as a likely merger remnant showing signs of circumstellar material without a stellar or substellar companion. The nature of ZTF J2008+4449 as a merger remnant is supported by its physical properties: it is hot (35 500 ± 300 K) and massive (1.12 ± 0.03 M⊙), rapidly rotating with a period of ≈6.6 minutes, and likely possesses exceptionally strong magnetic fields (∼400−600 MG) at its surface. Remarkably, we detect a significant period derivative of (1.80 ± 0.09)×10−12 s/s, indicating that the white dwarf is spinning down, and a soft X-ray emission that is inconsistent with photospheric emission. As the presence of a mass-transferring stellar or brown dwarf companion is excluded by infrared photometry, the detected spin-down and X-ray emission could be tell-tale signs of a magnetically driven wind or of interaction with circumstellar material, possibly originating from the fallback of gravitationally bound merger ejecta or from the tidal disruption of a planetary object. We also detect Balmer emission, which requires the presence of ionized hydrogen in the vicinity of the white dwarf, showing Doppler shifts as high as ≈2000 km s−1. The unusual variability of the Balmer emission on the spin period of the white dwarf is consistent with the trapping of a half ring of ionized gas in the magnetosphere of the white dwarf.

Observations of highly irradiated gas giant exoplanets have shown helium escaping from their atmospheres. There is limited evidence for atmospheres on rocky exoplanets, perhaps because they have already escaped. We report near-infrared spectroscopic observations of LHS 1140b, a rocky exoplanet that orbits in the habitable zone of a nearby low-mass star. The transit spectra show absorption by helium escaping from the planet’s atmosphere. Helium absorption is detected in 2024 but not in 2025, indicating time-variable atmospheric escape. We interpret these results as indicating an upper atmosphere dominated by helium and depleted in hydrogen, with other volatile species trapped at lower altitudes, which is consistent with atmospheric fractionation models. No helium absorption is detected for LHS 1140c, a smaller and more strongly irradiated exoplanet in the same system.

The Sloan Digital Sky Survey V (SDSS-V) is pioneering panoptic spectroscopy: it is the first all-sky, multiepoch, optical-to-infrared spectroscopic survey. SDSS-V is mapping the sky with multiobject spectroscopy (MOS) at telescopes in both hemispheres (the 2.5 m Sloan Foundation Telescope at Apache Point Observatory and the 100-inch du Pont Telescope at Las Campanas Observatory), where 500 zonal robotic fiber positioners feed light from a wide-field focal plane to an optical (R similar to 2000, 500 fibers) and a near-infrared (R similar to 22,000, 300 fibers) spectrograph. In addition to these MOS capabilities, the survey is pioneering ultra-wide-field (similar to 4000 deg(2)) integral field spectroscopy enabled by a new dedicated facility (LVM-I) at Las Campanas Observatory, where an integral field spectrograph (IFS) with 1801 lenslet-coupled fibers arranged in a 0 degrees.5-diameter hexagon feeds multiple R similar to 4000 optical spectrographs that cover 3600-9800 angstrom. SDSS-V's hardware and multiyear survey strategy are designed to decode the chemodynamical history of the Milky Way and tackle fundamental open issues in stellar physics in its Milky Way Mapper program, trace the growth physics of supermassive black holes in its Black Hole Mapper program, and understand the self-regulation mechanisms and the chemical enrichment of galactic ecosystems at the energy injection scale in its Local Volume Mapper program. The survey is well timed to multiply the scientific output from major all-sky space missions. The SDSS-V MOS programs began robotic operations in 2021; IFS observations began in 2023 with the completion of the LVM-I facility. SDSS-V builds on decades of heritage of SDSS's pioneering advances in data analysis, collaboration spirit, infrastructure, and product deliverables in astronomy.

Double-degenerate white dwarf (WD) merger remnants can exhibit extreme magnetic fields exceeding 108 G and rapid rotation, but their spectral energy distributions and high-energy emission mechanisms remain poorly characterised. ZTF J1901+1458 stands out as the most compact and strongly magnetised object discovered in this class to date. Intriguingly, recent Chandra observations have revealed that the white dwarf is also a source of soft X-ray emission that is too bright and hard to be of photospheric origin. We analysed new phase-resolved ultraviolet (UV) spectroscopy from the Hubble Space Telescope, together with optical and near-infrared photometry and spectroscopy, using new magnetic atmosphere models to determine its effective temperature, radius, mass, average surface magnetic-field strength, and cooling age. The spectral break at ≈3000 Å, observed in several highly magnetised WDs, is well reproduced by our new models, which account for the effect of magnetic opacities on the atmospheric structure. Our best-fit parameters for the WD yield a cooler effective temperature ( Teff = 27,445+680−1390 T eff = 27 , 445 − 1390 + 680 $ T_{\mathrm{eff}}=27,445^{+680}_{-1390} $ K) and a larger radius than previously reported. Furthermore, the near-infrared data exclude the presence of a stellar or brown dwarf companion hotter than ≈700 K. We jointly analysed published Chandra/Advanced CCD Imaging Spectrometer Imaging array (ACIS-I) data and new XMM-Newton/European Photon Imaging Camera (EPIC) X-ray spectra. The faint X-ray emission, LX = (1.44 ± 0.13)×1027 erg s−1, is highly pulsed at the rotation period of the WD, and the soft spectrum can be modelled by a power-law model with photon index Γ = 2.43+0.17−0.15 Γ = 2 . 43 − 0.15 + 0.17 $ \Gamma=2.43^{+0.17}_{-0.15} $ . We suggest that the X-rays are powered by accretion or by interaction between the WD magnetosphere and circumstellar material. A rapidly rotating magnetic field could power a weak wind along open field lines and extract material from the surface of the WD. Alternatively, low-level accretion of fallback material from the past merger event or the tidal disruption of a planetary body could supply the circumstellar material.

Atmospheric carbon has been detected in the optical spectra of six hydrogen-rich ultra-massive white dwarfs, revealing large carbon abundances (log(C/H) > −0.5) attributable to the convective dredge-up of internal carbon into thin hydrogen surface layers. These rare white dwarfs likely originate from stellar mergers, making them ‘smoking guns’ for one of the binary evolution channels leading to thermonuclear supernovae. However, optical spectroscopy can uncover only the most carbon-enriched objects, suggesting that many more merger remnants may masquerade as normal pure-hydrogen-atmosphere white dwarfs. Here we report the discovery of atmospheric carbon in a Hubble Space Telescope far-ultraviolet spectrum of WD 0525+526, a long-known hydrogen-rich ultra-massive white dwarf. The carbon abundance (log(C/H) = −4.62) is 4–5 dex lower than in the six counterparts and thus detectable only at ultraviolet wavelengths. We find that the total masses of hydrogen and helium in the envelope (10 −13.8 and 10 −12.6 of the total white dwarf mass, respectively) are substantially lower than those expected from single-star evolution, implying that WD 0525+526 is a merger remnant. Our modelling indicates that the low surface carbon abundance arises from an envelope structure in which a thin hydrogen-rich layer floats atop a semi-convection zone—a process that has been largely overlooked in white dwarfs. Our study highlights the importance of ultraviolet spectroscopy in identifying and characterizing merger remnants.

We compare three methods of deriving the local Galactic star formation history, using as a benchmark the Gaia-defined 40 pc white dwarf sample, currently the largest volume complete sample of stellar remnants with medium-resolution spectroscopy. We create a population synthesis model to 1) reproduce the observed white dwarf luminosity function, 2) reproduce the observed absolute Gaia G magnitude distribution, and 3) directly calculate the ages of all individual white dwarfs in the 40 pc volume. We then compare the star formation histories determined from each method. Previous studies using these methods were based on different white dwarf samples and as such were difficult to compare. Uncertainties in each method such as the initial mass function, initial-final mass relation, main sequence lifetimes, stellar metallicity, white dwarf cooling ages and binary evolution are accounted for to estimate the precision and accuracy of each method. We conclude that no method is quantitatively better at determining the star formation history and all three produce star formation histories that agree within uncertainties of current external astrophysical relations.

Double white dwarf binaries are an important remnant of binary evolution as they are possible type Ia supernova progenitors and strong sources of gravitational waves in the low-frequency regime. The double-lined double white dwarf (DBL) survey searches for compact double white dwarfs where both stars are spectrally disentangleable. Candidates are identified by being overluminous compared to the cooling sequence of a typical mass, single white dwarf. In this second DBL survey instalment, we present full orbital solutions of 15 double white dwarf binaries from our ongoing campaign to accurately measure a magnitude-limited mass-period distribution. 12 of these systems are fully solved for the first time. A long-standing bias in the full population has been evident, favouring systems with orbital periods up to a few hours, with little exploration of the majority of the compact double white dwarf population, whose orbital period distribution centres at approximately 20hr. The 15 systems in this study span the orbital period range 5-75hr, significantly augmenting the number of well-characterised systems over these periods, and in general have two similar mass stars combining to approximately 1.0 solar masses. We witness that the orbitally derived mass ratios generally show an excellent agreement with those deduced from atmospheric fits to double-lined spectra in previous work, emphasising the power of wide-scale spectroscopic surveys to efficiently locate the highest mass, double-lined double white dwarfs in the local Galaxy.

We report the discovery of two new magnetic cataclysmic variables with brown dwarf companions and long orbital periods (P-orb = 95 +/- 1 and 104 +/- 2 min). This discovery increases the sample of candidate magnetic period bouncers with confirmed sub-stellar donors from four to six. We also find their X-ray luminosity from archival XMM-Newton observations to be in the range L-X approximate to 10(28)-10(29) ergs(-1) in the 0.25-10 keV band. This low luminosity is comparable with the other candidates, and at least an order of magnitude lower than the X-ray luminosities typically measured in cataclysmic variables. The X-ray fluxes imply mass transfer rates that are much lower than predicted by evolutionary models, even if some of the discrepancy is due to the accretion energy being emitted in other bands, such as via cyclotron emission at infrared wavelengths. Although it is possible that some or all of these systems formed directly as binaries containing a brown dwarf, it is likely that the donor used to be a low-mass star and that the systems followed the evolutionary track for cataclysmic variables, evolving past the period bounce. The donor in long period systems is expected to be a low-mass, cold brown dwarf. This hypothesis is supported by near-infrared photometric observations that constrain the donors in the two systems to be brown dwarfs cooler than approximate to 1100 K (spectral types T5 or later), most likely losing mass via Roche Lobe overflow or winds. The serendipitous discovery of two magnetic period bouncers in the small footprint of the XMM-Newton catalogue implies a large space density of these type of systems, possibly compatible with the prediction of 40-70 per cent of magnetic cataclysmic variables to be period bouncers.