arXiv subjects

Antoine Bédard

Publications and source records attributed to Antoine Bédard.

At least 19 recordsLinked to original sources

arXiv2026

White dwarfs within 13 pc: insights from ultraviolet spectroscopy

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.

astro-ph.SR↗

arXiv2026

Improved Stark Broadened Profiles for Neutral Helium Lines Using Computer Simulations

The study of Stark broadening of neutral helium lines, despite significant advances over recent decades, has not led to updated large grids of helium line profiles relevant to the spectroscopic study of helium-rich stars. While the semi-analytical approach based on the standard Stark broadening theory is efficient for generating such grids, it presents challenges in incorporating additional physical effects into the model. Motivated by recent studies that highlight potential issues with line profiles in the context of white dwarf stars, this paper leverages advances in computer simulations to create a new grid of line profiles for 13 neutral helium lines in the optical range. These profiles cover densities ranging from 10^14 to 6 x 10^17 cm^-3 and temperatures from 10,000 K to 40,000 K, with the exception of the narrower He I 4713 line, for which the profile grid begins at 10^15.5 cm^-3. The primary goal of this research is to present the new grid and compare it with both the semi-analytical approach and other simulation results. By doing so, corrections to the previous grid will be explored, providing a foundation for future studies that utilize this updated grid. We also examine the impact of these new profiles on the determination of physical parameters for a range of astrophysical objects, including DB white dwarfs and other helium-rich stars.

astro-ph.SR↗

arXiv2026

High Velocity Circumstellar Gas Orbiting a White Dwarf Star

Numerous white dwarf stars are known to be orbited by disks of gas and dust. To date, broad, about 300 km s-1 wide, gaseous circumstellar absorption features have only been reported for the already iconic WD 1145+017, where one is witnessing the breakup of an extrasolar asteroid in real time. We report here the discovery of absorption from circumstellar gas around a second white dwarf (WD J0234-0406) with similarly broad features. The observed lines are carried by ions of Ca, Cr, Fe, Ti, Mg, Mn, Na, O, Si, Sc, Sr, Ti, and V. In addition, deep, non-photospheric lines of Si IV are seen in the ultraviolet; we compare these with Si IV lines previously seen in the ultraviolet spectra of various other white dwarfs. The apparent broadband flux of WD 1145+017 is known to change often and rapidly as chunks of the asteroid pass between the star and Earth. No such variations are seen in the brightness of WD J0234-0406. In addition, while the strength/structure of circumstellar absorption features at WD 1145+017 has changed dramatically with time, nothing similar is seen at WD J0234-0406. Excess infrared emission at WD J0234-0406 indicates the presence of circumstellar dust particles.

astro-ph.SR↗

arXiv2026

Assessing the star formation history of all-sky and part-sky 100pc white dwarf samples

Thanks to Gaia and large-scale spectroscopic follow-up surveys (4MOST, DESI, WEAVE, SDSS-V), it is now possible to build representative and minimally biased samples of the local white dwarf population. Here we analyse several volume-limited 100pc samples of white dwarfs, constructed from different surveys and studies, to evaluate their completeness and residual biases. We model the underlying star formation history and Galactic disc age via comparison with simulated populations of white dwarfs to quantitatively characterise completeness. We assess whether the benefit of Gaia XP spectra in datasets outweighs the reduction in sample size, and to what extent targeted, part-sky, and magnitude limited surveys can be used in comparison to all-sky volume limited surveys. Additionally, we simulate the 4MOST 100PC sub-survey and discuss its use to better understand the local star formation history.

astro-ph.SR↗

arXiv2026

Classifying white dwarfs from multi-object spectroscopy surveys with machine learning

With tens to hundreds of spectra of white dwarfs being taken each night from multi-object spectroscopic surveys, automated spectral classification is essential as part of efficient data processing. In this study, we design a neural network to classify the spectral type of white dwarfs using a combination of spectra from the Dark Energy Spectroscopic Instrument (DESI) data release~1 and imaging from Pan-STARRS photometry. The trained network has a near 100% accuracy at identifying DA and DB white dwarf spectral types, while having an 85-95% accuracy for identifying all other primary types, including metal pollution. Distinct spectral or photometric features map into separate structures when performing a Uniform Manifold Approximation and Projection (UMAP) dimensionality reduction. Investigating further and looking at multiple epoch spectra, we performed a separate search for objects that have strongly changing spectral signatures using UMAP, discovering 3 new inhomogeneous surface composition ('double-faced') white dwarfs in the process. We lastly show how machine learning has the potential to separate single white dwarfs from double white dwarf binary star systems in a large dataset, ideal for isolating a single star population. The results from all of these techniques show a compelling use of machine learning to boost efficiency in analysing white dwarfs observed in multi-object spectroscopy surveys, at times replacing the need for human-driven spectral classifications. This demonstrates our techniques as powerful tools for batch population analyses, finding outliers as a form of rare subclass detection, and in conducting multi-epoch spectral analyses.

astro-ph.SR↗

arXiv2025

Exogeological inferences from white dwarf pollutants: the impact of stellar physics

Many white dwarfs have accreted material from their own planetary systems. These objects can be used to infer the composition of exoplanetary material and identify evidence for key geological processes. However, the white dwarf atmospheric physics distorts the inferred material composition away from the true composition, mainly through differential atomic diffusion of the accreted metals. Correcting for this effect is essential, but is dependent on various physical assumptions associated with the white dwarf itself. We first focus on the effect of assumptions related to convective overshoot and thermohaline mixing on the atomic diffusion timescales. For white dwarfs with H-dominated atmospheres between 12000 K and 18000 K, we find that including a complete treatment of convective overshoot decreases the inferred Fe and O abundances in accreted material. For these white dwarfs, we also find that including thermohaline mixing decreases Fe and O abundances. For He-dominated systems, the effect of convective overshoot is comparatively minor. We then explore the overall effect of other physical assumptions by comparing publicly available grids of diffusion timescales. We find that the choice of model grid can have a large impact for white dwarfs with He-dominated atmospheres, notably on the inferred core to mantle ratio of accreted material. We identify several systems for which the geological interpretation is robust against these systematics. We also present a `discrepancy metric' which can be used to estimate the potential impact of changing the stellar physics without requiring detailed modelling.

astro-ph.EP↗

arXiv2025

Tracing Planetary Accretion in a 3 Gyr-old Hydrogen-Rich White Dwarf: The Extremely Polluted Atmosphere of LSPM J0207+3331

We report the detection of 13 heavy elements (Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, and Sr) in the photosphere of LSPM J0207+3331, a ~3 Gyr old hydrogen-rich white dwarf with an effective temperature comparable to that of the Sun. Upper limits on carbon, obtained through the absence of molecular CH, suggest accretion from a carbon-volatile-depleted source. The accreted parent body exhibits slight deficits of Mg and Si relative to Fe but otherwise bulk Earth-like abundance patterns; a reasonable interpretation is that LSPM J0207+3331 is accreting a massive differentiated rocky body that had a core mass fraction higher than the Earth's. The high level of pollution indicates that substantial accretion events can still occur even after 3 Gyr of cooling. We also detect weak Ca II H & K line-core emission, making this only the second known isolated polluted white dwarf to exhibit this phenomenon and suggesting the presence of additional physical processes in or above the upper atmosphere. Our analysis also highlights the critical importance of including heavy elements in the model atmosphere structure calculations for highly polluted hydrogen-rich white dwarfs. Neglecting their contribution significantly impacts the inferred thermodynamic structure, leading to inaccuracies in derived stellar parameters. Finally, we show that the observed 11.3 microns infrared excess can be explained by a single silicate dust disk rather than a two-ring disk model.

astro-ph.SR↗

arXiv2025

Precise Parameters for Two LISA Sources

We present precise parameters for two compact double white dwarf 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 HST/STIS, Keck I/LRIS, and Keck II/ESI. 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 of $i$ = 25$^{+4.5}_{-3.0}$ deg, while for J0634+3803, we obtain $i$ = 43$^{+7.0}_{-5.6}$ 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 multi-messenger observations in characterizing double white dwarf binaries and maximizing LISA's early scientific capabilities.

astro-ph.SR↗

arXiv2025

A hot white dwarf merger remnant revealed by an ultraviolet detection of carbon

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) 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 characterising merger remnants.

astro-ph.SR↗

arXiv2025

The DBL Survey I: discovery of 34 double-lined double white dwarf binaries

We present the first discoveries of the double-lined double white dwarf (DBL) survey that targets over-luminous sources with respect to the canonical white dwarf cooling sequence according to a set of well-defined criteria. The primary goal of the DBL survey is to identify compact double white dwarf binary star systems from a unique spectral detection of both stars, which then enables a precise quantification of the atmospheric parameters and radial velocity variability of a system. Our search of 117 candidates that were randomly selected from a magnitude limited sample of 399 yielded a 29% detection efficiency with 34 systems exhibiting a double-lined signature. A further 38 systems show strong evidence of being single-lined or potentially-double-lined double white dwarf binaries and 7 single-lined sources from the full observed sample are radial velocity variable. The 45 remaining candidates appear as a single WD with no companion or a non-DA white dwarf, bringing the efficiency of detecting binaries to 62%. Atmospheric fitting of all double-lined systems reveals a large fraction that have two similar mass components that combine to a total mass of 1.0-1.3 solar masses - a class of double white dwarf binaries that may undergo a sub-Chandrasekhar mass type Ia detonation or merge to form a massive O/Ne WD, although orbital periods are required to infer on which timescales. One double-lined system located 49pc away, WDJ181058.67+311940.94, is super-Chandrasekhar mass, making it the second such double white dwarf binary to be discovered.

astro-ph.SR↗

arXiv2025

A super-Chandrasekhar mass type Ia supernova progenitor at 49 pc set to detonate in 23 Gyr

Double white dwarf binaries are a leading explanation to the origin of type Ia supernovae, but no system exceeding the Chandrasekhar mass limit (1.4 M$_\odot$) has been found that will explode anywhere close to a Hubble time. Here, we present the super-Chandrasekhar mass double white dwarf WDJ181058.67+311940.94 whose merger time ($22.6\pm1.0$ Gyr) is of the same order as a Hubble time. The mass of the binary is large, combining to $1.555\pm0.044$ M$_\odot$, while being located only 49 pc away. We predict that the binary will explode dynamically via a double detonation destroying both stars just before they merge, appearing as a subluminous type Ia supernova with a peak apparent magnitude of about $m_V=-16$ (200,000 times brighter than Jupiter). The observationally-derived birthrate of super-Chandrasekhar mass double white dwarfs is now at least $6.0\times10^{-4}$ yr$^{-1}$ and the observed rate of type Ia supernovae in the Milky Way from such systems is approximately $4.4\times10^{-5}$ yr$^{-1}$, while the predicted type Ia supernova rate in the Milky Way from all progenitor channels is about sixty times larger. Hence, WDJ181058.67+311940.94 mitigates the observed deficit of massive double white dwarfs witnessed in volume-complete populations, but further evidence is required to determine the majority progenitors of type Ia supernovae.

astro-ph.SR↗

arXiv2025

Comparison of methods used to derive the Galactic star formation history from white dwarf samples

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.

astro-ph.SR↗

arXiv2025

Crystal nucleation rates in one-component Yukawa systems

Nucleation in the supercooled Yukawa system is relevant for addressing current challenges in understanding a range of crystallizing systems including white dwarf (WD) stars. We use both brute force and seeded molecular dynamics simulations to study homogeneous nucleation of crystals from supercooled Yukawa liquids. With our improved approach to seeded simulations, we obtain quantitative predictions of the crystal nucleation rate and cluster size distributions as a function of temperature and screening length. These quantitative results show trends towards fast nucleation with short-ranged potentials. They also indicate that for temperatures $T > 0.9T_m$, where $T_m$ is the melt temperature, classical homogeneous nucleation is too slow to initiate crystallization but transient clusters of around 100 particles should be common. We apply these general results to a typical WD model and obtain a delay of approximately 0.6 Gyr in the onset of crystallization that may be observable.

physics.plasm-ph↗

arXiv2025

The prototype double-faced white dwarf has a thin hydrogen layer across its entire surface

Some white dwarfs undergo significant changes in atmospheric composition owing to the diffusion and mixing of residual hydrogen in a helium-rich envelope. Of particular interest are a few objects exhibiting hydrogen and helium line variations modulated by rotation, revealing surface composition inhomogeneities. Recently, the hot ultramassive white dwarf ZTF J203349.80+322901.1 emerged as the most extreme such specimen, with hydrogen and helium lines successively appearing and vanishing in anti-phase, suggesting a peculiar double-faced configuration. However, standard atmosphere models fail to reproduce the observed spectrum at all rotation phases, hampering further interpretation. Here, we perform a new analysis of ZTF J203349.80+322901.1 using stratified atmosphere models, where hydrogen floats above helium, and obtain excellent fits to the phase-resolved spectra. Our results imply that an extremely thin hydrogen layer covers the entire surface but varies from optically thick to optically thin across the surface, thus producing the observed spectral variations. We present new envelope models indicating that the hydrogen layer arises from a delicate interplay between diffusion and convection. We discuss possible explanations for the surface layer asymmetry, including an asymmetric magnetic field and a non-uniform internal hydrogen distribution. Finally, we highlight implications for expanding and understanding the emerging class of inhomogeneous white dwarfs.

astro-ph.SR↗

arXiv2025

Long-lived Habitable Zones around White Dwarfs undergoing Neon-22 Distillation

White dwarf stars have attracted considerable attention in the past 15 years as hosts for potentially habitable planets, but their low luminosity and continuous cooling are major challenges for habitability. Recently, astronomers have found that about 6% of massive white dwarfs seem to have "paused" their cooling for up to ~10 Gyr. The leading explanation for this cooling delay is the distillation of neutron-rich isotopes such as $^{22}$Ne in the white dwarf's interior, which releases a considerable amount of gravitational energy as the star's internal structure rearranges. Here, we consider the impact of $^{22}$Ne distillation on the evolution of white dwarf habitable zones. We find that $^{22}$Ne distillation in the white dwarf host dramatically increases the time that a planet can continuously reside within the habitable zone (giving more time for life to arise) and that long-lasting habitable zones are located farther from the star (decreasing the impact of tidal forces). These properties may make white dwarfs undergoing $^{22}$Ne distillation more promising locations for habitability than white dwarfs undergoing standard cooling.

astro-ph.EP↗

arXiv2024

Buoyant crystals halt the cooling of white dwarf stars

White dwarfs are stellar remnants devoid of a nuclear energy source, gradually cooling over billions of years and eventually freezing into a solid state from the inside out. Recently, it was discovered that a population of freezing white dwarfs maintains a constant luminosity for a duration comparable to the age of the universe, signaling the presence of a powerful yet unknown energy source that inhibits the cooling. For certain core compositions, the freezing process is predicted to trigger a solid-liquid distillation mechanism, due to the solid phase being depleted in heavy impurities. The crystals thus formed are buoyant and float up, thereby displacing heavier liquid downward and releasing gravitational energy. Here we show that distillation interrupts the cooling for billions of years and explains all the observational properties of the unusual delayed population. With a steady luminosity surpassing that of some main-sequence stars, these white dwarfs defy their conventional portrayal as dead stars. Our results highlight the existence of peculiar merger remnants and have profound implications for the use of white dwarfs in dating stellar populations.

astro-ph.SR↗

arXiv2024

The Gaia white dwarf revolution

This review highlights the role of the Gaia space mission in transforming white dwarf research. These stellar remnants constitute 5-7% of the local stellar population in volume, yet before Gaia the lack of trigonometric parallaxes hindered their identification. The mission's Data Release 2 in 2018 provided the first unbiased colour-absolute magnitude diagram of the local stellar population, identifying 260,000 white dwarfs, with the number later increasing to over 355,000 in Data Release 3. Since then, more than 400 white dwarf studies have made critical use of Gaia data, establishing it as a fundamental resource for white dwarf identification, fundamental parameter determination and more recently spectral type characterisation. The review underscores the routine reliance on Gaia parallaxes and extensive use of its photometry in white dwarf surveys. We also discuss recent discoveries firmly grounded in Gaia data, including white dwarf mergers, exotic compact binaries and evolved planetary systems.

astro-ph.SR↗

arXiv2024

The spectral evolution of white dwarfs: where do we stand?

White dwarfs are the dense, burnt-out remnants of the vast majority of stars, condemned to cool over billions of years as they steadily radiate away their residual thermal energy. To first order, their atmosphere is expected to be made purely of hydrogen due to the efficient gravitational settling of heavier elements. However, observations reveal a much more complex situation, as the surface of a white dwarf (1) can be dominated by helium rather than hydrogen, (2) can be polluted by trace chemical species, and (3) can undergo significant composition changes with time. This indicates that various mechanisms of element transport effectively compete against gravitational settling in the stellar envelope. This phenomenon is known as the spectral evolution of white dwarfs and has important implications for Galactic, stellar, and planetary astrophysics. This invited review provides a comprehensive picture of our current understanding of white dwarf spectral evolution. We first describe the latest observational constraints on the variations in atmospheric composition along the cooling sequence, covering both the dominant and trace constituents. We then summarise the predictions of state-of-the-art models of element transport in white dwarfs and assess their ability to explain the observed spectral evolution. Finally, we highlight remaining open questions and suggest avenues for future work.

astro-ph.SR↗