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

TL;DR

DESI DR1 enables a Gaia-based, large-scale spectroscopic census of hot white dwarfs. The authors perform a comprehensive model-atmosphere analysis across DA, DB/DBA, DQ, DZ/DZ, and magnetic WDs, combining DESI spectra with Gaia parallaxes and multi-band photometry to derive $T_{eff}$, $\log g$, masses, and atmospheric compositions, while accounting for 3D corrections and NLTE effects where relevant. A key finding is a systematic photometric-spectroscopic mass discrepancy for DA WDs of about $0.05$–$0.06\,M_\odot$ likely linked to broad Balmer-line profiles and flux-calibration issues in DESI DR1, alongside a photometric mass peak near the canonical $0.6\,M_\odot$. The work also uncovers a notable population of magnetized ultramassive DAs and a substantial warm DQ component, and it quantifies the spectral evolution of WDs across $T_{eff}$ from $10^5$ to $10^4$ K. These results demonstrate the power and the limitations of next-generation, multiplexed spectroscopic surveys (DESI, SDSS-V, 4MOST, WEAVE) for advancing our understanding of white dwarf demographics, evolution, and accretion phenomena.

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 with $G_{\rm BP}-G_{\rm RP}\leq0$. We use the DESI spectra along with Gaia parallaxes and 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-$0.06~M_\odot$), 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 $0.6~M_\odot$. 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 9 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 $10^5-10^4$ K. 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 like DESI, SDSS-V, 4MOST, and WEAVE over the next several years.

Figures (32)

• Figure 1: Our selected DESI DR1 white dwarf sample in the H-R diagram. Evolutionary tracks for pure H atmosphere white dwarfs with $\log{g}=$ 7, 8, and 9 holberg06bedard20 are shown for comparison.
• Figure 2: Cumulative distribution of the signal-to-noise ratios of the DESI DR1 spectra of our sample of targets.
• Figure 3: Sky coordinates of our DESI DR1 targets.
• Figure 4: Model fits to a hot (left) and a warm (right) DA white dwarf. The top panels show the best-fitting pure H (filled dots) atmosphere models to the photometry (error bars). These panels also include the white dwarf name, Gaia Source ID, the file name of the spectrum (including the DESI observation date), and the photometry used in the fitting. The bottom panels show the fits to the DESI spectra. Since the photometric method becomes unreliable for hot white dwarfs, we force the spectroscopic temperature on their photometric fits. Here and in the following figures, we smooth the DESI spectra by 5 points (with 0.8 Å per point) for display purposes only.
• Figure 5: Physical parameters of DA white dwarfs estimated based on the SDSS $u$ and Pan-STARRS $grizy$ photometry versus using the full set of SDSS $ugriz$, Pan-STARRS $grizy$, and SkyMapper $uvgriz$ photometry. Here the sample is restricted to objects with distance accuracy better than 10%.