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

Abstract

We present a detailed model atmosphere analysis of cool white dwarfs in the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1). Our sample includes 25,642 unique targets with $G_{\rm BP}-G_{\rm RP}>0$. Unlike the hot DA white dwarf sample in DESI DR1, we do not find a significant discrepancy between the photometric and spectroscopic masses for cool DAs. Hence, DESI's calibration problems for broad lines have a negligible effect for cooler DAs with narrower lines. Magnetic DAs are found everywhere, and not just on the crystallization sequence, indicating that crystallization induced dynamos cannot solely explain the origin of magnetism in white dwarfs. A detailed analysis of cool DC and DZ white dwarfs indicates that the H/He abundance ratio in He-atmosphere white dwarfs increases at lower temperatures. Based on the currently available models, this is the only way to keep the DC masses consistent with the average white dwarf mass of $0.6~M_\odot$. Combined with the analysis of the hot white dwarfs presented previously, this paper completes the analysis of 44,963 white dwarf candidates with DESI DR1 spectra. We use this sample to constrain the fraction of He-atmosphere white dwarfs as a function of temperature, and demonstrate that the He-fraction increases significantly below 10,000 K due to convective mixing. We also highlight rare systems, including new extremely low-mass, DA+DB, and DA+DQ binaries.