Constraints on White Dwarf Hydrogen Layer Masses Using Gravitational Redshifts

TL;DR

This study uses gravitational redshift data and Gaia measurements to empirically constrain the hydrogen envelope masses of white dwarfs, confirming the importance of progenitor evolution in accurate modeling.

Contribution

It provides the first large-scale observational constraint on white dwarf hydrogen envelope masses using gravitational redshifts and Gaia data, validating evolutionary models.

Findings

Mass-radius relations agree with state-of-the-art models.

Thick, mass-dependent hydrogen envelopes are favored.

Gravitational redshifts are effective probes of hydrogen layer masses.

Abstract

The hydrogen envelope is the outermost layer of a DA white dwarf; it makes up the entirety of the stellar photosphere, and yet its typical extent is difficult to model theoretically and remains poorly observationally constrained. As a result, hydrogen envelope mass is a substantial source of systematic uncertainty in physical properties of white dwarf, including overall masses and cooling ages. In this work, we fit a Gaussian mixture model to gravitational redshifts from high-resolution spectroscopy, paired with radius measurements from Gaia BP/RP spectra, to measure the mass-radius relation for a sample of 468 white dwarfs. Our results are in excellent agreement with the predicted mass-radius relations of state-of-the-art evolutionary models, including those from the MESA Isochrones and Stellar Tracks (MIST) library. We find that mass-radius relations such as MIST which assume a thick and mass-dependent hydrogen envelope are preferred by the observed probability density function over models which assume a constant hydrogen envelope mass. Proper treatment of the evolution of white dwarf progenitors is thus important for accurately modeling the mass-radius relation. Our results indicate that gravitational redshift measurements of large samples of white dwarfs in wide binaries are promising probes of the hydrogen envelope masses of DA white dwarfs.