Astrophysical Implications of a New Dynamical Mass for the Nearby White Dwarf 40 Eridani B

TL;DR

This study revises the mass of the nearby white dwarf 40 Eri B using new measurements, analyzes its properties with models, and finds consistency with theoretical predictions assuming a thin hydrogen layer, impacting understanding of white dwarf evolution.

Contribution

The paper provides a revised dynamical mass for 40 Eri B based on new astrometric data and compares observational results with interior models, highlighting the significance of thin hydrogen layers in white dwarf structure.

Findings

Revised mass of 40 Eri B is 0.573±0.018 M_sun.

The star's properties are consistent with models assuming a thin hydrogen layer.

Current theory matches observations for multiple nearby white dwarfs.

Abstract

The bright, nearby DA-type white dwarf (WD) 40 Eridani B is orbited by the M dwarf 40 Eri C, allowing determination of the WD's mass. Until recently, however, the mass depended on orbital elements determined four decades ago, and that mass was so low that it created several astrophysical puzzles. Using new astrometric measurements, the binary-star group at the U.S. Naval Observatory has revised the dynamical mass upward, to $0.573\pm0.018\,M_\odot$. In this paper we use model-atmosphere analysis to update other parameters of the WD, including effective temperature, surface gravity, radius, and luminosity. We then compare these results with WD interior models. Within the observational uncertainties, theoretical cooling tracks for CO-core WDs of its measured mass are consistent with the position of 40 Eri B in the H-R diagram; equivalently, the theoretical mass-radius relation (MRR) is consistent with the star's location in the mass-radius plane. This consistency is, however, achieved only if we assume a "thin" outer hydrogen layer, with $q_{\rm H}=M_{\rm H}/M_{\rm WD}\simeq10^{-10}$. We discuss other evidence that a significant fraction of DA WDs have such thin H layers, in spite of expectation from canonical stellar-evolution theory of "thick" H layers with $q_{\rm H}\simeq10^{-4}$. The cooling age of 40 Eri B is $\sim$122 Myr, and its total age is $\sim$1.8 Gyr. We present the MRRs for 40 Eri B and three other nearby WDs in visual binaries with precise mass determinations, and show that the agreement of current theory with observation is excellent in all cases.