Observational Constraints on the Degenerate Mass-Radius Relation

TL;DR

This study tests the white dwarf mass-radius relation using precise measurements from 12 binary systems, confirming theoretical models at a 1-2 sigma level and highlighting the potential of Gaia data for future constraints.

Contribution

It provides the first detailed observational constraints on the white dwarf mass-radius relation using multiple independent measurements for a select set of systems.

Findings

White dwarfs conform to theoretical mass-radius relations at 1-2 sigma.

Strong evidence for a thin hydrogen envelope in 40 Eri B.

Future Gaia parallaxes will enable more precise tests of the mass-radius relation.

Abstract

The white dwarf mass-radius relationship is fundamental to modern astrophysics. It is central to routine estimation of DA white dwarf masses derived from spectroscopic temperatures and gravities. It is also the basis for observational determinations of the white dwarf initial-final mass relation. Nevertheless, definitive and detailed observational confirmations of the mass-radius relation (MRR) remain elusive due to a lack of sufficiently accurate white dwarf masses and radii. Current best estimates of masses and radii allow only broad conclusions about the expected inverse relation between masses and radii in degenerate stars. In this paper we examine a restricted set of 12 DA white dwarf binary systems for which accurate (1) trigonometric parallaxes, (2) spectroscopic effective temperatures and gravities, and (3) gravitational redshifts are available. We consider these three independent constraints on mass and radius in comparison with an appropriate evolved MRR for each star. For the best-determined systems it is found that the DA white dwarfs conform to evolved theoretical MRRs at the 1-{\sigma} to 2-{\sigma} level. For the white dwarf 40 Eri B (WD0413-077) we find strong evidence for the existence of a "thin" hydrogen envelope. For other stars improved parallaxes will be necessary before meaningful comparisons are possible. For several systems current parallaxes approach the precision required for the state-of-the-art mass and radius determinations that will be obtained routinely from the Gaia mission. It is demonstrated here how these anticipated results can be used to firmly constrain details of theoretical mass-radius determinations.