A temperature scale of 1∼2 eV in the mass–radius relationship of white dwarfs of type DA

Abstract

The mass–radius relationship of white dwarfs (WDs) is one of their defining characteristics, largely derived from electron degeneracy pressure. We present a model-independent study of the observed mass–radius relationship in WD binaries of Parsons et al. (2017), listing data over a broad temperature range up to about 60,000 K (5 eV). The data show an appreciable temperature sensitivity with pronounced intrinsic scatter (beyond measurement uncertainty) for the canonical He-models with proton$-$to$-$neutron ratio 1:1. We characterize temperature sensitivity by a temperature scale $T_0$ in model-agnostic power-law relations with temperature normalized radius. For low-mass WDs, the results identify a remarkably modest $T_0=1\sim 2$  eV. We comment on a potential interpretation for atmospheres insulating super-Eddington temperature cores from the sub-Eddington photospheres of low-mass WDs.