Near-UV absorption in very cool DA white dwarfs

TL;DR

This study uses Hubble Space Telescope spectra and comprehensive modeling to confirm that the red wing of the Lyman alpha line explains the near-UV flux decline in very cool hydrogen-rich white dwarfs, improving atmospheric parameter estimates.

Contribution

The paper provides a rigorous validation of the Lyman alpha red wing opacity model for very cool white dwarfs using combined spectral and photometric data.

Findings

Lyman alpha red wing opacity accurately models near-UV flux decline.

Improved estimates of temperature and gravity for cool white dwarfs.

Upper limits on helium abundance in their atmospheres.

Abstract

The atmospheres of very cool, hydrogen-rich white dwarfs (Teff <6000 K) are challenging to models because of the increased complexity of the equation of state, chemical equilibrium, and opacity sources in a low-temperature, weakly ionized dense gas. In particular, many models that assume relatively simple models for the broadening of atomic levels and mostly ideal gas physics overestimate the flux in the blue part of their spectra. A solution to this problem that has met with some success is that additional opacity at short wavelengths comes for the extreme broadening of the Lyman alpha line of atomic H by collisions primarily with H2. For the purpose of validating this model more rigorously, we acquired Hubble Space Telescope STIS spectra of 8 very cool white dwarfs (5 DA and 3 DC stars). Combined with their known parallaxes, BVRIJHK and Spitzer IRAC photometry, we analyze their entire spectral energy distribution (from 0.24 to 9.3 micron) with a large grid of model atmospheres and synthetic spectra. We find that the red wing of the Lyman alpha line reproduces the rapidly decreasing near-UV flux of these very cool stars very well. We determine better constrained values of Teff and gravity as well as upper limits to the helium abundance in their atmospheres.