Atmospheric Temperature Inversions and He I 5876 Core Profile Structure in White Dwarfs

TL;DR

This study investigates the core profiles of the He I 5876 line in DB white dwarfs, linking their appearance to atmospheric composition and temperature inversions, and suggests their potential as diagnostics for atmospheric modeling.

Contribution

It reveals the relationship between helium line core structures and atmospheric pollution levels, and proposes temperature inversions as the underlying cause, advancing understanding of white dwarf atmospheres.

Findings

Self-reversed emission cores occur in stars with low hydrogen and heavy element pollution.

Higher pollution levels lead to a single absorption core, indicating suppressed temperature inversions.

Current models partially explain the observations but lack some physics like 3D effects and non-LTE processes.

Abstract

We report distinctive core profiles in the strongest optical helium line, He I 5876, from high-resolution high-sensitivity observations of spectral type DB white dwarfs. By analyzing a sample of 40 stars from Keck/HIRES and VLT/UVES, we find the core appearance to be related to the degree of hydrogen and heavy element content in the atmosphere. New Ca K-line measurements or upper limits are reported for about half the sample stars. He I 5876 emission cores with a self-reversed central component are present for those stars with relatively low hydrogen abundance, as well as relatively low atmospheric heavy element pollution. This self-reversed structure disappears for stars with higher degrees of pollution and/or hydrogen abundance, giving way to a single absorption core. From our model atmospheres, we show that the self-reversed emission cores can be explained by temperature inversions in the upper atmosphere. We propose that the transition to a single absorption core is due to the additional opacity from hydrogen and heavy elements that inhibits the temperature inversions. Our current models do not exactly match the effective temperature range of the phenomenon nor the amplitude of the self-reversed structure, which is possibly a result of missing physics such as 3D treatment, convective overshoot, and/or non-LTE effects. The He I 5876 line structure may prove to be a useful new diagnostic for calibrating temperature profiles in DB atmosphere models.