Towards an Understanding of the Atmospheres of Cool White Dwarfs

TL;DR

This paper investigates the atmospheres of cool white dwarfs using observational data and models, confirming the reliability of pure-hydrogen models in the UV, analyzing carbon features in DQ/DQp stars, and highlighting current modeling limitations in IR spectra.

Contribution

It provides new spectral fits for cool white dwarfs, confirms the nature of DQp stars as pressure-distorted DQ stars, and discusses limitations in current atmospheric models for dense He/H mixtures.

Findings

Pure-hydrogen models reliably fit near-UV spectra.

DQp stars have carbon abundances consistent with DQ stars.

Current models struggle to reproduce IR spectra of mixed He/H atmospheres.

Abstract

Cool white dwarfs with Teff < 6000 K are the remnants of the oldest stars that existed in our Galaxy. Their atmospheres, when properly characterized, can provide valuable information on white dwarf evolution and ultimately star formation through the history of the Milky Way. Understanding the atmospheres of these stars requires joined observational effort and reliable atmosphere modeling. We discuss and analyze recent observations of the near-ultraviolet (UV) and near-infrared (IR) spectrum of several cool white dwarfs including DQ/DQp stars showing carbon in their spectra. We present fits to the entire spectral energy distribution (SED) of selected cool stars, showing that the current pure-hydrogen atmosphere models are quite reliable, especially in the near-UV spectral region. Recently, we also performed an analysis of the coolest known DQ/DQp stars investigating further the origin of the C2 Swan bands-like spectral features that characterize the DQp stars. We show that the carbon abundances derived for DQp stars fit the trend of carbon abundance with Teff seen in normal cool DQ stars. This further supports the recent conclusion of Kowalski A&A (2010) that DQp stars are DQ stars with pressure distorted Swan bands. However, we encounter some difficulties in reproducing the IR part of the SED of stars having a mixed He/H atmosphere. This indicates limitations in current models of the opacity in dense He/H fluids.