On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model-Atmosphere Analysis of Hot White Dwarfs from SDSS DR12

TL;DR

This study analyzes 1806 hot white dwarfs from SDSS DR12 to understand their spectral evolution, revealing the fraction of helium-rich stars that transition to hydrogen atmospheres and identifying hybrid stars, with implications for stellar evolution models.

Contribution

It provides the first detailed statistical analysis of the spectral evolution of hot white dwarfs using new model atmospheres and cooling tracks, constraining the DO-to-DA transition process.

Findings

24% of white dwarfs start as DO stars

Approximately two-thirds of DO stars become DA stars

DO-to-DA transition occurs over a broad temperature range

Abstract

As they evolve, white dwarfs undergo major changes in surface composition, a phenomenon known as spectral evolution. In particular, some stars enter the cooling sequence with helium atmospheres (type DO) but eventually develop hydrogen atmospheres (type DA), most likely through the upward diffusion of residual hydrogen. Our empirical knowledge of this process remains scarce: the fractions of white dwarfs that are born helium-rich and that experience the DO-to-DA transformation are poorly constrained. We tackle this issue by performing a detailed model-atmosphere investigation of 1806 hot ($T_{\rm eff} \ge 30,000$ K) white dwarfs observed spectroscopically by the Sloan Digital Sky Survey. We first introduce our new generations of model atmospheres and theoretical cooling tracks, both appropriate for hot white dwarfs. We then present our spectroscopic analysis, from which we determine the atmospheric and stellar parameters of our sample objects. We find that $\sim$24% of white dwarfs begin their degenerate life as DO stars, among which $\sim$2/3 later become DA stars. We also infer that the DO-to-DA transition occurs at substantially different temperatures ($75,000 \ {\rm K} > T_{\rm eff} > 30,000$ K) for different objects, implying a broad range of hydrogen content within the DO population. Furthermore, we identify 127 hybrid white dwarfs, including 31 showing evidence of chemical stratification, and we discuss how these stars fit in our understanding of the spectral evolution. Finally, we uncover significant problems in the spectroscopic mass scale of very hot ($T_{\rm eff} > 60,000$ K) white dwarfs.