The ubiquity of carbon dredge-up in hydrogen-deficient white dwarfs as revealed by GALEX

TL;DR

This study uses GALEX ultraviolet data to demonstrate that most hydrogen-deficient white dwarfs, including featureless DC types, undergo carbon dredge-up, revealing widespread internal mixing in these stars.

Contribution

It provides the first observational evidence of ubiquitous carbon pollution in DC white dwarfs, extending the understanding of convective dredge-up beyond DQ stars.

Findings

Most DC white dwarfs show ultraviolet signatures of carbon lines.

Carbon dredge-up occurs in the majority of hydrogen-deficient white dwarfs.

The process is more widespread than previously confirmed.

Abstract

The convective dredge-up of carbon from the interiors of hydrogen-deficient white dwarfs has long been invoked to explain the presence of carbon absorption features in the spectra of cool DQ stars ($T_{\rm eff} < 10{,}000\,{\rm K}$). It has been hypothesized that this transport process is not limited to DQ white dwarfs and also operates, albeit less efficiently, in non-DQ hydrogen-deficient white dwarfs within the same temperature range. This non-DQ population is predominantly composed of DC white dwarfs, which exhibit featureless optical spectra. However, no direct observational evidence of ubiquitous carbon pollution in DC stars has thus far been uncovered. In this Letter, we analyze data from the Galaxy Evolution Explorer (GALEX) to reveal the photometric signature of ultraviolet carbon lines in most DC white dwarfs in the $8500\,{\rm K} \leq T_{\rm eff} \leq 10{,}500\,{\rm K}$ temperature range. Our results show that the vast majority of hydrogen-deficient white dwarfs experience carbon dredge-up at some point in their evolution.