Hot DQ White Dwarfs: Something Different

TL;DR

This paper provides a comprehensive analysis of Hot DQ white dwarfs with carbon-dominated atmospheres, revealing their temperature range, surface composition, magnetic fields, and pulsation properties, suggesting they are a distinct evolutionary phase.

Contribution

It offers the first detailed spectroscopic and photometric characterization of all known Hot DQ white dwarfs, proposing their connection to a new class of hydrogen and helium deficient stellar remnants.

Findings

All Hot DQ stars have temperatures between 18,000 and 24,000 K.

Most have normal surface gravity around log g = 8.0.

One star shows a weak magnetic field and pulsations.

Abstract

We present a detailed analysis of all the known Hot DQ white dwarfs in the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to have carbon dominated atmospheres. Our spectroscopic and photometric analysis reveals that these objects all have effective temperatures between ~18,000 and 24,000 K. The surface composition is found to be completely dominated by carbon, as revealed by the absence of Hbeta and HeI 4471 lines (or determination of trace amount in a few cases). We find that the surface gravity of all objects but one seems to be ''normal'' and around log g = 8.0 while one is likely near log g = 9.0. The presence of a weak magnetic field is directly detected by spectropolarimetry in one object and is suspected in two others. We propose that these strange stars could be cooled down versions of the weird PG1159 star H1504+65 and form a new family of hydrogen and helium deficient objects following the post-AGB phase. Finally, we present the results of full nonadiabatic calculations dedicated specifically to each of the Hot DQ that show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity variations. This result is in excellent agreement with recent observations by Montgomery et al. who find that J142625.70+575218.4 is the only pulsator among 6 Hot DQ white dwarfs surveyed in February 2008.