Taking Advantage of the COS Time-Tag Capability: Observations of Pulsating Hot DQ White Dwarfs and Discovery of a New One

TL;DR

This study utilizes HST/COS time-tag data to analyze ultraviolet pulsations in Hot DQ white dwarfs, confirming known modes, discovering a new pulsator, and highlighting the potential of space-based observations for this class.

Contribution

First space-based detection of pulsations in a Hot DQ white dwarf, expanding understanding of their variability and demonstrating COS time-tag capabilities.

Findings

Recovered known pulsation modes in three stars

Discovered pulsations in SDSS J1153+0056, a new Hot DQ pulsator

High UV amplitudes suggest most Hot DQ stars may pulsate at some level

Abstract

We present an analysis of the ultraviolet light curves of five Hot DQ white dwarfs recently observed with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). These light curves were constructed by extracting the time-tag information from the FUV and NUV spectroscopic data. Single-color light curves were thus produced in 60 s time bins. The Fourier analysis of these data successfully recovers the main pulsation modes of the three stars previously known to be variable from ground-based observations. We also report the discovery of pulsations in another object, SDSS J1153+0056, making it only the fifth member of the new class of variable Hot DQ stars, and the first pulsating white dwarf to be discovered from space-based observations. The relatively high amplitudes of the modes observed in the FUV -- 2 to 4 times that observed in the optical -- as well as the high fraction of stars variable in our sample suggest that most, if not all, Hot DQ white dwarfs might be pulsating at some level when observed at high enough sensitivity. Our results also underline the vast potential of the time-tag capability of the HST/COS combination.