Discovery of an ultramassive pulsating white dwarf

TL;DR

This paper reports the discovery of the most massive pulsating white dwarf, GD 518, providing a unique opportunity to study its interior and core composition, which has implications for stellar evolution and supernova progenitors.

Contribution

It presents the first detection of pulsations in an ultramassive white dwarf with an oxygen-neon core, enabling interior probing of such stars.

Findings

Mass of GD 518 is 1.20 solar masses.

Pulsation periods range from 425 to 595 seconds.

Star shows multi-periodic luminosity variations.

Abstract

We announce the discovery of the most massive pulsating hydrogen-atmosphere (DA) white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12,030 +/- 210 K WD with a log(g) = 9.08 +/- 0.06, which corresponds to a mass of 1.20 +/- 0.03 Msun. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425-595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains SNe Ia progenitor systems.