Discovery of an Ultramassive Pulsating White Dwarf

Date
2013-07
Authors
Hermes, J. J.
Kepler, S. O.
Castanheira, Barbara G.
Gianninas, A.
Winget, D. E.
Montgomery, M. H.
Brown, Warren R.
Harrold, Samuel T.
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Abstract

We announce the discovery of the most massive pulsating hydrogen-atmosphere 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 +/- 0.03M(circle dot). 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 to 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 Type Ia supernovae progenitor systems.

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Citation
Hermes, James J., Souza Oliveira Kepler, Barbara G. Castanheira, Alexandros Gianninas, Donald Earl Winget, Michael Houston Montgomery, Warren R. Brown, and Samuel T. Harrold. "Discovery of an ultramassive pulsating white dwarf." The Astrophysical Journal Letters, Vol. 771, No. 1 (Jul., 2013): L2.