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arXiv · 1211.1022

Discovery of pulsations, including possible pressure modes, in two new extremely low mass, He-core white dwarfs

J. J. Hermes·M. H. Montgomery·D. E. Winget·Warren R. Brown·A. Gianninas·Mukremin Kilic·Scott J. Kenyon·Keaton J. Bell·Samuel T. Harrold
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Abstract

We report the discovery of the second and third pulsating extremely low mass white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 Msun and effective temperatures below 10,000 K, establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti stars). The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic (p-mode) pulsations in any WD, which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g-modes also present. J1112 is a Teff = 9590 +/- 140 K and log(g) = 6.36 +/- 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792-2855 s. In this star we also see short-period variability, strongest at 134.3 s, well short of expected g-modes for such a low-mass WD. The other new pulsating WD, J1518, is a Teff = 9900 +/- 140 K and log(g) = 6.80 +/- 0.05 WD. The light curve of J1518 is highly non-sinusoidal, with at least seven significant periods between 1335-3848 s. Consistent with the expectation that ELM WDs must be formed in binaries, these two new pulsating He-core WDs, in addition to the prototype SDSS J184037.78+642312.3, have close companions. However, the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars.

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J. J. Hermes, M. H. Montgomery, D. E. Winget, Warren R. Brown, A. Gianninas, Mukremin Kilic, Scott J. Kenyon, Keaton J. Bell, Samuel T. Harrold. 2012-11-05. Discovery of pulsations, including possible pressure modes, in two new extremely low mass, He-core white dwarfs. https://doi.org/10.1088/0004-637x%2F765%2F2%2F102

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