Discovery Of Pulsations, Including Possible Pressure Modes, In Two New Extremely Low Mass, He-Core White Dwarfs




Hermes, J. J.
Montgomery, Michael H.
Winget, D. E.
Brown, Warren R.
Gianninas, A.
Kilic, Mukremin
Kenyon, S. J.
Bell, Keaton J.
Harrold, Samuel T.

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We report the discovery of the second and third pulsating extremely low mass (ELM) white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 M-circle dot 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 that are also present. J1112 is a T-eff = 9590 +/- 140 K and log g = 6.36 +/- 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792 and 2855 s. In this star, we also see short-period variability, strongest at 134.3 s, well short of the 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 and 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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Hermes, J. J., M. H. Montgomery, D. E. Winget, Warren R. Brown, A. Gianninas, Mukremin Kilic, Scott J. Kenyon, Keaton J. Bell, and Samuel T. Harrold. "Discovery of Pulsations, Including Possible Pressure Modes, in Two New Extremely Low Mass, He-core White Dwarfs." The Astrophysical Journal, Vol. 765, No. 2 (Mar., 2013): 102.