White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8
Abstract
We present photometry and spectroscopy for 27 pulsating hydrogenatmosphere white dwarfs (DAVs; a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The spacebased photometry reveals pulsation properties previously inaccessible to groundbased observations. We observe a sharp dichotomy in oscillation mode line widths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode line widths, more reminiscent of a damped harmonic oscillator than a heatdriven pulsator. Extended Kepler coverage also permits extensive mode identification: we identify the spherical degree of 87 out of 201 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4 mclass telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra, we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low and intermediatemass stars. We find that 0.510.73 M _{⊙} white dwarfs, which evolved from 1.73.0 M _{⊙} ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for highermass white dwarfs. Finally, we announce an online repository for our Kepler data and followup spectroscopy, which we collect at http://k2wd.org.
 Publication:

The Astrophysical Journal Supplement Series
 Pub Date:
 October 2017
 DOI:
 10.3847/15384365/aa8bb5
 arXiv:
 arXiv:1709.07004
 Bibcode:
 2017ApJS..232...23H
 Keywords:

 stars: oscillations;
 stars: variables: general;
 white dwarfs;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 33 pages, 31 figures, 5 tables