DA White Dwarfs in the Kepler Field

TL;DR

This study identifies and characterizes 23 DA white dwarfs in the Kepler field, confirming their pulsation status and refining the empirical instability strip through spectroscopy and Kepler photometry.

Contribution

The paper reports 16 new DA white dwarfs, confirms 7 previous ones, and analyzes their pulsation properties to better define the ZZ Ceti instability strip.

Findings

12 of 23 WDs are outside the instability strip and are non-pulsating.

7 WDs within the strip show no pulsation in Kepler data.

4 WDs in the strip lack Kepler photometry, requiring ground-based follow-up.

Abstract

We present 16 new, and confirm 7 previously identified, DA white dwarfs in the Kepler field through ground-based spectroscopy with the Hale 200", Kitt Peak 4-meter, and Bok 2.3-meter telescopes. Using atmospheric models we determine their effective temperatures and surface gravities to constrain their position with respect to the ZZ Ceti (DA pulsator) instability strip, and look for the presence or absence of pulsation with Kepler's unprecedented photometry. Our results are as follows: i) From our measurements of temperature and surface gravity, 12 of the 23 DA white dwarfs from this work fall well outside of the instability strip. The Kepler photometry available for 11 of these WDs allows us to confirm that none are pulsating. One of these eleven happens to be a presumed binary, KIC 11604781, with a period of ~5 days. ii) The remaining 11 DA white dwarfs are instability strip candidates, potentially falling within the current, empirical instability strip, after accounting for uncertainties. These WDs will help constrain the strip's location further, as eight are near the blue edge and three are near the red edge of the instability strip. Four of these WDs do not have Kepler photometry, so ground-based photometry is needed to determine the pulsation nature of these white dwarfs. The remaining seven have Kepler photometry available, but do not show any periodicity on typical WD pulsation timescales.