The Occurrence of Non-Pulsating Stars in the gamma Dor and delta Set Pulsation Instability Regions: Results from Kepler Quarter 14-17 Data

Abstract

Abstract In our October 2013 Astronomical Review article, we discussed the statistics of variability for 633 relatively faint (Kepler magnitude 14-16) spectral type A-F stars observed by the Kepler spacecraft as part of the Guest Observer program during Quarters 6-13 (June 2010-June 2012). We found six stars that showed no variability with with amplitude 20 ppm level or greater in the range 0.2 to 24.4 cycles/day, but whose positions in the log g – Teff plane put them in the delta Set or gamma Dor pulsation instability regions established from pre-Kepler groundbased observations. Here we present results for an additional 2100+ stars observed during Quarters 14-17 (June 2012-December 2013), until the second of Kepler’s four reaction-wheels failed, ending Kepler’s planet-search mission in the Cygnus-Lyra region. This sample is not unbiased, as we limited our target list to stars showing variability in Quarter 0 full-frame images to enhance our variable star discovery rate. We find that 46% of these stars show no frequencies in the Fourier transform of their light curves down to the 20-ppm level. We find 35 additional stars that lie within the ground-based gamma Dor /delta Set pulsation instability regions; their lack of pulsations requires explanation. In the analysis for our first paper, we included a +229 K offset to the Kepler Input Catalog Teff to take into account an average systematic difference between the KIC values and the Teff derived from SDSS color photometry for main-sequence F stars (Pinsonneault et al. 2012). Here we compare the KIC catalog Teff value and the Teff derived from spectroscopy taken by the LAMOST instrument (Molenda-Zakowicz et al. 2013) for 51 stars in common to determine whether the 229 K offset should be applied to our target stars, and the implications for the location of the constant stars relative to the instability region boundaries. Finally, we also review possible explanations for the constant stars that lie within the pulsation instability regions, and consider whether super-Nyquist frequencies (>24.4 cycles/day), if they existed, would have been detected in the long-cadence data via their reflections at low frequencies, as discussed by Murphy et al. (2013).