The occurrence of non-pulsating stars in the γ Dor and δ Sct pulsation instability regions: results from Kepler quarter 14–17 data

The high precision long time-series photometry of the NASA Kepler spacecraft provides an excellent means to discover and characterize variability in main-sequence stars, and to make progress in interpreting the pulsations to derive stellar interior structure and test stellar models. For stars of spectral types A–F, the Kepler data revealed a number of surprises, such as more hybrid pulsating Sct and Dor pulsators than expected, pulsators lying outside of the instability regions predicted by theory, and stars that were expected to pulsate, but showed no variability. In our 2013 Astronomical Review article, we discussed the statistics of variability for 633 faint (Kepler magnitude 14–16) spectral type A–F stars observed by Kepler during Quarters 6–13 (June 2010–June 2012). We found six stars that showed no variability with amplitude 20 ppm or greater in the range 0.2–24.4 cycles/day, but whose positions in the log diagram place them in the Sct or Dor pulsation instability regions established from pre-Kepler ground-based observations. Here, we present results for an additional 2137 stars observed during Quarters 14–17 (June 2012–May 2013). 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 990 stars, or 46%, show no frequencies in the Fourier transform of their light curves down to the 20-ppm level, a smaller percentage than the 60% of our Q6–13 sample. We find 34 additional stars that lie within the ground-based Dor/ Sct pulsation instability regions; their lack of pulsations requires explanation. In the analysis for our first paper, we included a K offset to the Kepler Input Catalog to take into account an average systematic difference between the KIC values and the derived from SDSS color photometry for main-sequence F stars. We compare the KIC value and the derived from spectroscopy taken by the LAMOST instrument for 54 stars common to both samples. We find no trend to support applying this offset; the trend instead shows that a small average temperature decrease relative to the KIC may be more appropriate for the stars in our spectral-type range. If the 229 K offset is omitted, only 17 of our 34 “constant” stars fall within the pulsation instability regions. The comparisons with LAMOST-derived log also show that the KIC log may be too large for stars with KIC log values . For the two “constant” stars in the instability region that were also observed by LAMOST, the LAMOST values are cooler than the KIC by several hundred K, and would move these stars out of the instability regions. It is possible that a more accurate determination of their and log would move some of the other “constant” stars outside of the instability regions. However, if average (random) errors in ( K) and an offset in log from the KIC values are taken into account, 15–52 stars still persist within the instability regions. Explanations for these “constant” stars, both theoretical and observational, remain to be investigated.