Gaia-derived luminosities of Kepler A/F stars and the pulsator fraction across the δ Scuti instability strip

We study the fraction of stars in and around the delta Scuti instability strip that are pulsating, using Gaia DR2 parallaxes to derive precise luminosities. We classify a sample of over 15 000 Kepler A and F stars into delta Sct and non-delta Sct stars, paying close attention to variability that could have other origins. We find that 18 per cent of the delta Sct stars have their dominant frequency above the Kepler long-cadence Nyquist frequency (periods < 1 h), and 30 per cent have some super-Nyquist variability. We analyse the pulsator fraction as a function of effective temperature and luminosity, finding that many stars in the delta Sct instability strip do not pulsate. The pulsator fraction peaks at just over 70 per cent in the middle of the instability strip. The results are insensitive to the amplitude threshold used to identify the pulsators. We define a new empirical instability strip based on the observed pulsator fraction that is systematically hotter than theoretical strips currently in use. The stellar temperatures, luminosities, and pulsation classifications are provided in an online catalogue.