Characterising the observational properties of δ Sct stars in the era of space photometry from the Kepler mission

Abstract

The δ Sct stars are a diverse group of intermediate-mass pulsating stars located on and near the main sequence within the classical instability strip in the Hertzsprung–Russell diagram. Many of these stars are hybrid stars pulsating simultaneously with pressure and gravity modes that probe the physics at different depths within a star’s interior. Using two large ensembles of δ Sct stars observed by the Kepler Space Telescope, the instrumental biases inherent to Kepler mission data and the statistical properties of these stars are investigated. An important focus of this work is an analysis of the relationships between the pulsational and stellar parameters, and their distribution within the classical instability strip. It is found that a non-negligible fraction of main sequence δ Sct stars exist outside theoretical predictions of the classical instability boundaries, which indicates the necessity of a mass-dependent mixing length parameter to simultaneously explain low- and high-radial order pressure modes in δ Sct stars within the Hertzsprung–Russell diagram. Furthermore, a search for regularities in the amplitude spectra of these stars is also presented, specifically the frequency difference between pressure modes of consecutive radial order. In this work, it is demonstrated that an ensemble-based approach using space photometry from the Kepler mission is not only plausible for δ Sct stars, but that it is a valuable method for identifying the most promising stars for mode identification and asteroseismic modelling. The full scientific potential of studying δ Sct stars is as yet unrealised. The ensembles discussed in this paper represent a high-quality data set for future studies of rotation and angular momentum transport inside A and F stars using asteroseismology.