Gaia luminosities of pulsating A-F stars in the Kepler field

TL;DR

This study uses Gaia DR2 parallaxes to precisely define the luminosity and instability regions of δ Scuti and γ Doradus stars in the Kepler field, revealing that γ Dor stars are within the δ Sct instability strip and challenging existing models.

Contribution

It provides the first precise luminosity-based definitions of the instability strips for δ Sct and γ Dor stars, showing γ Dor stars are not a separate class and questioning current stellar pulsation theories.

Findings

γ Dor stars are within the δ Sct instability strip.

Observed instability regions do not match recent models.

Most high-amplitude δ Sct stars are normal, not transitional.

Abstract

All stars in the Kepler field brighter than 12.5 magnitude have been classified according to variability type. A catalogue of {\delta} Scuti and {\gamma} Doradus stars is presented. The problem of low frequencies in {\delta} Sct stars, which occurs in over 98 percent of these stars, is discussed. Gaia DR2 parallaxes were used to obtain precise luminosities, enabling the instability strips of the two classes of variable to be precisely defined. Surprisingly, it turns out that the instability region of the {\gamma} Dor stars is entirely within the {\delta} Sct instability strip. Thus {\gamma}Dor stars should not be considered a separate class of variable. The observed red and blue edges of the instability strip do not agree with recent model calculations. Stellar pulsation occurs in less than half of the stars in the instability region and arguments are presented to show that this cannot be explained by assuming pulsation at a level too low to be detected. Precise Gaia DR2 luminosities of high-amplitude {\delta} Sct stars (HADS) show that most of these are normal {\delta} Sct stars and not transition objects. It is argued that current ideas on A star envelopes need to be revised.