Spectrum Analysis of Bright Kepler late B- to early F- Stars

TL;DR

This study uses high-resolution spectroscopy to analyze the fundamental parameters of Kepler-observed variable stars, revealing discrepancies in temperature estimates and suggesting a need to revise the instability strips for Gamma Dor and Delta Sct stars.

Contribution

It provides new spectroscopic classifications and insights into the pulsation characteristics of variable stars in the Kepler field, highlighting the prevalence of GD-like oscillations among DSct stars.

Findings

Hot star temperatures are underestimated by KIC.

GD-like oscillations are common among DSct stars.

Revision of GD and DSct instability strips is needed.

Abstract

The Kepler satellite mission delivers single band-pass light curves of a huge number of stars observed in the Cygnus-Lyra region opening a new window for asteroseismology. In order to accomplish one of the preconditions for the asteroseismic modelling of the stars, we aim to derive fundamental parameters and individual abundances for a sample of 18 Gamma Dor (GD)/Delta Sct (DSct) and 8 SPB/beta Cep candidate stars in the Kepler satellite field of view. We use the spectral synthesis method to model newly obtained, high-resolution spectra of 26 stars in order to derive their fundamental parameters and individual abundances. The stars are then placed into the log(Teff)-log(g) diagram and the obtained spectroscopic classification is compared to the existing photometric one. For hot stars, the KIC temperatures appear to be systematically underestimated, in agreement with previous findings. We also find that the temperatures derived from our spectra agree reasonably well with those derived from the SED fitting. According to their position in the log(Teff)-log(g) diagram, two stars are expected GD stars, four stars are expected DSct stars, and four stars are possibly DSct stars at the blue edge of the instability strip. Two stars are confirmed SPB variables, and one star falls into the SPB instability region but its parameters might be biased by binarity. Two of the four stars that fall into the DSct instability region show GD-type oscillation in their light curves implying that GD-like oscillations are much more common among the DSct stars than is theoretically expected. Moreover, one of the stars located at the hot border of the DSct instability strip is classified as DSct-GD hybrid pulsator from its light curve analysis. Given that these findings are fully consistent with recent investigations, we conclude that a revision of the GD and DSct instability strips is essential.