Spectrum analysis of bright Kepler late B- to early F-stars

摘要

The Kepler satellite mission was designed to search for transiting exoplanets and delivers single band-pass light curves of a huge number of stars observed in the Cygnus Lyra region. At the same time, it opens a new window for asteroseismology. In order to accomplish one of the required preconditions for the asteroseismic modelling of the stars, namely knowledge of their precise fundamental parameters, ground-based spectroscopic and/or photometric follow-up observations are needed. We aim to derive fundamental parameters and individual abundances for a sample of 18 γ Dor/δ Sct and 8 slowly pulsating B (SPB)/β 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 like Teff, log g, vsini, ξ, [M/H] and individual abundances with high accuracy. 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 most A- and F-type stars, the derived Teff values agree within the measurement errors with the values given in the Kepler Input Catalog (KIC). 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 spectral energy distribution fitting. According to their position in the log (Teff)-log (g) diagram, two stars are expected γ Dor stars, four stars are expected δ Sct stars and four stars are possibly δ Sct 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 δ Sct instability region show γ Dor-type oscillation in their light curves implying that γ Dor-like oscillations are much more common among the δ Sct stars than what is theoretically expected. Moreover, one of the stars located at the hot border of the δ Sct instability strip is classified as δ Sct-γ Dor hybrid pulsator from its light-curve analysis. Given that these findings are fully consistent with recent investigations, we conclude that a revision of the γ Dor and δ Sct instability strips is essential.