A spectroscopic study of the hybrid pulsator Gamma Pegasi

Abstract

The recent detection of both pressure and high-order gravity modes in the clas- sical B-type pulsator γ Pegasi offers promising prospects for probing its internal structure through seismic studies. To aid further modelling of this star, we present the results of a detailed NLTE abundance analysis based on a large number of time-resolved, high-quality spectra. A chemical composition typical of nearby B-type stars is found. The hybrid nature of this star is consistent with its location in the overlapping region of the instability strips for β Cephei and slowly pulsating B stars computed using OP opacity tables, although OPAL cal- culations may also be compatible with the observations once the uncertainties in the stellar parameters and the current limitations of the stability calcula- tions are taken into account. The two known frequencies f1 = 6.58974 and f2 = 0.68241 d−1 are detected in the spectroscopic time series. A mode iden- tification is attempted for the low-frequency signal, which can be associated to a high-order g-mode. Finally, we re-assess the binary status of γ Peg and find no evidence for variations that can be ascribed to orbital motion, contrary to previous claims in the literature.