Variabilities and pulsations in the B supergiant Epsilon Ori (HD 37128)

Abstract

High precision time series photometry from space telescopes together with high-resolution spectroscopy has opened up new prospects in the study of massive B-type stars. These observations have revealed significant and diverse variabilities in the class of B supergiants. The origin of these variabilities is not fully understood but some of them may be associated with stellar pulsation. HD37128 (Epsilon Orionis) is a highly luminous B-type supergiant located in the constellation of Orion. The variability of this star has been most extensively studied in the optical domain. In several spectroscopic studies, the star has been observed to exhibit significant line profile variations. These variations, which occur in both photospheric and wind lines, have periods of the order of a few days and bear no obvious connection with the rotational modulation. In an attempt to understand the origin of these variabilities, we have constructed models of HD 37128 using the observed surface temperature and luminosity and performed non-adiabatic linear stability analysis. Several low-order radial and non-radial modes are found to be unstable in the considered models. Mode interactions are present in models having mass below 35 solar mass which is an indication of the presence of strange mode instabilities. To understand the final fate of the instabilities, nonlinear simulations have been carried out for selected models. It is found to lead to finite amplitude pulsations with periods which are consistent with the observed values, thus allowing a possible interpretation of the variabilities in terms of stellar pulsation.