The pulsation spectra of hot subdwarf stars in the core-He burning stage

Abstract

Studying pulsation spectra through asteroseismology allows probing the inner structure of stars. Core helium burning stars, specifically subdwarf type B stars, have been observed to harbour pressure and gravity mode pulsations, allowing respectively to probe the envelope and core of such stars. In particular, observations show a dichotomy in gravity modes pulsation spectra, with sometimes structures called trapped modes, which induce variable period spacings between observed periods, and other times no trapped modes at all, with a rather smooth pulsation spectra showing more or less constant spacings instead. In this master thesis, we model subdwarf type B stars with both 4th generation static models and evolutionary models, using the STELUM and PULSE codes. Through this, we aim to gain insights on the influence of core helium burning on the pulsation spectra. We highlight as well the influence of the chemical and thermal structures on the behavior of pulsation spectra, in particular the mass of the core and envelope, as well as the thermal gradients prescriptions. A clear distinction is made between evolutionary and static models. The latter are studied first, and we discuss the origin of trapped modes from chemical transitions and temperature gradients in such models. In evolutionary models, we focus on the overshooting and semi-convection phenomena, which are not found in static models, and study their impact on the chemical and thermal structure of the star, as well as on the pulsation spectra. This master thesis gives the theoretical basis of pulsation spectra computed from current available stellar models of subdwarf B stars, now to be compared in detail with observations of such stars, in particular those observed by the Kepler and TESS satellites.