Adiabatic survey of subdwarf B star oscillations. I. Pulsation properties of a representative evolutionary model

We present the first results of a large, systematic adiabatic survey of the pulsation properties of models of subdwarf B (sdB) stars. This survey is aimed at providing the most basic theoretical data with which to analyze the asteroseismological properties of the recently discovered class of pulsating sdB stars (the EC 14026 stars). Such a theoretical framework has been lacking up to now. In this paper, the first of a series of three, an adiabatic pulsation code is used to compute, in the 80-1500 s period window, the radial (l = 0) and nonradial (from l = 1 up to l = 3) oscillation modes for a representative evolutionary model of subdwarf B stars. Quantities such as the periods, kinetic energies, first-order rotational splitting coefficients, eigenfunctions, and weight functions are given by the code, providing a complete set of very useful diagnostic tools with which to study the mode properties. The main goal is to determine how these quantities relate to the internal structure of B subdwarfs, a crucial and necessary step if one wants to eventually apply the tools of asteroseismology to EC 14026 stars. All modes (p, f, and g) were considered in order to build the most complete picture we can have on pulsations in these stars. In that context, we show that 9-modes are essentially deep interior modes oscillating mainly in the radiative helium-rich core (but not in the convective nucleus), while p-modes are shallower envelope modes. We demonstrate that g-modes respond to a trapping/confinement phenomenon induced mainly by the He/H chemical transition between the H-rich envelope and the He-rich core of subdwarf B stars. This phenomenon is very similar in nature to the g-mode trapping and confinement mechanisms observed in pulsating white dwarf models. We emphasize that p-modes may also experience distortions of their period distribution due to this He/H transition, although these are not as pronounced as in the g-mode case. These phenomena are of great interest as they can potentially provide powerful tools for probing the internal structure of these objects, in particular, with respect to constraining the mass of their H-rich envelope. The results given in this first paper form the minimal background on pulsation mode characteristics in sdB stars. Upcoming discussions on additional mode properties in subdwarf B star models (Paper II and Paper III of this series) will strongly rely on these basic results since they provide essential guidance in understanding mode period behaviors as functions of B subdwarf stellar parameters and/or evolution.