The Potential of Asteroseismology for Hot, Subdwarf B Stars: A New Class of Pulsating Stars?

Abstract

We present key sample results of a systematic survey of the pulsation properties of models of hot B subdwarfs. We use equilibrium structures taken from detailed evolutionary sequences of solar metallicity (Z = 0.02) supplemented by grids of static envelope models of various metallicities (Z = 0.02, 0.04, 0.06, 0.08, and 0.10). We consider all pulsation modes with l = 0, 1, 2, and 3 in the 80-1500 s period window, the interval currently most suitable for fast photometric detection techniques. We establish that significant driving is often present in hot B subdwarfs and is due to an opacity bump associated with heavy-element ionization. We find that models with Z ≥ 0.04 show low radial order unstable modes; both radial and nonradial (p, f, and g) pulsations are excited. The unstable models have T_eff 30,000 K and log g 5.7, depending somewhat on the metallicity. We emphasize that metal enrichment need only occur locally in the driving region. On this basis, combined with the accepted view that local enrichments and depletions of metals are commonplace in the envelopes of hot B subdwarfs, we predict that some of these stars should show luminosity variations resulting from pulsational instabilities.