Distinguishing Core and Shell Helium-burning Subdwarf B Stars by Asteroseismology

Abstract

Abstract Subdwarf B stars are generally considered to be core helium-burning stars. However, most canonical sdB models have shown that they leave the sdB band (i.e., the region with 20,000 K ≤ T eff ≤ 40,000 K and ) after they have ignited helium in the shell. This means there will be a few shell helium-burning stars that have exhausted their helium in the core yet may still be located on the sdB band and should be considered as sdB stars too. But because they have effective temperature and gravity close to those of core helium-burning sdB stars, we cannot distinguish the two kinds of star just by atmospheric parameters. Some sdB stars display multimode pulsations. We thought that asteroseismology could offer some methods to distinguish the two kinds of sdB star. Core and shell helium-burning sdB stars have some obvious differences in structure and evolutionary track, which results in some clear differences in their pulsation properties. So, we may distinguish them by their pulsation properties such as the rates of change of period of the p-mode sdB pulsators, the numbers of mixed modes, and the period spacings of the g-mode sdB pulsators. We suggest that there are two special sdB stars (V391 Pegasi and PG 1605+072) that are likely shell helium-burning sdB stars.