What asteroseismology can teach us about stellar evolution: the case of subdwarf B stars

Subdwarfs B (sdB) stars are hot (Teff=20,000-40,000 K) and compact (log g= 5.0-6.2) evolved objects that form the very hot end of the horizontal branch, the so-called Extreme Horizontal Branch (EHB). Understanding the formation of sdB stars is one of the remaining challenges of stellar evolution theory. Competing scenarios have been proposed to account for the existence of such evolved objects, and give quite different mass distributions for resulting sdB stars.

Detailed asteroseismic analyses, including mass estimates, of 15 pulsating hot B subdwarfs have been published since a decade. The masses have also been reliably determined by light curve modeling and spectroscopy for 7 sdB components of eclipsing or reflection binaries. I will present in the talk the empirical mass distributions of sdB stars on the basis of these samples.

I will discuss how these empirical mass distributions, although still based on small-number statistics, compare with the expectations of stellar evolution theory. In particular, the two He-white dwarfs merger scenario does not seem to be the dominant channel to form isolated sdB stars, while the post-red giant branch scenario is reinforced. This opens new questions on the extreme mass loss of red giants to form extreme horizontal branch stars, possibly in connection with the recently discovered close planets orbiting sdB stars.