Asteroseismic signatures of the helium core flash

Abstract

All evolved stars of up to 2 solar masses undergo a helium core flash at the end of their first stage as a giant star. Although theoretically predicted more than 50 years ago1,2, this core flash phase has yet to be observationally probed. We show here that gravity modes stochastically excited by helium-flash-driven convection are able to reach the stellar surface and induce periodic photometric variabilities in hot subdwarf stars with amplitudes of the order of a few thousandths of a magnitude. As such, they can now be detected by space-based photometry with the Transiting Exoplanet Survey Satellite in relatively bright stars (for example, Johnson–Cousins magnitudes of IC ≲ 13 mag)3. The range of predicted periods spans from a few thousand seconds to tens of thousands of seconds, depending on the details of the excitation region. In addition, we find that stochastically excited pulsations reproduce the pulsations observed in a few helium-rich hot subdwarf stars. These stars, particularly the future Transiting Exoplanet Survey Satellite target Feige 46, are the most promising candidates to probe the helium core flash for the first time.