Ensemble seismic study of red-clump stars

Stars in the Red Clump (RC) are high-metallicity, low-mass, core-helium burning stars which went through the helium flash. They are astrophysically important, since they can be used as standard candles that trace the chemical evolution of the Galaxy. Yet, due to the many uncertainties associated with the physical processes that happen during the helium flash, as well as in the central regions of RC stars, modelling these stars is particularly challenging. In this work, we use asteroseismology to constrain these processes, with a focus on nuclear reactions and convection. RC stars are solar-like oscillators and exhibit mixed modes, whose period spacing is a very good probe of the properties of the region around the convective core. We performed an ensemble seismic study of the RC stars observed by Kepler and compared the observed period spacing distribution to a simulated distribution. This simulated distribution incorporates the mass as well as the metallicity distribution of the observed stars; for this we use models computed with the MESA stellar evolution code. These comparisons allow us to test different prescriptions of core boundary mixing, as well as different values of the carbon-alpha nuclear reaction rate. Notably, we find that assuming mode trapping in the semiconvective region as well as a nominal carbon-alpha nuclear reaction rate yields a period spacing distribution that is compatible with that of observations.