The effects of stellar population synthesis on the distributions of the asteroseismic observables νmax and Δν of red-clump stars

Abstract

The distributions of the frequency of maximum oscillation power (νmax) and the large frequency separation (Δν) of red giant stars observed by the COnvection ROtation and planetary Transits (CoRoT) mission have a dominant peak. Miglio et al. identified that these stars are red-clump stars. Using a stellar population-synthesis method, we studied the effects of Reimers mass loss, binary interactions, star-formation rate and mixing-length parameter on the distributions of νmax and Δν of red-clump stars. Reimers mass loss can result in an increase in the νmax and Δν of the old population, which has lost a considerable amount of mass. However, it leads to a small decrease in the parameters of the ‘middle-age’ population, which has lost only a little mass. Furthermore, a high mass-loss rate impedes low-mass and low-metal stars from evolving into the core-helium-burning (CHeB) stage. Both Reimers mass loss and star-formation rate affect mainly the number of CHeB stars with νmax and Δν, and barely affect the peak locations of νmax and Δν. Binary interactions can also lead to an increase or decrease in the νmax and Δν of some stars. However, the fraction of CHeB stars undergoing binary interactions is very small in our simulations. Therefore, the peak locations are also not affected by binary interactions. The non-uniform distributions of νmax and Δν are mainly caused by most of the red-clump stars having an approximate radius rather than mass. The radius of red-clump stars decreases with increasing mixing-length parameter. The peak locations of νmax and Δν can, thus, be affected by the mixing-length parameter.