Asteroseismology of RR Lyrae stars with non-radial modes

Abstract

ABSTRACT The additional signals observed in the frequency spectra of the first-overtone RR Lyrae stars, which form a period ratio around 0.61 with the period of the first overtone, are a common phenomenon for RRc and RRd stars, as well as for first-overtone classical Cepheids. The recently proposed model explains these signals as harmonics of non-radial modes of degrees 8 or 9 in the case of RR Lyrae stars and 7, 8, or 9 in the case of classical Cepheids. We selected at least triple-mode RR Lyrae stars pulsating in radial and non-radial modes for asteroseismic modelling. We assume the identification of the non-radial modes as predicted by the model. We calculated a dense grid of models for RR Lyrae stars using envelope pulsation code. By matching first-overtone period and period ratios, we obtained physical parameters for the selected sample of triple-mode stars. It is the very first attempt of modelling RR Lyrae stars with non-radial modes. We compared our results with predictions of stellar evolution theory, which resulted in a mass discrepancy more noticeable for long-period stars: Pulsation masses seem higher than evolutionary masses. We compared metallicity estimates for RRc stars from a modelled sample with metallicities determined spectroscopically for a sample of RRc stars in the solar neighbourhood: Both distributions are consistent.