Asteroseismology of RR Lyrae stars with non-radial modes

TL;DR

This study models RR Lyrae stars exhibiting non-radial pulsation modes to derive their physical parameters, marking the first such asteroseismic modeling and revealing a mass discrepancy with evolutionary predictions.

Contribution

It presents the first asteroseismic modeling of RR Lyrae stars with non-radial modes, using a dense grid of models to match observed period ratios and derive stellar parameters.

Findings

Derived physical parameters for triple-mode RR Lyrae stars.

Identified a mass discrepancy between pulsation and evolutionary masses.

Confirmed metallicity estimates are consistent with spectroscopic data.

Abstract

The additional signals observed in the frequency spectra of the first-overtone RR Lyrae stars, that 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 modeling. 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 modeling 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 modeled sample with metallicities determined spectroscopically for a sample of RRc stars in the solar neighbourhood: both distributions are consistent.