On a new theoretical framework for RR Lyrae stars I: the metallicity dependence

TL;DR

This paper introduces a new theoretical framework for RR Lyrae stars, exploring how their pulsation properties depend on metallicity through advanced hydrodynamical models and deriving new relations for their observable characteristics.

Contribution

It presents comprehensive nonlinear hydrodynamical models of RR Lyrae stars across various metallicities, providing new analytical relations for instability strip edges, pulsation masses, and Period-Luminosity relations.

Findings

The topology of the instability strip varies with metallicity.

New analytical relations for pulsation mass and Period-Radius-Metallicity are established.

Derived Period-Luminosity and Period-Wesenheit-Metallicity relations are consistent with existing data.

Abstract

We present new nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z=0.0001--0.02). The stellar masses and luminosities adopted to construct the pulsation models were fixed according to detailed central He burning Horizontal Branch evolutionary models. The pulsation models cover a broad range in stellar luminosity and effective temperatures and the modal stability is investigated for both fundamental and first overtones. We predict the topology of the instability strip as a function of the metal content and new analytical relations for the edges of the instability strip in the observational plane. Moreover, a new analytical relation to constrain the pulsation mass of double pulsators as a function of the period ratio and the metal content is provided. We derive new Period-Radius-Metallicity relations for fundamental and first-overtone pulsators. They agree quite well with similar empirical and theoretical relations in the literature. From the predicted bolometric light curves, transformed into optical (UBVRI) and near-infrared (JHK) bands, we compute the intensity-averaged mean magnitudes along the entire pulsation cycle and, in turn, new and homogenous metal-dependent (RIJHK) Period-Luminosity relations. Moreover, we compute new dual and triple band optical, optical--NIR and NIR Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical Period-W(V,B-V) is independent of the metal content and that the accuracy of individual distances is a balance between the adopted diagnostics and the precision of photometric and spectroscopic datasets.