RR Lyrae the Stellar Beacons of the Galactic Structure

TL;DR

This paper reviews recent advances in using RR Lyrae stars as distance indicators and stellar tracers, focusing on their pulsation properties, the Period-Luminosity relation, and helium content estimation methods.

Contribution

It provides a comprehensive overview of theoretical and observational findings on RR Lyrae stars, including their pulsation models, PL relation dependence on metallicity, and helium content diagnostics.

Findings

RR Lyrae pulsation properties are minimally affected by helium content.

The K-band Period-Luminosity slope shows mild metallicity dependence.

RR Lyrae stars can be used to estimate helium content via specific parameters.

Abstract

We present some recent findings concerning the use of RR Lyrae as distance indicators and stellar tracers. We outline pros and cons of field and cluster RR Lyrae stars and discuss recent theoretical findings concerning the use of the Bailey (amplitude vs pulsation period) diagram to constrain the possible occurrence of Helium enhanced RR Lyrae stars. Nonlinear, convective RR Lyrae models indicate that the pulsation properties of RR Lyrae stars are minimally affected by the helium content. The main difference between canonical and He enhanced models is due to the increase in luminosity predicted by evolutionary models. Moreover, we focus our attention on the near-infrared Period-Luminosity (PL) relation of RR Lyrae and summarize observational evidence concerning the slope of the K-band PL relation in a few globulars (M92, Reticulum, M5, Omega Cen) covering a range in metallicity of ~1 dex. Current findings suggest that the slope has a mild dependence on the metal content when moving from the metal-poor to the metal-intermediate regime. Finally, we also discuss the use of RR Lyrae stars either to estimate (helium indicator: A-parameter) or to measure (absorption and emission lines) the helium content.