Theoretical Modeling of the RR Lyrae Variables in NGC 1851

TL;DR

This paper presents a theoretical model explaining the pulsational properties of RR Lyrae stars in NGC 1851, linking their periods and amplitudes to minor helium variations within the cluster.

Contribution

The study introduces a model that reproduces RR Lyrae properties by considering a helium-enriched stellar component, suggesting these variables belong to a second-generation star population.

Findings

Helium variations explain RR Lyrae period differences.

RR Lyrae stars with higher helium resemble OoII variables.

The model aligns observed pulsational properties with stellar composition.

Abstract

The RR Lyrae instability strip (IS) in NGC 1851 is investigated, and a model is presented which reproduces the pulsational properties of the RR Lyrae population. In our model, a stellar component within the IS that displays minor helium variations (Y~0.248-0.270) is able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae variables. The RR Lyrae variables therefore may belong to an He-enriched second generation of stars. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables.