Variable Stars in Local Group Galaxies. I: Tracing the Early Chemical Enrichment and Radial Gradients in the Sculptor dSph with RR Lyrae Stars

TL;DR

This study analyzes a large sample of RR Lyrae stars in the Sculptor dwarf galaxy to understand its early chemical enrichment and radial population gradients, providing new distance and metallicity estimates.

Contribution

It offers the largest RR Lyrae sample in Sculptor, constrains its metallicity distribution, and reveals early chemical evolution and radial gradients in stellar populations.

Findings

Distance estimate of 19.62 mag consistent with previous studies

Metallicity range from -2.3 to -1.5 dex for old stars

Radial gradients in HB morphology indicating early chemical evolution

Abstract

We identified and characterized the largest (536) RR Lyrae (RRL) sample in a Milky Way dSph satellite (Sculptor) based on optical photometry data collected over $\sim$24 years. The RRLs display a spread in V-magnitude ($\sim$0.35 mag) which appears larger than photometric errors and the horizontal branch (HB) luminosity evolution of a mono-metallic population. Using several calibrations of two different reddening free and metal independent Period-Wesenheit relations we provide a new distance estimate $\mu$=19.62 mag ($\sigma_{\mu}$=0.04 mag) that agrees well with literature estimates. We constrained the metallicity distribution of the old population, using the $M_I$ Period-Luminosity relation, and we found that it ranges from -2.3 to -1.5 dex. The current estimate is narrower than suggested by low and intermediate spectroscopy of RGBs ($\Delta$[Fe/H] $\le$ 1.5). We also investigated the HB morphology as a function of the galactocentric distance. The HB in the innermost regions is dominated by red HB stars and by RRLs, consistent with a more metal-rich population, while in the outermost regions it is dominated by blue HB stars and RRLs typical of a metal-poor population. Our results suggest that fast chemical evolution occurred in Sculptor, and that the radial gradients were in place at an early epoch.