Variable Stars in Local Group Galaxies. IV. RR Lyrae stars in the central regions of the low-density galaxy Crater II

TL;DR

This study identifies and analyzes 34 RR Lyrae stars in the low-density galaxy Crater II, determining its distance, metallicity, and stellar population characteristics using optical data from the Isaac Newton Telescope.

Contribution

First detailed analysis of RR Lyrae stars in Crater II, providing new distance and metallicity estimates and insights into its stellar population.

Findings

Distance modulus of 20.30 mag for Crater II

Mean metallicity of [Fe/H] = -1.64 with low dispersion

Notable lack of blue horizontal branch stars

Abstract

We present a search and analysis of variable stars in the recently discovered Crater~II dwarf galaxy. Based on $B$, $V$, $I$ data collected with the Isaac Newton Telescope (FoV$\sim$0.44 square degrees) we detected 37 variable stars, of which 34 are bone-fide RR Lyrae stars of Crater~II (28 RRab, 4 RRc, 2 RRd). We applied the metal-independent ($V$, $B-V$) Period--Wesenheit relation and derived a true distance modulus ($\mu$ = 20.30$\pm$0.08 mag ($\sigma$=0.16 mag). Individual metallicities for RR Lyrae stars were derived by inversion of the predicted $I$-band Period-Luminosity relation. We find a mean metallicity of [Fe/H]=-1.64 and a standard deviation of $\sigma_{[Fe/H]}$ =0.21 dex, compatible with either negligible or vanishing intrinsic metallicity dispersion. The analysis of the Colour-Magnitude Diagram reveals a stark paucity of blue horizontal branch stars, at odds with other Galactic dwarfs, and globular clusters with similar metal abundances.