Variable stars in local group galaxies - IV. RR Lyrae stars in the central regions of the low-density galaxy Crater II

Description

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∼0.44 deg²), 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 (μ = 20.30 ± 0.08 mag (σ = 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 σ _{[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.

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