RR Lyrae Variable Stars in the Crater II Dwarf Galaxy

TL;DR

This study identifies and analyzes RR Lyrae stars in the Crater II dwarf galaxy, revealing its classification as an Oosterhoff-intermediate type and providing insights into its stellar population and relation to the Milky Way.

Contribution

First detailed detection and characterization of RR Lyrae stars in Crater II, establishing its Oosterhoff classification and stellar population properties.

Findings

Crater II hosts 83 ab-type and 13 c-type RR Lyrae stars.

Crater II is classified as Oosterhoff-intermediate based on RR Lyrae properties.

Crater II's stellar population is old but relatively younger than the MW's oldest globular clusters.

Abstract

We report the detection of RR Lyrae variable stars in Crater II, a recently discovered large and diffuse satellite dwarf galaxy of the Milky Way (MW). Based on B, V time-series photometry obtained with the Korea Microlensing Telescope Network (KMTNet) 1.6-m telescope at CTIO, we identified 83 ab-type and 13 c-type pulsators by fitting template light curves. The detected RR Lyrae stars are centrally concentrated, which ensures that most of them are members of Crater II. In terms of the distribution of RRab stars in the period-amplitude diagram, Crater II is clearly different from ultra-faint dwarf (UFD) galaxies, but very similar to the two classical MW dwarf spheroidal (dSph) galaxies Draco and Carina with Oosterhoff-intermediate (Oo-int) properties. Combined with the mean period of ab-type variables (<Pab> = 0.631+-0.004 d) and the c-type fraction (~0.14) in Crater II, this suggests an Oo-int classification for Crater II and implies that its nature is more like a dSph rather than a UFD. We also estimated the mean metallicity, reddening, and distance of Crater II, from the photometric and pulsation properties of the RR Lyrae stars. The stellar population model we have constructed indicates that Crater II is dominated by an old population, but is relatively younger than the oldest globular clusters in the MW. With a lack of high-amplitude short-period RRab stars, Crater II, like most of the other less massive dSphs, is probably not a surviving counterpart of the major building blocks of the MW halo.