RR Lyrae Variables in the Crater II Dwarf Galaxy

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 1.6 m telescope at Cerro Tololo Inter-American Observatory, 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 (< P-ab > = 0.631 +/- 0.004 days) and the c-type fraction (similar to 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.