A DECam View of the Diffuse Dwarf Galaxy Crater II: Variable Stars

TL;DR

This study used DECam time series data to identify variable stars in Crater II, deriving its distance, shape, and metallicity distribution, and providing insights into its stellar populations and evolution.

Contribution

It presents the first detailed variable star analysis of Crater II, including distance measurement, shape, and metallicity insights, based on extensive RR Lyrae and other variable star data.

Findings

Distance modulus of (m-M)_0=20.333±0.004 (stat) ± 0.07 (sys)

Elliptical shape with ellipticity 0.24 and position angle 153°

Small metallicity dispersion of 0.17 dex among old stars

Abstract

Time series observations of a single dithered field centered on the diffuse dwarf satellite galaxy Crater II were obtained with the Dark Energy Camera (DECam) at the 4m Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile, uniformly covering up to two half-light radii. Analysis of the $g$ and $i$ time series results in the identification and characterization of 130 periodic variable stars, including 98 RR Lyrae stars, 7 anomalous Cepheids, and 1 SX Phoenicis star belonging to the Crater II population, and 24 foreground variables of different types. Using the large number of ab-type RR Lyrae stars present in the galaxy, we obtained a distance modulus to Crater II of $(m-M)_0=20.333\pm 0.004$ (stat) $\pm 0.07$ (sys). The distribution of the RR Lyrae stars suggests an elliptical shape for Crater II, with an ellipticity of 0.24 and a position angle of $153^\circ$. From the RR Lyrae stars we infer a small metallicity dispersion for the old population of Crater II of only 0.17 dex. There are hints that the most metal-poor stars in that narrow distribution have a wider distribution across the galaxy, while the slightly more metal rich part of the population is more centrally concentrated. Given the features in the color-magnitude diagram of Crater II, the anomalous Cepheids in this galaxy must have formed through a binary evolution channel of an old population.