A DECam view of the diffuse dwarf galaxy Crater II – Variable 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 ± 0.004 (stat) ±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 • . 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.

Identifying RR Lyrae Variable Stars in Six Years of the Dark Energy Survey

“…Thus, we train another random forest classifier to select potential RRL. We input the RSS, amplitudes, and von Mises-Fisher concentration param- Fabrizio et al (2019) and scaled to the g band by Vivas et al (2020b) (see their Fig. 7).…”

Zwicky Transient Facility and Globular Clusters: the Period–Luminosity and Period–Luminosity–Color Relations for Late-type Contact Binaries

“…Recently, Vivas et al (2020) conducted time-series observations in gi-band on a dwarf galaxy Crater II using the DECam installed on the 4m Blanco Telescope. These authors found 14 eclipsing binaries within the survey area.…”

“…Visual inspecting the light curves of these eclipsing binaries revealed 6 of them (V118, V119, V120, V121, V124 & V126) have CB-like light curves. As illustrated in Figure 15, the reported periods in Vivas et al (2020) need to be doubled for V119, V121, V124 & V126, because CB light curves are expected to exhibit two minima with nearly equal depth. Since the photometry of these DECam observations were calibrated to the SDSS (Sloan Digital Sky Survey) system (Vivas et al 2020), we converted the mean g-band magnitudes for these six CB to the Pan-STARRS1 (PS1) system via the transformation equations provided in Abbott et al (2021).…”

“…As illustrated in Figure 15, the reported periods in Vivas et al (2020) need to be doubled for V119, V121, V124 & V126, because CB light curves are expected to exhibit two minima with nearly equal depth. Since the photometry of these DECam observations were calibrated to the SDSS (Sloan Digital Sky Survey) system (Vivas et al 2020), we converted the mean g-band magnitudes for these six CB to the Pan-STARRS1 (PS1) system via the transformation equations provided in Abbott et al (2021). We did not convert the corresponding mean i-band magnitudes because Vivas et al (2020) observations did not include the r-band, hence no (r − i) colors available to perform such transformation.…”

“…Since the photometry of these DECam observations were calibrated to the SDSS (Sloan Digital Sky Survey) system (Vivas et al 2020), we converted the mean g-band magnitudes for these six CB to the Pan-STARRS1 (PS1) system via the transformation equations provided in Abbott et al (2021). We did not convert the corresponding mean i-band magnitudes because Vivas et al (2020) observations did not include the r-band, hence no (r − i) colors available to perform such transformation. Assuming these eclipsing binaries are late-type CB, using the extinction provided by Vivas et al (2020) and the gband PL relation given in equation ( 19) we found that the distance moduli of these 6 CB ranged from 11.28 mag to 13.48 mag (with errors of ∼ 0.45 mag, dominated by the dispersion of the g-band PL relation), which are much closer than the distance modulus of Crater II (20.33 ± 0.07 mag, Vivas et al 2020).…”