Variable stars in sagittarius globular clusters. I. Arp 2

Abstract

We present new photometry and analysis of the twelve variable stars (nine RR Lyrae (RRL), three SX Phoenicis (SX Phe)) belonging to the Sagittarius globular cluster Arp 2. Of the nine RRL stars in the cluster, eight are RRab and one is RRc. From the RRab stars, we determined a mean period of 〈Pab〉 = 0.581 ± 0.047 days, where the error is the standard error of the mean. This places Arp 2 at the border between the Oosterhoff I and Oosterhoff-Int clusters. Using the V-band data from the RRL stars, a distance modulus of (m -M)0 = 17.24 ± 0.17 was determined. From the I-band data, we found (m -M)0 = 17.34 ± 0.07. We also used the SX Phe variables to determine a distance modulus of (m -M)0 = 17.27 ± 0.04. Color excesses were determined from the RRL light curves using both the (B - V) and (V - I) colors. The mean reddening values were in line with or were a little higher than those found in the literature. Both methods indicated star-to-star variability in the reddening toward Arp 2. Of the nine RRL stars, seven were flagged as variables by Gaia, with three having periods determined. We used the Gaia data to investigate the membership of the seven Gaia RRL. Although Arp 2 is too distant for reliable Gaia parallaxes, the current data do not exclude any of the variables discussed in this paper from being members of Arp 2.