# Variable Stars in Sagittarius Globular Clusters I. Arp~2

## TL;DR

This study analyzes variable stars in the Sagittarius globular cluster Arp 2, providing new photometry, distance estimates, and insights into its variable star population and reddening characteristics.

## Contribution

First detailed analysis of variable stars in Arp 2, including new photometry, distance measurements, and reddening assessment, linking Gaia data for membership evaluation.

## Key findings

- Mean RRab period of 0.581 days, placing Arp 2 between Oosterhoff I and II.
- Distance modulus estimates around 17.24 to 17.34, consistent across methods.
- Star-to-star reddening variability observed in the cluster.

## Abstract

We present new photometry and analysis of the twelve variable stars (nine RR Lyrae, three SX Phoenicis) belonging to the Sagittarius globular cluster Arp 2. Of the nine RR Lyrae stars in the cluster, eight are RRab and one is RRc. From the RRab stars, we determined a mean period of $\langle P_{ab}\rangle=0.581\pm0.047$ 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 RR Lyrae stars, a distance modulus of $(m-M)_0=17.24\pm0.17$ was determined. From the $I$-band data, we found $(m-M)_0=17.34\pm0.07$. We also used the SX Phoenicis variables to determine a distance modulus of $(m-M)_0=17.27\pm0.04$. Color excesses were determined from the RR Lyrae 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 RR Lyrae 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 RR Lyrae. Although Arp 2 is too distant for reliable Gaia parallax, the current data do not exclude any of the variables discussed in this paper from being members of Arp 2.