HST/ACS Observations of RR Lyrae Stars in Six Ultra-deep Fields of M31

TL;DR

This study uses HST/ACS observations to identify and analyze RR Lyrae stars across six ultra-deep fields in M31, expanding the understanding of their distribution, properties, and metallicities in different galactic components.

Contribution

It provides new spatial coverage of RR Lyrae stars in M31, including in the disk and stellar stream, and compares multiple methods for deriving their properties.

Findings

Detected 55 RR Lyrae stars in the initial field at 11kpc from M31's nucleus.

Found additional RR Lyrae stars in four other fields, with varying populations.

Most RR Lyrae stars are of Oosterhoff I type, with some mixed or intermediate types.

Abstract

We present HST/ACS observations of RR Lyrae variable stars in six ultra deep fields of the Andromeda galaxy (M31), including parts of the halo, disk, and giant stellar stream. Past work on the RR Lyrae stars in M31 has focused on various aspects of the stellar populations that make up the galaxy's halo, including their distances and metallicities. This study builds upon this previous work by increasing the spatial coverage (something that has been lacking in previous studies) and by searching for these variable stars in constituents of the galaxy not yet explored. Besides the 55 RR Lyrae stars we found in our initial field located 11kpc from the galactic nucleus, we find additional RR Lyrae stars in four of the remaining five ultra deep fields as follows: 21 in the disk, 24 in the giant stellar stream, 3 in the halo field 21kpc from the galactic nucleus, and 5 in one of the halo fields at 35kpc. No RR Lyrae were found in the second halo field at 35kpc. The RR Lyrae populations of these fields appear to mostly be of Oosterhoff I type, although the 11kpc field appears to be intermediate or mixed. We will discuss the properties of these stars including period and reddening distributions. We calculate metallicities and distances for the stars in each of these fields using different methods and compare the results, to an extent that has not yet been done. We compare these methods not just on RR Lyrae stars in our M31 fields, but also on a data set of Milky Way field RR Lyrae stars.