The Swift/UVOT Stars Survey. II. RR Lyrae Stars in M 3 and M 15

TL;DR

This study uses the Swift/UVOT telescope to conduct the first systematic near-ultraviolet survey of RR Lyrae stars in globular clusters M 3 and M 15, revealing new insights into their properties and relationships.

Contribution

It presents the first NUV survey of RR Lyrae stars in these clusters, mapping their pulsation characteristics and establishing a period-metallicity-luminosity relation in the NUV.

Findings

Identified 280 variable stars, including 275 RR Lyrae, with 2 new discoveries.

Mapped the NUV Bailey diagram for the first time.

Showed NUV sensitivity to temperature and surface gravity of RR Lyrae.

Abstract

We present the first results of an near-ultraviolet (NUV) survey of RR Lyrae stars from the Ultraviolet Optical Telescope (UVOT) aboard the Swift Gamma-Ray Burst Mission. It is well-established that RR Lyrae have large amplitudes in the far- and near-ultraviolet. We have used UVOT's unique wide-field NUV imaging capability to perform the first systematic NUV survey of variable stars in the Galactic globular clusters M 3 and M 15. We identify 280 variable stars, comprising 275 RR Lyrae, two anomalous Cepheids, one classical Cepheid, one SX Phoenicis star and one possible long-period or irregular variable. Only two of these are new discoveries. We compare our results to previous investigations and find excellent agreement in the periods with significantly larger amplitudes in the NUV. We map out, for the first time, an NUV Bailey diagram from globular clusters, showing the usual loci for fundamental mode RRab and first overtone RRc pulsators. We show the unique sensitivity of NUV photometry to both the temperatures and the surface gravities of RR Lyrae stars. Finally, we show evidence of an NUV period-metallicity-luminosity relationship. Future investigations will further examine the dependence of NUV pulsation parameters on metallicity and Oosterhoff classification.