Short period variability in the globular cluster NGC 419 and the SMC field

Abstract

Delta Scuti ($\hat{I}´$ Sct) stars have been extensively studied in our Galaxy, but far less in extragalactic systems. Here we study the population of $\hat{I}´$ Sct variables in NGC 419, an intermediate-age globular cluster of the Small Magellanic Cloud (SMC), using $g,r,i$ Gemini-S/GMOS time series observations. Our goal is to study the role of such variables in the cluster extended main-sequence turnoff (MSTO). We report the discovery of 54 $\hat{I}´$ Sct stars and three eclipsing binaries in the NGC 419 field. We find only a handful of the $\hat{I}´$ Sct stars at the MSTO of NGC 419 while the majority is fainter, indicating that the cluster is younger ($\lesssim 1.2$ Gyr) than previously thought. Considering their radial distribution, we identify only six $\hat{I}´$ Sct stars as probable members of NGC 419 while the 48 remaining are likely $\hat{I}´$ Sct stars of the SMC field. Cluster $\hat{I}´$ Sct stars appear close to the red edge of the MSTO, supporting the idea that the extended MSTO has its origin in an age spread. The 48 field $\hat{I}´$ Sct stars represent the largest detection of $\hat{I}´$ Sct stars made in the SMC. The period distribution of these newly detected $\hat{I}´$ Sct stars ($0.04\lesssim P\lesssim 0.15$ d) is similar to that detected in other systems. The amplitude distribution ($0.05\lesssim \hat{I}”r\lesssim 0.60$ mag) is likely biased because of the lack of low-amplitude stars. We finally use the $\hat{I}´$ Sct stars to calculate distances using different period-luminosity relations. The average distance moduli obtained are $18.76\pm 0.14$ mag for NGC 419 and $18.86\pm 0.11$ mag for the SMC field, which agree with previous measurements.

23 pages, 7 figures, 5 tables. Accepted for publication in AJ