FSR 1716: A new Milky Way Globular Cluster confirmed using VVV RR Lyrae stars

Abstract

We use deep multi-epoch near-IR images of the VISTA Variables in the Vía Láctea (VVV) Survey to search for RR Lyrae stars towards the Southern Galactic plane. Here we report the discovery of a group of RR Lyrae stars close together in VVV tile d025. Inspection of the VVV images and PSF photometry reveals that most of these stars are likely to belong to a globular cluster, that matches the position of the previously known star cluster FSR\,1716. The stellar density map of the field yields a $>100$ sigma detection for this candidate globular cluster, that is centered at equatorial coordinates $R{A}_{J2000}=$16:10:30.0, $DE{C}_{J2000}=-$53:44:56; and galactic coordinates $l=$329.77812, $b=-$1.59227. The color-magnitude diagram of this object reveals a well populated red giant branch, with a prominent red clump at $K_s=13.35\pm 0.05$, and $J-K_s=1.30\pm 0.05$. We present the cluster RR Lyrae positions, magnitudes, colors, periods and amplitudes. The presence of RR Lyrae indicates an old globular cluster, with age $>10$ Gyr. We classify this object as an Oosterhoff type I globular cluster, based on the mean period of its RR Lyrae type ab, $<P>=0.540$ days, and argue that this is a relatively metal-poor cluster with $[Fe/H]=-1.5\pm 0.4$ dex. The mean extinction and reddening for this cluster are $A_{K_s}=0.38\pm 0.02$, and $E(J-K_s)=0.72\pm 0.02$ mag, respectively, as measured from the RR Lyrae colors and the near-IR color-magnitude diagram. We also measure the cluster distance using the RR Lyrae type ab stars. The cluster mean distance modulus is $(m-M{)}_0=14.38\pm 0.03$ mag, implying a distance $D=7.5\pm 0.2$ kpc, and a Galactocentric distance $R_G=4.3$ kpc.

7 pages, 4 figures. ApJL, in press