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 forRR Lyrae stars toward the Southern Galactic plane. Here, we report the discovery of a group of RR Lyrae starsclose together in VVV tile d025. Inspection of the VVV images and PSF photometry reveals that most of thesestars are likely to belong to a globular cluster that matches the position of the previously known star clusterFSR 1716. The stellar density map of thefield yields a>100σdetection for this candidate globular cluster that iscentered at equatorial coordinates R.A.J2000=16:10:30.0, decl.J2000=−53:44:56 and galactic coordinatesl=329.77812,b=−1.59227. The color–magnitude diagram of this object reveals a well-populated red giantbranch, with a prominent red clump atKs=13.35±0.05, andJ−Ks=1.30±0.05. We present the clusterRR Lyrae positions, magnitudes, colors, periods, and amplitudes. The presence of RR Lyrae indicates an oldglobular cluster, with an age>10 Gyr. We classify this object as an Oosterhoff type I globular cluster, based onthe mean period of its RR Lyrae type ab,áñ=P0.540days, and argue that this is a relatively metal-poor clusterwith[Fe/H]=−1.5±0.4 dex. The mean extinction and reddening for this cluster are=A0.38 0.02KsandE(J−Ks)=0.72±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 meandistance modulus is(m−M)0=14.38±0.03 mag, implying a distanceD=7.5±0.2 kpc and aGalactocentric distanceRG=4.3 kpc