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 toward 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 >100σ detection for this candidate globular cluster that is centered at equatorial coordinates R.A.J2000 = 16:10:30.0, decl.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 Ks = 13.35±0.05, and J−Ks = 1.30±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 an 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, áPñ = 0.540 days, and argue that this is a relatively metal-poor cluster with [Fe/H] = −1.5±0.4 dex. The mean extinction and reddening for this cluster are AKs = 0.38 0.02 and E( 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 mean distance modulus is (m−M)0 = 14.38±0.03 mag, implying a distance D = 7.5±0.2 kpc and a Galactocentric distance RG = 4.3 kpc.