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

Minniti, Dante; Palma, Tali; Dékány, Istvan; Hempel, Maren; Rejkuba, Marina; Pullen, Joyce; Alonso-García, Javier; Barbá, Rodolfo; Barbuy, Beatriz; Bica, Eduardo; Bonatto, Charles; Borissova, Jura; Catelan, Marcio; Carballo-Bello, Julio A.; Chene, Andre Nicolas; Clariá, Juan J.; Cohen, Roger E.; Contreras-Ramos, Rodrigo; Dias, Bruno; Emerson, Jim; Froebrich, Dirk; Buckner, Anne S. M.; Geisler, Douglas; Gonzalez, Oscar A.; Gran, Felipe; Hagdu, Gergely; Irwin, Mike; Ivanov, Valentin D.; Kurtev, Radostin; Lucas, Philip W.; Majaess, Daniel; Mauro, Francesco; Moni-Bidin, Christian; Navarrete, Camila; Ramírez Alegría, Sebastian; Saito, Roberto K.; Valenti, Elena; Zoccali, Manuela
The Astrophysical Journal Letters, Volume 838, Issue 1, article id. L14, 7 pp. (2017).


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 K = 13.35 ± 0.05, and J ‑ K = 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, < P> =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 {A}{Ks}=0.38+/- 0.02 and E(J ‑ K ) = 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 R = 4.3 kpc.