FSR 1716: A New Milky Way Globular Cluster Confirmed Using VVV RR Lyrae Stars

Abstract

<p>We use deep multi-epoch near-IR images of the VISTA Variables in the V&iacute;a L&aacute;ctea (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 &gt;100&sigma; detection for this candidate globular cluster that is centered at equatorial coordinates R.A.<em>_J2000</em> = 16:10:30.0, decl._{<em>J2000</em>} = &minus;53:44:56 and galactic coordinates <em>l</em> = 329.77812, <em>b</em> = &minus;1.59227. The color&ndash;magnitude diagram of this object reveals a well-populated red giant branch, with a prominent red clump at K_s = 13.35 &plusmn; 0.05, and <em>J</em> &minus; <em>K_s</em> = 1.30 &plusmn; 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 &gt;10 Gyr. We classify this object as an Oosterhoff type I globular cluster, based on the mean period of its RR Lyrae type ab,&lsaquo;P&rsaquo;&nbsp;= 0.540 days, and argue that this is a relatively metal-poor cluster with [Fe/H] = &minus;1.5 &plusmn; 0.4 dex. The mean extinction and reddening for this cluster are <em>A_Ks</em> = &plusmn; 0.38 0.02 s and <em>E</em>( <em>J</em> &minus; <em>K</em>_s) = 0.72 &plusmn; 0.02 mag, respectively, as measured from the RR Lyrae colors and the near-IR color&ndash; magnitude diagram. We also measure the cluster distance using the RR Lyrae type ab stars. The cluster mean distance modulus is (<em>m</em> &minus; M)₀ = 14.38 &plusmn; 0.03 mag, implying a distance <em>D</em> = 7.5 &plusmn; 0.2 kpc and a Galactocentric distance <em>R_G</em> = 4.3 kpc.</p>