Three Galactic globular cluster candidates

Abstract

Context. The census of Galactic globular clusters (GCs) is still incomplete, and about ten new objects are supposed to await discovery, hidden behind the crowded and reddened regions of the Galactic bulge and disk. Aims. We investigated the nature of three new GC candidates, discovered in the frames collected by the Vista Variables in the Via Lactea (VVV) near-infrared survey. They will be called VVV CL002, VVV CL003, and VVV CL004. Methods. We studied the results of point-spread-function near-infrared photometry from VVV data for the three objects and their surrounding fields, the proper motion information available in the literature and, when possible, we derived the cluster parameters by means of calibrated indices measured on the color-magnitude diagrams. Results. The evidence shows that VVV CL002 is a newly discovered, small, moderately metal-rich ([Fe/H] ≈ −0.4) Galactic GC. It is located at a Galactocentric distance of 0.7 ± 0.9 kpc, and it could be one of the nearest GC to the Galactic center. Its characteristics are more similar to those of low-mass, Palomar-like GCs than to more classical, old, and massive bulge GCs. VVV CL003 is the first star cluster discovered in the Galactic disk on the opposite side of the center with respect to the Sun, at a Galactocentric distance of ∼5 kpc. Its high metallicity ([Fe/H] ≈ −0.1) and location point to an open cluster, but a GC cannot be excluded. VVV CL004, on the contrary, is most probably only a random clump of field stars, as indicated by both its low statistical significance and by the impossibility to distinguish its stars from the surrounding field population. Conclusions. We claim the detection of i) a new Galactic GC, deriving an estimate of its basic parameters; ii) a stellar aggregate, probably an open cluster, in the disk directly beyond the Galactic center; and iii) an overdensity of stars, most probably an asterism.

Deriving metallicities from calcium triplet spectroscopy in combination with near-infrared photometry

“…We performed photometry through fitting of point spread function using the VVV-SkZ_pipeline code (Mauro et al 2013), which is based on DAOPHOT suite (Stetson 1987(Stetson , 1994, on the single 2048 × 2048 pixel chips extracted from the stacked VVV pawprints (Saito et al 2012). The photometry was tied to the 2MASS system, as described in Moni Bidin et al (2011) and Chené et al (2012). The use of the 2MASS system as the standard photometric system permitted us to integrate our photometric database with the 2MASS PSC.…”

Effects of helium enrichment in globular clusters

“…Nowadays, ∼160 GCs are known in the Milky Way, with metallicities [M/H] = log(Z/X)−log(Z/X) between ∼−2.3 and ∼−0.25 (Harris 1996(Harris , revision 2010 and with ages ranging from ∼6.5 to ∼13 Gyr (Aparicio et al 2001;Chaboyer 2001;Marín-Franch et al 2009;Dotter et al 2010;Moni Bidin et al 2011a).…”

VVV: The near-IR Milky Way bulge and plane survey

“…Already during the first year many new open and globular cluster candidates were discovered [1,6,7], in only a subset of the whole area. The high resolution and uniform coverage of the whole bulge in 5 broad-band filters will enable construction of homogeneous and high resolution extinction and metallicity distribution maps of the bulge [3], which can then be used to study in detail the structure of the inner and outer bulge [4,9].…”