Digging into the Galactic Bulge: Stellar Population and Structure of the Poorly Studied Cluster NGC 6316

Abstract

High-resolution Hubble Space Telescope optical observations have been used to analyze the stellar population and the structure of the poorly investigated bulge globular cluster NGC 6316. We constructed the first high-resolution reddening map in the cluster direction, which allowed us to correct the evolutionary sequences in the color–magnitude diagram (CMD) for the effects of differential reddening. A comparison between the CMDs of NGC 6316 and 47 Tucanae revealed strikingly similar stellar populations, with the two systems basically sharing the same turnoff, subgiant branch, and horizontal branch morphologies, indicating comparable ages. The red giant branch in NGC 6316 appears slightly bluer than in 47 Tucanae, suggesting a lower metal content. This has been confirmed by the isochrone fitting of the observed CMD, which provided us with updated values of the cluster age, distance, average color excess, and metallicity. We estimated an absolute age of 13.1 ± 0.5 Gyr, consistent with the age of 47 Tucanae, an average color excess E(B − V) = 0.64 ± 0.01, and a true distance modulus (m − M)0 = 15.27 ± 0.03 that sets the cluster distance at 11.3 kpc from the Sun. In addition, the photometric estimate of the cluster metallicity suggests [Fe/H] ≈ −0.9, which is ∼0.2 dex smaller than that of 47 Tucanae. We also determined the gravitational center and the density profile of the system from resolved stars. The latter is well reproduced by a King model. Our results confirm that NGC 6316 is another extremely old relic of the assembly history of the Galaxy.

Internal Kinematics and Structure of the Bulge Globular Cluster NGC 6569 ∗

“…For this reason several years ago we initiated a systematic photometric and spectroscopic investigation of bulge GCs and their stellar populations (see Origlia et al 1997Origlia et al , 2001Origlia et al , 2002Origlia & Rich 2004;Origlia et al 2005;Valenti et al 2005Valenti et al , 2007Ferraro et al 2009;Valenti et al 2010Valenti et al , 2011Ferraro et al 2015;Saracino et al 2015Saracino et al , 2016Cadelano et al 2017b;Pallanca et al 2019;Saracino et al 2019;Pallanca et al 2021bPallanca et al , 2021aCadelano et al 2022;Leanza et al 2022a;Deras et al 2023). In this respect, the discovery that Terzan5 and Liller1 are not genuine GCs but host multi-iron and multiage stellar populations has opened a new line of investigation, providing the first detection of Bulge Fossil Fragments, the possible remnants of primordial giant clumps that contributed to the bulge formation, surviving the violent phase of its assembling (Ferraro et al 2009;Lanzoni et al 2010;Origlia et al 2011Origlia et al , 2013Massari et al 2014;Ferraro et al 2016;Origlia et al 2019;Ferraro et al 2021;Dalessandro et al 2022).…”

Empirical Measurement of the Dynamical Ages of Three Globular Clusters and Some Considerations on the Use of the Dynamical Clock *

“…To characterize the BSS distribution in each of the investigated clusters, we took advantage of recent photometric studies performed by our group: Pallanca et al (2019Pallanca et al ( , 2021 for NGC 6440, Deras et al (2023) for NGC 6316, Ferraro et al (2018a) andB. Lanzoni et al (2023, in preparation) for NGC 3201.…”

“…Deep optical observations obtained with the HST WFC3 in the F555W and the F814W filters have recently been used to analyze the stellar population and the structure of this poorly investigated GC in the Galactic bulge (Deras et al 2023). The data reduction procedure is very similar to that described above for NGC 6440, and all the details can be found in Deras et al (2023). Also in this case, a high-resolution extinction map in the direction of the system was determined and used to correct the CMD for the effects of differential reddening.…”

“…In all the other cases, when only optical CMDs are available, we used theoretical evolutionary tracks from the BaSTI database (Pietrinferni et al 2006(Pietrinferni et al , 2021 to match the MS-TO region of each system. Since all the three clusters are very old stellar systems, with ages of approximately 12-13 Gyr (Pallanca et al 2021;Deras et al 2023), we selected the 12.5 Gyr BaSTI isochrones with the appropriate metallicity. In particular, considering that Galactic GCs typically show αelement enhancements of the order of [α/Fe] ∼ +0.3, we adopted the BaSTI α-enhanced model with Z = 1 × 10 −3 for NGC 3201 (having [Fe/H] ∼ −1.6; Harris 1996) and with Z = 8 × 10 −3 for the two metal-rich systems (NGC 6440 and NGC 6316), where [Fe/H] ; −0.5/−0.6 (Harris 1996;Origlia et al 1997Origlia et al , 2008.…”

“…In the case of NGC 6316, for which a PM-based membership selection is not feasible yet, we also evaluated the impact on A rh + induced by the potential contamination of Galactic field stars in the adopted BSS and REF samples. To this end, we used the Gaia catalog between 500″ and 600″ from the center (i.e., well beyond the cluster tidal radius, r t = 345″; Deras et al 2023) to count the number of field stars that fall within the two selection boxes (see, e.g., Dalessandro et al 2019). By taking into account the areas covered by this field sample and the area included within the cluster half-mass radius, we estimate that a total of one BSS (out of 38) and nine REF stars (out of 3707) could be field contaminants.…”