Revealing the stellar population of the ultra-obscured Galactic globular cluster Glimpse-C02

In this paper, we present the results of a detailed photometric analysis of Glimpse-C02, one of the most extincted globular clusters of the Milky Way. We built a deep color magnitude diagram spanning ≈ 10 magnitudes and enabling the very first identification of the main sequence turnoff of the cluster. Due to the extreme reddening conditions of the region where the stellar system is located, a differential reddening correction was necessary. The resulting reddening map shows color excess variations up to δ E(B-V) ≈ 2.5 mag in the direction of the target. From isochrone-fitting of the differential reddening corrected color-magnitude diagram, we obtained a new estimate of the mean color excess, E(B-V)=6.33^ +0.05 _ -0.04 , and a distance modulus $(m-M)_0=14.00^ +0.26 _ -0.11 $, corresponding to a distance of d=6.3^ +0.8 _ -0.3 kpc from the Sun, and a Galactocentric distance of 2.6^ +0.6 _ -0.7 kpc. This distance value, within the associated uncertainties, suggests that the cluster may be located closer to the Galactic Center compared to previous estimates, possibly supporting its classification as a bulge globular cluster. Furthermore, we obtained a photometric metallicity estimate of Fe/H $=-0.30^ +0.10 _ -0.08 $ and the first absolute age determination for Glimpse-C02, resulting in t=11.9^ +0.7 _ -0.6 Gyr, as typically measured for Galactic globular clusters at this metallicity. We also obtained a new estimate of the center of gravity of the cluster and determined its projected density profile from resolved star counts, finding a high King concentration parameter (c = 1.97_ -0.67 ^ +0.51 ) and a core radius r_c =8.72^ +0.40 _ -0.35 arcsec. Finally, from the surface brightness profile of the system, we derived an integrated H-band magnitude M_ ̊m H =-7.9, corresponding to a mass of M=3.57^ +0.22 _ -0.19 10^4 M_⊙. Thus, our work classifies Glimpse-C02 as an old and metal-rich globular cluster that is in an advanced stage of its dynamical evolution.