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 rc =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. 19 10⁴ 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.