A new identity card for the bulge globular cluster NGC 6440 from resolved star counts

Cristina Pallanca, Barbara Lanzoni, Francesco R. Ferraro, Luca Casagrande, Sara Saracino, Bhavana Purohith Bhaskar Bhat, Silvia Leanza, Emanuele Dalessandro, Enrico Vesperini, --

Published 2021-04-19Version 1

We present a new identity card for the cluster NGC 6440 in the Galactic Bulge. We have used a combination of high-resolution Hubble Space Telescope images, wide-field ground-based observations performed with the ESO-FORS2, and the public survey catalog Pan-STARRS, to determine the gravitational center, projected density profile and structural parameters of this globular from resolved star counts. The new determination of the cluster center differs by ~ 2" (corresponding to 0.08 pc) from the previous estimate, which was based on the surface brightness peak. The star density profile, extending out to 700" from the center and suitably decontaminated from the Galactic field contribution, is best-fitted by a King model with significantly larger concentration ($c=1.86\pm0.06$) and smaller core radius ($r_c=6.4"\pm0.3"$) with respect to the literature values. By taking advantage of high-quality optical and near-infrared color-magnitude diagrams, we also estimated the cluster age, distance and reddening. The luminosity of the RGB-bump was also determined. This study indicates that the extinction coefficient in the bulge, in the direction of the cluster has a value ($R_V=2.7$) that is significantly smaller than that traditionally used for the Galaxy ($R_V=3.1$). The corresponding best-fit values of the age, distance and color excess of NGC 6440 are 13 Gyr, 8.3 kpc and $E(B-V)\sim 1.27$, respectively. These new determinations also allowed us to update the values of the central ($t_{rc}=2.5\ 10^7$ yr) and half-mass ($t_{rh}=10^9$ yr) relaxation times, suggesting that NGC 6440 is in a dynamically evolved stage.