A New Identity Card for the Bulge Globular Cluster NGC 6440 from Resolved Star Counts*

Abstract

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 fit by a King model with a significantly higher concentration ( c = 1.86 ± 0.06) and smaller core radius ( r c = 6.″4 ± 0.″3) with respect to the literature values. By taking advantage of high-quality optical and near-IR color–magnitude diagrams, we also estimated the cluster age, distance, and reddening. The luminosity of the red giant branch 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 lower 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 ) ∼ 1.27. 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.