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