Survey for transiting extrasolar planets in stellar systems. I. Fundamental parameters of the open cluster NGC 1245

We derive fundamental parameters for the old, rich, open cluster NGC 1245 using BVI photometry from the MDM 1.3 m and 2.4 m telescopes. Based on detailed isochrone fitting, we find that NGC 1245 has a slightly subsolar metallicity, [Fe/H] = - 0.05 +/- 0.03 (statistical) +/- 0.08 (systematic) and an age of 1.04 +/- 0.02 +/- 0.09 Gyr. In contrast to previous studies, we find no evidence of significant differential reddening. We determine an extinction of A(V) = 0.68 +/- 0.02 +/- 0.09 mag and a distance modulus of (m - M)(0) = 12.27 +/- 0.02 +/- 0,12 mag, which corresponds to a distance of 2.8 +/- 0.2 kpc. We derive a logarithmic mass function slope for the cluster of alpha = - 3.12 +/- 0.27, where a Salpeter slope is alpha = - 1.35. Fits to the radial surface density profile yield a core radius of r(c) = 3.'10 +/- 0.'52 (2.57 +/- 0.47 pc). NGC 1245 is highly relaxed and contains a strongly mass-segregated population. The mass function for the inner cluster has a very shallow slope, alpha = - 0.56 +/- 0.28. In contrast, the outer periphery of the cluster is enriched with low-mass members and devoid of high-mass members out to the tidal radius, r(t) = 20' (16.5 pc). Based on the observed surface density profile and an extrapolated mass function, we derive a total cluster mass, M = 1300 +/- 90 +/- 170 M-..