Parameter properties and stellar population of the old open cluster NGC 3960

Abstract

We present a $BVI$ photometric and astrometric catalogue of the open cluster NGC 3960, down to limiting magnitude $V\sim22$, obtained from observations taken with the Wide Field Imager camera at the MPG/ESO 2.2 m Telescope at La Silla. The photometry of all the stars detected in our field of view has been used to estimate a map of the strong differential reddening affecting this area. Our results indicate that, within the region where the cluster dominates, the $E(V-I)$ values range from 0.21 up to 0.78, with $E(V-I)=0.36$ ($E(B-V)=0.29$) at the nominal cluster centroid position; color excesses $E(V-I)$ up to 1 mag have been measured in the external regions of the field of view where field stars dominate. The reddening corrected color-magnitude diagram (CMD) allows us to conclude that the cluster has an age between 0.9 and 1.4 Gyr and a distance modulus of $(V-M_V)_0=11.35$. In order to minimize field star contamination, their number has been statistically subtracted based on the surface density map. The empirical cluster main sequence has been recovered in the $V$ vs. $V-I$ and in the $J$ vs. $J-K_S$ planes, using optical and infrared data, respectively. From these empirical cluster main sequences, two samples of candidate cluster members were derived in order to obtain the luminosity distributions as a function of the $V$ and $J$ magnitudes. The Luminosity Functions have been transformed into the corresponding Mass Functions; for $M>1 M_\odot$, the two distributions have been fitted with a power law of index $\alpha_V=2.95\pm0.53$ and $\alpha_J=2.81\pm0.84$ in $V$ and in $J$, respectively, while the Salpeter Mass Function in this notation has index $\alpha=2.35$.