Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960

Abstract

Aims. We derive accurate parameters related to the CMD, structure and dynamical state of M 52 and NGC 3960, whose fields are affected by differential reddening. Previous works estimated their ages in the ranges 35-135 Myr and 0.5-1.0 Gyr, respectively. Methods. J, H and Ks 2MASS photometry with errors <0.2 mag is used to build CMDs, radial density profiles, luminosity and mass functions, and correct for differential reddening. Field-star decontamination is applied to uncover the cluster's intrinsic CMD morphology, and colour-magnitude filters are used to isolate stars with high probability of being cluster members. Results. The differential-reddening corrected radial density profile of M 52 follows King's law with core and limiting radii of Rcore = 0.91 ± 0.14 pc and Rlim = 8.0 ± 1.0 pc. NGC 3960 presents an excess of the stellar density over King's profile (Rcore = 0.62 ± 0.11 pc and Rlim = 6.0 ± 0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 are Rtidal = 13.1 ± 2.2 pc and Rtidal = 10.7 ± 3.7 pc. Cluster ages of M 52 and NGC 3960 derived with Padova isochrones are constrained to 60 ± 10 Myr and 1.1 ± 0.1 Gyr. In M 52 the core MF ( χcore = 0.89 ± 0.12) is flatter than the halo's ( χhalo = 1.65 ± 0.12). In NGC 3960 they are χcore = -0.74 ± 0.35 and χhalo = 1.26 ± 0.26. The mass locked up in MS/evolved stars in M 52 is ̃1200 M⊙, and the total mass (extrapolated to 0.08 M⊙) is ̃3800 M⊙. The total mass in NGC 3960 is ̃1300 M⊙. Conclusions. Compared to open clusters in different dynamical states studied with similar methods, the core and overall parameters of M 52 are consistent with an open cluster more massive than 1000 M ⊙ and ̃60 Myr old, with some mass segregation in the inner region. The core of NGC 3960 is in an advanced dynamical state with strong mass segregation in the core/halo region, while the somewhat flat overall MF ( χ ≈ 1.07) suggests low-mass star evaporation. The excess stellar density in the core may suggest post-core collapse. The dynamical evolution of NGC 3960 may have been accelerated by the tidal Galactic field, since it lies ≈0.5 kpc inside the Solar circle. © ESO 2006.