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 R_core =0.91 ± 0.14 pc and R_lim =8.0 ± 1.0 pc. NGC 3960 presents an excess of the stellar density over King's profile (R_core = 0.62 ± 0.11 pc and R_lim =6.0 ± 0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 are R_tidal=13.1 ± 2.2 pc and R_tidal=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 (χ_core=0.89 ± 0.12) is flatter than the halo's (χ_halo=1.65 ± 0.12). In NGC 3960 they are χ_core=-0.74 ± 0.35 and χ_halo=1.26 ± 0.26. The mass locked up in MS/evolved stars in M 52 is ~1200 M_☉, and the total mass (extrapolated to 0.08M_☉) 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.
Astronomy and Astrophysics
- Pub Date:
- September 2006
- Galaxy: open clusters and associations: individual: M 52;
- Galaxy: structure;
- Galaxy: open clusters and associations: individual: NGC 3960;
- 15 pages and 8 figures. Astronomy and astrophysics, in press