Methods for improving open cluster fundamental parameters applied to M 52 and NGC 3960
Universidade Federal do Rio Grande do Sul, Instituto de Física, CP 15051, Porto Alegre 91501-970, RS, Brazil e-mail: [charles;bica]@if.ufrgs.br
Accepted: 5 May 2006
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 Myr and 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 0.14 pc and ± 1.0 pc. NGC 3960 presents an excess of the stellar density over King's profile ( ± 0.11 pc and ± 0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 are ± 2.2 pc and ± 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 ( 0.12) is flatter than the halo's ( ± 0.12). In NGC 3960 they are ± 0.35 and ± 0.26. The mass locked up in MS/evolved stars in M 52 is ~1200 , and the total mass (extrapolated to 0.08) is ~3800 . The total mass in NGC 3960 is ~1300 .
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 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 () 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.
Key words: Galaxy: open clusters and associations: individual: M 52 / Galaxy: structure / Galaxy: open clusters and associations: individual: NGC 3960
© ESO, 2006