Astronomy & Astrophysics

Issue		A&A Volume 455, Number 3, September I 2006
Page(s)		931 - 942
Section		Galactic structure, stellar clusters, and populations
DOI		10.1051/0004-6361:20065315

A&A 455, 931-942 (2006)
DOI: 10.1051/0004-6361:20065315

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

C. Bonatto and E. Bica

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

(Received 29 March 2006 / Accepted 5 May 2006)

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 $K_{\rm s}$ 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 $R_{\rm core} =0.91 \pm$ 0.14 pc and $R_{\rm lim} =8.0$ $\pm$ 1.0 pc. NGC 3960 presents an excess of the stellar density over King's profile ( $R_{\rm core} = 0.62$ $\pm$ 0.11 pc and $R_{\rm lim} =6.0$ $\pm$ 0.8 pc) at the center. The tidal radii of M 52 and NGC 3960 are $R_{\rm tidal}=13.1$ $\pm$ 2.2 pc and $R_{\rm tidal}=10.7$ $\pm$ 3.7 pc. Cluster ages of M 52 and NGC 3960 derived with Padova isochrones are constrained to 60 $\pm$ 10 Myr and 1.1 $\pm$ 0.1 Gyr. In M 52 the core MF ( $\chi_{\rm core}=0.89 \pm$ 0.12) is flatter than the halo's ( $\chi_{\rm halo}=1.65$ $\pm$ 0.12). In NGC 3960 they are $\chi_{\rm core}=-0.74$ $\pm$ 0.35 and $\chi_{\rm halo}=1.26$ $\pm$ 0.26. The mass locked up in MS/evolved stars in M 52 is ~1200 $M_\odot$ , and the total mass (extrapolated to 0.08 $M_\odot$ ) is ~3800 $M_\odot$ . The total mass in NGC 3960 is ~1300 $M_\odot$ .
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 $M_\odot$ 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 ( $\chi\approx 1.07$ ) 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 $\approx$ 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

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