SCYON Abstract

Received on August 4 2006

Methods for improving open cluster fundamental parameters applied to M52 and NGC3960

AuthorsCharles Bonatto and Eduardo Bica
Universidade Federal do Rio Grande do Sul, Instituto de Física,
CP 15051, Porto Alegre 91501-970, RS, Brazil
Accepted byAstronomy & Astrophysics
LinksNGC 3960 / M52


We derive accurate parameters related to the CMD, structure and dynamical state of M52 and NGC3960, whose fields are affected by differential reddening. Previous works estimated their ages in the ranges 35 - 135Myr and 0.5 - 1.0Gyr, respectively. 2MASS photometry with errors <0.2mag 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. The differential-reddening corrected radial density profile of M52 follows King's law with core and limiting radii of rc=0.91+/-0.14pc and rl=8.0+/-1.0pc. NGC3960 presents an excess of the stellar density over King's profile (rc=0.62+/-0.11pc and rl=6.0+/-0.8pc) at the center. The tidal radii of M52 and NGC3960 are rt=13.1+/-2.2pc and rt=10.7+/-3.7pc. Cluster ages of M52 and NGC3960 derived with Padova isochrones are constrained to 60+/-10Myr and 1.1+/-0.1Gyr. In M52 the core MF (x=0.89+/-0.12) is flatter than the halo's (x=1.65+/-0.12). In NGC3960 they are x=-0.74+/-0.35 and x=1.26+/-0.26. The mass locked up in MS/evolved stars in M52 is ~1200M(sun), and the total mass (extrapolated to 0.08M(sun)) is ~3800M(sun). The total mass in NGC3960 is ~1300M(sun). Compared to open clusters in different dynamical states studied with similar methods, the core and overall parameters of M52 are consistent with an open cluster more massive than 1000M(sun) and ~60Myr old, with some mass segregation in the inner region. The core of NGC3960 is in an advanced dynamical state with strong mass segregation in the core/halo region, while the somewhat flat overall MF (x~1.07) suggests low-mass star evaporation. The excess stellar density in the core may suggest post-core collapse. The dynamical evolution of NGC3960 may have been accelerated by the tidal Galactic field, since it lies ~0.5kpc inside the Solar circle.