Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane

Free access article

Issue		A&A Volume 437, Number 2, July II 2005


Page(s)		483 - 500
Section		Galactic structure, stellar clusters and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20042516

A&A 437, 483-500 (2005)
DOI: 10.1051/0004-6361:20042516

Detailed analysis of open clusters: A mass function break and evidence of a fundamental plane

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@if.ufrgs.br

(Received 10 December 2004 / Accepted 22 March 2005 )

Abstract
We derive photometric, structural and dynamical evolution-related parameters of 11 nearby open clusters with ages in the range 70 Myr to 7 Gyr and masses in the range $\approx$ 400 $M_\odot$ to $\approx$ 5300 $M_\odot$ . The clusters are homogeneously analysed by means of J, H and $K_{\rm S}$ 2MASS photometry, which provides spatial coverage wide enough to properly take into account the contamination of the cluster field by Galaxy stars. Structural parameters such as core and limiting radii are derived from the background-subtracted radial density profiles. Luminosity and mass functions (MFs) are built for stars later than the turnoff and brighter than the 2MASS PSC 99.9% completeness limit. The total mass locked up in stars in the core and the whole cluster, as well as the corresponding mass densities, are calculated by taking into account the observed stars (evolved and main sequence) and extrapolating the MFs down to the H-burning mass limit, 0.08 $M_\odot$ . We illustrate the methods by analysing for the first time in the near-infrared the populous open clusters NGC 2477 and NGC 2516. For NGC 2477 we derive an age of 1.1 $\pm$ 0.1 Gyr, distance from the Sun $\mbox{$d_\odot$ }=1.2$ $\pm$ 0.1 kpc, core radius $\mbox{$R_{{\rm core}}$ }=1.4$ $\pm$ 0.1 pc, limiting radius $\mbox{$R_{{\rm lim}}$ }=11.6$ $\pm$ 0.7 pc and total mass $\mbox{$m_{{\rm tot}}$ }\approx(5.3\pm1.6)$ $\times$ 10³ $M_\odot$ . Large-scale mass segregation in NGC 2477 is reflected in the significant variation of the MF slopes in different spatial regions of the cluster, and in the large number-density of giant stars in the core with respect to the cluster as a whole. For NGC 2516 we derive an age of 160 $\pm$ 10 Myr, $\mbox{$d_\odot$ }=0.44$ $\pm$ 0.02 kpc, $\mbox{$R_{{\rm core}}$ }=0.6$ $\pm$ 0.1 pc, $\mbox{$R_{{\rm lim}}$ }=6.2$ $\pm$ 0.2 pc and $\mbox{$m_{{\rm tot}}$ }\approx(1.3\pm0.2)$ $\times$ 10³ $M_\odot$ . Mass-segregation in NGC 2516 shows up in the MFs. Six of the 11 clusters present a slope break in the MF occurring at essentially the same mass as that found for the field stars in Kroupa's universal IMF. The MF break is not associated to cluster mass, at least in the clusters in this paper. In two clusters the low-mass end of the MF occurs above the MF break. However, in three clusters the MF break does not occur, at least for the mass range $m\geq0.7~\mbox{$M_\odot$ }$ . One possibility is dynamical evolution affecting the MF slope distribution. We also search for relations of structural and evolutionary parameters with age and Galactocentric distance. The main results for the present sample are: (i) cluster size correlates both with age and Galactocentric distance; (ii) because of size and mass scaling, core and limiting radii, and core and overall mass correlate; (iii) massive ( $m\geq1000$ $M_\odot$ ) and less-massive clusters follow separate correlation paths on the plane core radius and overall mass; (iv) MF slopes of massive clusters are restricted to a narrow range, while those of the less-massive ones distribute over a wider range. Core and overall MF flattening is related to the ratio ( $\tau$ ) of age to relaxation time. For large values of $\tau$ the effects of large-scale mass segregation and low-mass stars evaporation can be observed in the MFs. In this sense, $\tau$ appears to characterize the evolutionary state of the clusters. We conclude that appreciable slope flattenings in the overall MFs of the less-massive clusters take ~6 times longer to occur than in the core, while in the massive clusters they take a time ~13 times longer. We investigate cluster parameters equivalent to those determining the fundamental plane of ellipticals. These parameters are: overall mass, projected mass density and core radius. We conclude that in the present sample there is evidence of a fundamental plane. Larger samples are necessary to pin down this issue.

Key words: open clusters and associations: general

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