SCYON Abstract

Received on June 17 2002

Mass segregation in rich clusters in the Large Magellanic Cloud -- III. Implications for the initial mass function

AuthorsR. de Grijs (1), G.F. Gilmore (1), A.D. Mackey (1), M.I., Wilkinson (1), S.F. Beaulieu (2), R.A. Johnson (3), , B.X., Santiago (4)
Affiliation1 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA
2 Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 1A1, Canada
3 European Southern Observatory, Casilla 19001, Santiago 19, Chile
4 Universidade Federal do Rio Grande do Sul, Instituto de Física, 91501-970 Porto Alegre, RS Brazil
Submitted toMonthly Notices of the Royal Astronomical Society


The distribution of core radii of rich clusters in both the Large and Small Magellanic Clouds (LMC, SMC) systematically increases in both upper limit and spread with increasing cluster age. Cluster-to-cluster variations in the stellar initial mass function (IMF) have been suggested as an explanation. We discuss the implications of the observed degree of mass segregation for the shape of the initial mass function.

Our results are based on Hubble Space Telescope/WFPC2 observations of six rich star clusters in the LMC, selected to include three pairs of clusters of similar age, mass, metallicity, and distance from the LMC centre, exhibiting a large spread in core radii between the clusters in each pair.

All clusters show clear evidence of mass segregation: (i) their luminosity function slopes steepen with increasing cluster radius, and (ii) the brighter stars are characterized by smaller core radii. For all sample clusters, both the slope of the luminosity function in the cluster centres and the degree of mass segregation are similar to each other, within observational errors of a few tenths of power-law slope fits to the data. This implies that their initial mass functions must have been very similar, down to ~0.8 - 1.0 Mo.

We rule out variations in the IMF as the main driver of the increasing spread of cluster core radii with cluster age. We propose instead that external tidal effects generate the range of core properties seen.