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Astron. Astrophys. 336, 503-517 (1998)

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1. Introduction

The formation process and early dynamical evolution of star clusters can be inferred from the structure and stellar content of very young clusters. The star cluster system of the Large Magellanic Cloud is particularly suitable for this purpose since it has a large number of rich clusters of all ages.

Westerlund (1961) found that the young LMC cluster NGC 2100 showed evidence of having the most bright stars concentrated at its innermost region. Elson et al. (1987), also found indications of mass segregation for NGC 1866. Subramaniam et al. (1993) studied the spatial distribution of the stellar masses in five young LMC clusters. In one of these clusters (NGC 1711) they found evidence of mass segregation, which they interpreted as indicating the different sites of formation of stars of different mass in the protocluster. It is also worth noting that two of the prototypical `just formed' cluster i.e. the Trapezium cluster in the Orion Nebula in our Galaxy, and the 30 Doradus cluster in the LMC (which is actually two orders of magnitude larger than the Trapezium cluster) both have their most massive stars concentrated towards the centre (Larson 1993).

The observed mass segregation in young clusters could either indicate that the more massive stars were formed preferentially near the centres of these clusters (Lightman & Shapiro, 1978; Meylan & Heggie, 1997), or that energy equipartition was achieved faster than predicted by the conventional single-mass models. Indeed, it has been theoretically shown that the dynamical evolution of a cluster is related to the mass function (Lightman & Shapiro, 1978; Pryor et al., 1986). A steep mass function can apparently speed up significantly energy equipartition in a populous star cluster.

Mass segregation in the very old Galactic globulars is now under investigation with the HST and most recent studies summarised by Sosin (1997) show that the central density cusps in Galactic globular clusters might be the result of gravothermal oscillations of centrally concentrated populations of binaries or a black hole. However the case of young globulars with mass segregation needs equally thorough investigation.

Two young clusters in the LMC, NGC 2098 and SL 666, were studied by Kontizas et al. (1987b,1987c,1988), using the spatial distribution of their spectral types and star counts in photographic plates taken by the 1.2m U.K. Schmidt Telescope and the GPO astrographic camera of ESO. The clusters showed evidence of mass segregation, in the sense that the early type stars were more centrally concentrated than the late type stars. The estimated ages of these clusters appeared to be significantly lower than their mean relaxation time (Kontizas et al., 1987b,1987c,1988), thus apparently excluding two-body encounters as the principal cause of the apparent mass stratification.

In an attempt to further investigate these issues, we conducted a detailed analysis of the two populous clusters NGC 2098 and SL 666, which includes (i) derivation of more reliable ages for these clusters, based on isochrone fitting of their colour-magnitude diagrams and (ii) study of the distribution of stars of different mass in them. The observations were made with the 3.9-m Anglo-Australian Telescope. In Sect. 2, the observations and data reduction are described. In Sect. 3, we derive the ages of the clusters using the isochrone fitting technique. In Sect. 4, we investigate the number density profiles of the clusters, and derive the structural parameters. In Sect. 5, we give the luminosity and mass functions of the clusters, as a function of radius. The results for the clusters are discussed in Sect. 6. Finally the conclusions are given in Sect. 7.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998