Mass segregation in rich LMC clusters from modelling of deep HST colour-magnitude diagrams

作者：Kerber LO^*; Santiago BX

来源：Astronomy & Astrophysics, 2006, 452(1): 155-162.

DOI：10.1051/0004-6361:20054198

• galaxies : star clusters
• galaxies : Magellanic Clouds
• stars : luminosity function, mass function
• stars : Hertzsprung-Russell ( HR) and C-M diagrams
• methods : numerical

Aims. We used the deep colour-magnitude diagrams (CMDs) of five rich LMC clusters (NGC1805, NGC1818, NGC1831, NGC 1868, and Hodge 14) observed with HST/WFPC2 to derive their present day mass PDMF) and its variation with position within the cluster.
Methods. The PDMF was parameterized as a power law in the available main-sequence mass range of each cluster, typically 0.9 less than or similar to m/M-circle dot less than or similar to 2.5; its slope was determined at different positions spanning from the very centre out to several core radii. The CMDs in the central regions of the clusters were carefully studied earlier, resulting in accurate age, metallicity, distance modulus, and reddening values. The slope alpha (where Salpeter is 2.35) was determined in annuli by following two distinct methods: 1) a power law fit to the PDMF obtained from the systemic luminosity LF); 2) a statistical comparison between observed and model CMDs. In the second case, a is a free input parameter in the CMD modelling process where we incorporate photometric errors and the effect of binarity as a fraction of unresolved binaries (f(bin) = 100%) with random pairing of masses from the same PDMF.
Results. In all clusters, significant mass segregation is found from the positional dependence of the PDMF slope: a less than or similar to 1.8 for R <= 1.0 R-core and alpha similar to Salpeter inside R = 2 similar to 3 R-core (except for Hodge 14, where alpha similar to Salpeter for R similar to 4 R-core). The results are robust in the sense that they hold true for both methods used. The CMD method reveals that unresolved binaries flatten the PDMF obtained form the systemic LF, but this effect is smaller than the uncertainties in the a determination. For each cluster we estimated dynamical ages inside the core and for the entire system. In both cases we found a trend in the sense that older clusters have flatter PDMF, consistent with a dynamical mass segregation and stellar evaporation.

• 出版日期2006-6