Journal Article

Mass segregation and elongation of the starburst cluster Westerlund 1*

M. Gennaro, W. Brandner, A. Stolte and Th. Henning

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 4, pages 2469-2488
Published in print April 2011 | ISSN: 0035-8711
Published online April 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.18068.x
Mass segregation and elongation of the starburst cluster Westerlund 1*

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Massive stellar clusters are the best available laboratories to study the mass function of stars. Based on NTT/SofI near-infrared photometry, we have investigated the properties of the massive young cluster Westerlund 1. From comparison with stellar models, we derived an extinction mag, an age τ= 4 ± 0.5 Myr and a distance d= 4.0 ± 0.2 kpc for Westerlund 1, as well as a total mass of . Using spatially-dependent completeness corrections, we performed a two-dimensional study of the cluster’s initial mass function (IMF) and, in addition, of the stellar density profiles of the cluster as a function of mass. From both IMF slope variations and stellar density, we find strong evidence of mass segregation. For a cluster with some 105 stars, this is not expected at such a young age as the result of two-body relaxation alone. We also confirm previous findings on the elongation of Westerlund 1; assuming an elliptical density profile, we found an axial ratio of a:b= 3:2. Rapid mass segregation and elongation could be well explained as the results of subclusters merging during the formation of Westerlund 1.

Keywords: stars: evolution; stars: formation; Hertzsprung–Russell and colour–magnitude diagrams; stars: luminosity function, mass function; stars: pre-main-sequence; open clusters and associations: individual: Westerlund 1

Journal Article.  17410 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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