Intermediate to low-mass stellar content of Westerlund 1

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Issue		A&A Volume 478, Number 1, January IV 2008


Page(s)		137 - 149
Section		Galactic structure, stellar clusters, and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20077579

A&A 478, 137-149 (2008)
DOI: 10.1051/0004-6361:20077579

Intermediate to low-mass stellar content of Westerlund 1

W. Brandner^{1, 2}, J. S. Clark³, A. Stolte², R. Waters⁴, I. Negueruela⁵, and S. P. Goodwin⁶

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: brandner@mpia.de
² UCLA, Division of Astronomy, Los Angeles, CA 90095-1547, USA
³ Open University, UK
⁴ Sterrenkundig Instituut "Anton Pannekoek", Amsterdam, The Netherlands
⁵ Dpto. de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, 03080 Alicante, Spain
⁶ Dept. of Physics & Astronomy, University of Sheffield, Sheffield, UK

(Received 1 April 2007 / Accepted 31 October 2007)

Abstract
We have analysed near-infrared NTT/SofI observations of the starburst cluster Westerlund 1, which is among the most massive young clusters in the Milky Way. A comparison of colour-magnitude diagrams with theoretical main-sequence and pre-main sequence evolutionary tracks yields improved extinction and distance estimates of $A_{K{\rm s}}$ = 1.13 $\pm$ 0.03 mag and d = 3.55 $\pm$ 0.17 kpc (DM = 12.75 $\pm$ 0.10 mag). The pre-main sequence population is best fit by a Palla & Stahler isochrone for an age of 3.2 Myr, while the main sequence population is in agreement with a cluster age of 3 to 5 Myr. An analysis of the structural parameters of the cluster yields that the half-mass radius of the cluster population increases towards lower mass, indicative of the presence of mass segregation. The cluster is clearly elongated with an eccentricity of 0.20 for stars with masses between 10 and 32 $M_\odot$ , and 0.15 for stars with masses in the range 3 to 10 $M_\odot$ . We derive the slope of the stellar mass function for stars with masses between 3.4 and 27 $M_\odot$ . In an annulus with radii between 0.75 and 1.5 pc from the cluster centre, we obtain a slope of $\Gamma$ = -1.3. Closer in, the mass function of Westerlund 1 is shallower with $\Gamma$ = -0.6. The extrapolation of the mass function for stars with masses from 0.08 to 120 $M_\odot$ yields an initial total stellar mass of $\approx$ 52 000 $M_\odot$ , and a present-day mass of 20 000 to 45 000 $M_\odot$ (about 10 times the stellar mass of the Orion nebula cluster, and 2 to 4 times the mass of the NGC 3603 young cluster), indicating that Westerlund 1 is the most massive starburst cluster identified to date in the Milky Way.

Key words: stars: evolution -- stars: formation -- stars: luminosity function, mass function -- stars: pre-main sequence -- stars: supernovae: general -- Galaxy: open clusters and associations: individual: Westerlund 1

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