| Issue |
A&A
Volume 478, Number 1, January IV 2008
|
|
|---|---|---|
| Page(s) | 137 - 149 | |
| Section | Galactic structure, stellar clusters, and populations | |
| DOI | https://doi.org/10.1051/0004-6361:20077579 | |
| Published online | 20 November 2007 | |
Intermediate to low-mass stellar content of Westerlund 1*
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
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 
= 1.13 ± 0.03 mag and d = 3.55 ± 0.17 kpc (DM = 12.75 ± 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 
, and 0.15 for stars with masses in the range 3 to 10 .
We derive
the slope of the stellar mass function for stars with masses between 3.4 and
27 . In an annulus with radii between 0.75 and 1.5 pc from the
cluster centre, we obtain a slope of Γ = -1.3.
Closer in, the mass function of Westerlund 1 is shallower with
Γ = -0.6. The extrapolation of the mass function for stars with masses
from 0.08 to 120 yields an initial total stellar mass of
≈52 000 , and a present-day mass of 20 000 to 45 000
(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
© ESO, 2008
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