2008A&A...478..137B

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_Ks=1.13±0.03mag and d=3.55±0.17kpc (DM=12.75±0.10mag). The pre-main sequence population is best fit by a Palla & Stahler (1999ApJ...525..772P) isochrone for an age of 3.2Myr, while the main sequence population is in agreement with a cluster age of 3 to 5Myr. 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 32M_☉, and 0.15 for stars with masses in the range 3 to 10M_☉. We derive the slope of the stellar mass function for stars with masses between 3.4 and 27M_☉. In an annulus with radii between 0.75 and 1.5pc 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 120M_☉ yields an initial total stellar mass of ≃52000M_☉, and a present-day mass of 20000 to 45000M_☉ (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 Westerlund1 is the most massive starburst cluster identified to date in the Milky Way.