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Starburst clusters with ages of a few Myr represent unique astrophysical laboratories, as stars across the entire stellar mass range from the upper mass cut-off in the mass function down to the hydrogen burning limit (and possibly beyond) are present at the same time in a homogeneous environment. The galactic starburst cluster Westerlund 1 with its estimated 250 O-stars, W-R stars, supergiants and hypergiants is among the most massive young clusters in the Milky Way.
While previous studies of Westerlund 1 focused largely on optical and X-ray observations of its evolved massive stellar population, we have analyzed near-infrared data, resulting in the first in depth study of the "lower-mass" main sequence and pre-main sequence cluster population, i.e., of stars in the mass range 0.4 to 30 solar masses. We derive cluster properties like size and shape, investigate evidence for mass segregation, study the question of co-evality of the low- and high-mass stellar population, test theoretical evolutionary tracks, and determine the (initial) mass function (including the search for upper or lower mass cut-offs). We also address the question if Westerlund 1 and its galactic siblings like the NGC 3603 Young Cluster, the Arches and Quintuplet clusters in the Galactic center region, and the R136 cluster in the 30 Dor region in the LMC can be considered as proto-globular clusters.
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