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We have analyzed two epochs of HST/WFPC2 observations of the young Galacticstarburst cluster in NGC 3603 with the aim to study its internal dynamics andstellar population. Relative proper motions measured over 10.15 yrs of morethan 800 stars enable us to distinguish cluster members from field stars. Thebest-fitting isochrone yields Av=4.6-4.7 mag, a distance of 6.6-6.9 kpc, and anage of 1 Myr for NGC 3603 Young Cluster (NYC). We identifypre-main-sequence/main-sequence transition stars located in the short-livedradiative-convective gap, which in the NYC occurs in the mass range 3.5-3.8Msun. We also identify a sparse population of stars with an age of 4 Myr, whichappear to be the lower mass counterparts to previously discovered bluesupergiants located in the giant HII region NGC 3603. For the first time, weare able to measure the internal velocity dispersion of a starburst clusterfrom 234 stars with I < 18.5 mag to {\sigma}_pm1D=141+/-27 {\mu}as/yr(4.5+/-0.8 km/s at a distance of 6.75 kpc). As stars with masses between 1.7and 9 Msun all exhibit the same velocity dispersion, the cluster stars have notyet reached equipartition of kinetic energy (i.e., the cluster is not in virialequilibrium). The results highlight the power of combining high-precisionastrometry and photometry, and emphasize the role of NYC as a benchmark objectfor testing stellar evolution models and dynamical models for young clustersand as a template for extragalactic starburst clusters.

Internal dynamics and membership of the NGC 3603 Young Cluster from microarcsecond astrometry