Internal dynamics and membership of the NGC 3603 Young Cluster from
  microarcsecond astrometry
  We have analyzed two epochs of HST/WFPC2 observations of the young Galactic
starburst cluster in NGC 3603 with the aim to study its internal dynamics and
stellar population. Relative proper motions measured over 10.15 yrs of more
than 800 stars enable us to distinguish cluster members from field stars. The
best-fitting isochrone yields Av=4.6-4.7 mag, a distance of 6.6-6.9 kpc, and an
age of 1 Myr for NGC 3603 Young Cluster (NYC). We identify
pre-main-sequence/main-sequence transition stars located in the short-lived
radiative-convective gap, which in the NYC occurs in the mass range 3.5-3.8
Msun. We also identify a sparse population of stars with an age of 4 Myr, which
appear to be the lower mass counterparts to previously discovered blue
supergiants located in the giant HII region NGC 3603. For the first time, we
are able to measure the internal velocity dispersion of a starburst cluster
from 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 1.7
and 9 Msun all exhibit the same velocity dispersion, the cluster stars have not
yet reached equipartition of kinetic energy (i.e., the cluster is not in virial
equilibrium). The results highlight the power of combining high-precision
astrometry and photometry, and emphasize the role of NYC as a benchmark object
for testing stellar evolution models and dynamical models for young clusters
and as a template for extragalactic starburst clusters.
