Time-domain study of the young massive cluster Westerlund 2 with the
  Hubble Space Telescope. I
  Time-domain studies of pre-main sequence stars have long been used to
investigate star properties during their early evolutionary phases and to trace
the evolution of circumstellar environments. Historically these studies have
been confined to the nearest, low-density, star forming regions. We used the
Wide Field Camera 3 on board of the Hubble Space Telescope to extend, for the
first time, the study of pre-main sequence variability to one of the few young
massive clusters in the Milky Way, Westerlund 2. Our analysis reveals that at
least 1/3 of the intermediate and low-mass pre-main sequence stars in
Westerlund 2 are variable. Based on the characteristics of their light curves,
we classified ~11% of the variable stars as weak-line T-Tauri candidates, ~ 52%
as classical T-Tauri candidates, ~ 5% as dippers and ~26% as bursters. In
addition, we found that 2% of the stars below 6Mo (~6% of the variables) are
eclipsing binaries, with orbital periods shorter than 80 days. The spatial
distribution of the different populations of variable pre-main sequence stars
suggests that stellar feedback and UV-radiation from massive stars play an
important role on the evolution of circumstellar and planetary disks.
