Asymmetric Ejecta of Cool Supergiants and Hypergiants in the Massive
  Cluster Westerlund 1
  We report new 5.5 GHz radio observations of the massive star cluster
Westerlund 1, taken by the Australia Telescope Compact Array, detecting nine of
the ten yellow hypergiants (YHGs) and red supergiants (RSGs) within the
cluster. Eight of nine sources are spatially resolved. The nebulae associated
with the YHGs Wd1-4a, -12a and -265 demonstrate a cometary morphology - the
first time this phenomenon has been observed for such stars. This structure is
also echoed in the ejecta of the RSGs Wd1-20 and -26; in each case the cometary
tails are directed away from the cluster core. The nebular emission around the
RSG Wd1-237 is less collimated than these systems but once again appears more
prominent in the hemisphere facing the cluster. Considered as a whole, the
nebular morphologies provide compelling evidence for sculpting via a physical
agent associated with Westerlund 1, such as a cluster wind.
