A Radio Census of the Massive Stellar Cluster Westerlund 1
  Massive stars and their stellar winds are important for a number of feedback
processes. The mass lost in the stellar wind can help determine the end-point
of the star as a NS or a BH. However, the impact of mass-loss on the post-Main
Sequence evolutionary stage of massive stars is not well understood. Westerlund
1 is an ideal astrophysical laboratory in which to study massive stars and
their winds in great detail over a large range of different evolutionary
phases. Aims: We aim to study the radio emission from Westerlund 1, in order to
measure radio fluxes from the population of massive stars, and determine
mass-loss rates and spectral indices where possible. Methods: Observations were
carried out in 2015 and 2016 with the Australia telescope compact array (ATCA)
at 5.5 and 9 GHz using multiple configurations, with maximum baselines ranging
from 750m to 6km. Results: 30 stars were detected in the radio from the fully
concatenated dataset, 10 of which were WRs (predominantly late type WN stars),
5 YHGs, 4 RSGs, 1 LBV star, the sgB[e] star W9, and several O and B
supergiants. New source detections in the radio were found for 5 WR stars, and
5 OB supergiants. These detections have led to evidence for 3 new OB supergiant
binary candidates, inferred from derived spectral index limits. Conclusions:
Spectral indices and index limits were determined for massive stars in
Westerlund 1. For cluster members found to have partially optically thick
emission, mass-loss rates were calculated. Under the approximation of a
thermally emitting stellar wind and a steady mass-loss rate, clumping ratios
were then estimated for 8 WRs. Diffuse radio emission was detected throughout
the cluster. Detections of knots of radio emission with no known stellar
counterparts indicate the highly clumped structure of this intra-cluster
medium, likely shaped by a dense cluster wind.
