Inferring the parallax of Westerlund 1 from Gaia DR2
  Westerlund 1 (Wd1) is potentially the largest star cluster in the Galaxy.
That designation critically depends upon the distance to the cluster, yet the
cluster is highly obscured, making luminosity-based distance estimates
difficult. Using {\it Gaia} Data Release 2 (DR2) parallaxes and Bayesian
inference, we infer a parallax of $0.35^{+0.07}_{-0.06}$ mas corresponding to a
distance of $2.6^{+0.6}_{-0.4}$ kpc. To leverage the combined statistics of all
stars in the direction of Wd1, we derive the Bayesian model for a cluster of
stars hidden among Galactic field stars; this model includes the parallax
zero-point. Previous estimates for the distance to Wd1 ranged from 1.0 to 5.5
kpc, although values around 5 kpc have usually been adopted. The {\it Gaia} DR2
parallaxes reduce the uncertainty from a factor of 3 to 18\% and rules out the
most often quoted value of 5 kpc with 99\% confidence. This new distance allows
for more accurate mass and age determinations for the stars in Wd1. For
example, the previously inferred initial mass at the main-sequence turn-off was
around 40 M$_{\odot}$; the new {\it Gaia} DR2 distance shifts this down to
about 22 M$_{\odot}$. This has important implications for our understanding of
the late stages of stellar evolution, including the initial mass of the
magnetar and the LBV in Wd1. Similarly, the new distance suggests that the
total cluster mass is about four times lower than previously calculated.
