2007A&A...468..993K

In spite of a large number of recent publications about the massive stellar cluster Westerlund 1, its distance from the Sun remains uncertain with values as low as 1.1kpc, but largely between 4 and 5kpc. The goal of this study is to determine a distance to Westerlund 1 independent of the characteristics of the stellar population and to study its neutral environment, using observations of atomic hydrogen. The HI observations are taken from the Southern Galactic Plane Survey to study HI absorption in the direction of the HII region created by the members of Westerlund 1 and to investigate its environment as observed in the HI line emission. A Galactic rotation curve was derived using the recently revised values for the Galactic centre distance of R_☉=7.6kpc, and the velocity of the Sun around the Galactic centre of Θ_☉=214km/s. This rotation curve successfully predicts the location of the Tangent point gas and the velocity of the Sagittarius Arm outside the solar circle on the far side of the Galaxy to within 4km/s. Compared to the typically used values of R_☉=8.5kpc and Θ_☉=220km/s this reduces kinematically determined distances by more than 10%. The newly determined rotation model leads us to derive a distance of 3.9±0.7kpc to Westerlund 1, consistent with a location in the Scutum-Crux Arm. Included in this estimate is a very careful investigation of possible sources of error for the Galactic rotation curve. We also report on small expanding HI features around the cluster with a maximum dynamic age of 600 000 years and a larger bubble which has a minimum dynamic age of 2.5 million years. Additionally we re-calculated the kinematic distances to nearby HII regions and supernova remnants based on our new Galaxic rotation curve. We propose that in the early stages of the development of Wd 1 a large interstellar bubble of diameter about 50 pc was created by the cluster members. This bubble has a dynamic age similar to the age of the cluster. Small expanding bubbles, with dynamical ages ∼0.6Myr are found around Wd 1, which we suggest consist of recombined material lost by cluster members through their winds.