On the distance and molecular environment of Westerlund 2 and HESS J102-575

The extended TeV gamma-ray source HESS J1023-575 is coincident with the massive, young stellar cluster Westerlund 2 ( Wd2) and its surrounding H II region RCW 49. On the basis of an analysis of the CO emission and 21 cm absorption along the line of sight to Wd2, it is argued that this cluster, and by assumption the TeV source as well, must be associated with a giant molecular cloud in the far side of the Carina arm with a mass of 7.5 x 10(5) M circle dot. Analysis of the spatial and velocity structure of the cloud reveals clear evidence of interaction with Wd2. The cloud's kinematic distance of 6.0 +/- 1.0 kpc is shown to be consistent with distances inferred from both the radius-line width relation of molecular clouds and the foreground gas column derived from 230 X-ray sources in Wd2.