Temperature and Density Distribution in the Molecular Gas Toward Westerlund 2: Further Evidence for Physical Association

TL;DR

This study maps temperature and density in molecular gas near Westerlund 2, confirming its physical association with the HII region and providing data to test PDR theories.

Contribution

It offers detailed temperature and density distributions of molecular gas around Westerlund 2 using LVG analysis, supporting its physical connection to the HII region.

Findings

Molecular gas temperature ranges from 10 K to 150 K near Westerlund 2.

High temperatures confirm physical association with the HII region.

Results are consistent with photo dissociation region (PDR) models.

Abstract

Furukawa et al. 2009 reported the existence of a large mass of molecular gas associated with the super star cluster Westerlund 2 and the surrounding HII region RCW49, based on a strong morphological correspondence between NANTEN2 12CO(J=2-1) emission and Spitzer IRAC images of the HII region. We here present temperature and density distributions in the associated molecular gas at 3.5 pc resolution, as derived from an LVG analysis of the 12CO(J=2-1), 12CO(J=1-0) and 13CO(J=2-1) transitions. The kinetic temperature is as high as 60-150 K within a projected distance of 5-10 pc from Westerlund 2 and decreases to as low as 10 K away from the cluster. The high temperature provides robust verification that the molecular gas is indeed physically associated with the HII region, supporting Furukawa et al.'s conclusion. The derived temperature is also roughly consistent with theoretical calculations of photo dissociation regions (PDRs), while the low spatial resolution of the present study does not warrant a more detailed comparison with PDR models. We suggest that the molecular clouds presented here will serve as an ideal laboratory to test theories on PDRs in future higher resolution studies.