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Session 17 - Molecular Clouds and Star Formation.
Oral session, Monday, June 10
Humanities 2650,
Using the VLA in the C and A configurations, we have imaged the NH_3 (J,K) = (3,3) line toward the massive star forming region W49 at 0.1'' resolution. The presence of redshifted absorption in lower angular resolution images of HCO^+ and NH_3 has led to the suggestion that the ring of H II regions in W49 North was formed by the large-scale inside-out collapse of the entire 10^4 M_ødot molecular cloud core. We find redshifted NH_3 (3,3) absorption toward only two of the H II regions: B and G. The absorption toward source B is optically thick, and that toward source G displays a large velocity gradient over \sim 1 '' scales. None of the remaining H II regions show any NH_3 (3,3) absorption. These results suggest that the NH_3 (3,3) absorption is local to sources B and G. Since the global collapse interpretation requires the redshifted absorbing gas to lie in foreground material several pc in front of the H II regions, the NH_3 (3,3) absorbing gas cannot participate in such a large scale collapse. Therefore, our data do not support the hypothesis of large-scale collapse. Instead, they suggest that local accretion produces the redshifted absorption. We have also discovered two unresolved NH_3 (3,3) emission sources in the ring. One of these is optically thick and the other may represent the sixth known example of NH_3 (3,3) maser emission.
