THE MOLECULAR CLOUD ASSOCIATED WITH THE H-II REGION RCW-34

We present millimetric multi-line observations of the molecular cloud associated with the H II region RCW 34. We believe we are able to present a coherent description of the cloud: a molecular bar on top of the H II region is divided into three elements: a dense, heated part situated in the post-shock trail, a cold, less dense part besides, left unperturbed and a diffuse component either in front of the dense parts or possibly mixed with them if the dense parts are clumpy. This diffuse component has a density of a few hundreds per cm-3, a temperature in the range 30-60 K and its opacity in (CO)-C-12 is close to or less than unity. We show that this diffuse component, the existence of which is controversial, is the best explanation to the large (CO)-C-12 (j: 1 --> 0)/(J: 2 --> 1) ratio we have observed. A simple-minded model is presented to approach its properties. The cloud is on top and partly in front of the H II region and can explain the 4.2 mag of visual extinction measured by one of us (Heydari-Malayeri 1988). The very high velocity flow detected in Halpha by Heydari-Malayeri has no counterpart in radio but could be a direct visualisation of the so-called ''Champagne effect'' (Tenorio-Tagle 1979).