Cooling balance and thermal instability of a collapsing molecular protocloud

Molecular line emissions play an important role in the thermal balance of collapsing molecular clouds. In the present paper we calculate analytically the molecular cooling due to H-2 and HD, the last being the main cooling agent around 200 K (Puy & Signore, 1997 [NewA, 2, 299]). Our analysis is done under the hypothesis: we consider only the first two excited levels of the H-2 and HD molecules. We analyse the ratio between the molecular cooling due to HD and that due to H-2. The potentiality of fragmentation of these collapsing protoclouds through the thermal instability is also discussed. Finally, we suggest that this study could also be extended to the CO molecule, because the carbon reservoir molecule CO has already been observed in high redshift objects. (C) 1998 Elsevier Science B.V.