1998NewA....3..247P

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). 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.