Résumé
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.