Molecules in a gravitational collapse
The study of primordial chemistry of molecules adresses a number of interesting questions pertaining to the thermal balance of collapsing molecular protoclouds. In numerous astrophysical cases molecular cooling and heating influence dynamical evolution of the medium. The existence of a significant abundance of molecules can be crucial on the dynamical evolution of collapsing objects. Because the cloud temperature increases with contraction, a cooling mechanism can be important for the structure formation, by lowering pressure opposing gravity, i.e. by allowing continued collapse of Jeans unstable protoclouds. This is particularly true for the first generation of objects. It has been suggested that a finite amount of molecules such as $H_2$, $HD$ and $LiH$ can be formed immediately after the recombination of cosmological hydrogen (Lepp and Shull 1984, Puy et al 1993). The cosmological fractional abundance $HD/H_2$ is small. Nevertheless, the presence of a non-zero permanent electric dipole moment makes $HD$ a potentially more important coolant than $H_2$ at modest temperatures, although $HD$ is much less abundant than $H_2$. Recently Puy and Signore (1997) showed that during the early stages of gravitational collapse, for some collapsing masses, $HD$ molecules were the main cooling agent when the collapsing protostructure had a temperature of about 200 Kelvins. Thus, we analytically estimate the molecular cooling during the collapse of protoclouds Then, the possibility of thermal instability during the early phase of gravitational collapse is discussed.