Molecular collisions in circumstellar discs

Inelastic molecular collisions are incorporated into the statistical model for turbulent circumstellar discs (Hämeen-Anttila, 1991; Verronenet al., 1993), and are found to provide a more effective cooling mechanism than molecule-grain impacts. Examination of the viscous evolution of the disc shows that the chemical composition and the coefficient of viscosity are crucial for determining the existence and properties of a state of equilibrium. A detailed study is carried out for carbon monoxide and hydroxyl. Abundances as small as 10^–5 for CO and 10^–8 for OH are sufficient to balance the disc against the viscous production of heat. The disc is usually non-convective. Near the equilibrium state the dust layer becomes thin enough to be gravitationally unstable. Infall of interstellar material is not expected to change the occurrence of instability, since the disc was not found to be convective even in the homogeneous circumstances. The results also remain unaltered even if the abundances of coolants are reduced by several orders of magnitude.