Physical Properties of Molecular Clouds in the Inner Disk of M31

We discuss recent observations of CO, CS, and NH_3_ emission lines from two molecular clouds in the inner regions of M31. The intensities of the ^12^CO(1-0), ^12^CO(2-1), ^13^CO(1-0), and ^13^CO(2-1) lines have been measured with the IRAM 30 m telescope at the same angular resolution of 23". Upper limits have been obtained in the CS(2-1), CS(3-2) and in the NH_3_(1, 1) and (2, 2) lines (the latter two with the Effelsberg 100 m telescope). The line intensities and ratios are compared with the predictions of a one-side UV-illuminated plane-parallel model which treats consistently radiative, chemical, and excitation processes of CO and its isotopes. A satisfactory agreement cannot be achieved for a single-density model. It appears that the emission of the ^12^CO lines is dominated by a low-density, very cold gas while that of the ^13^CO lines comes largely from higher density clumps inside the clouds. The properties of the molecular clouds can be understood as resulting mainly from a very small rate of photodissociation due to a very low UV radiation field, together with a low cosmic-ray density. Typical kinematic temperatures inside the clouds can drop to values less than 5 K.