PHYSICAL-PROPERTIES OF MOLECULAR CLOUDS IN THE INNER DISK OF M31

We discuss recent observations of CO, CS, and NH, emission lines from two molecular clouds in the inner regions of M31. The intensities of t he (CO)-C-12(1-0), (CO)-C-12(2-1), (CO)-C-13(1-0), and (CO)-C-13(2-1) lines have been measured with the IRAM 30 m telescope at the same angu lar resolution of 23''. Upper limits have been obtained in the CS(2-1) , CS(3-2) and in the NH3(1, 1) and (2, 2) lines (the latter two with t he Effelsberg 100 m telescope). The line intensities and ratios are co mpared with the predictions of a one-side UV-illuminated plane-paralle l model which treats consistently radiative, chemical, and excitation precesses of CO and its isotopes. A satisfactory agreement cannot be a chieved for a single-density model. It appears that the emission of th e (CO)-C-12 lines is dominated by a low-density, very cold gas while t hat of the (CO)-C-13 lines comes largely from higher density clumps in side the clouds. The properties of the molecular clouds can be underst ood as resulting mainly from a very small rate of photodissociation du e to a very low UV radiation field, together with a low cosmic-ray den sity. Typical kinematic temperatures inside the clouds can drop to val ues less than 5 K.