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 the (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 angular 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 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 precesses of CO and its isotopes. A satisfactory agreement cannot be achieved for a single-density model. It appears that the emission of the (CO)-C-12 lines is dominated by a low-density, very cold gas while that of the (CO)-C-13 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.