RESULTS OF THE ESO-SEST KEY PROGRAM - CO IN THE MAGELLANIC CLOUDS .4.PHYSICAL-PROPERTIES OF MOLECULAR CLOUDS IN THE SMALL MAGELLANIC CLOUD

We discuss recent molecular observations of CO in the Small Magellanic Cloud, made with the SEST telescope. For 6 resolved CO clouds the int ensities of the (CO)-C-12(1-0), (CO)-C-12(2-1), (CO)-C-13(1-O) and (CO )-C-13(2-1) lines have been measured at the same angular resolution of 45 are seconds. The line ratios are compared with the predictions of a one-side illuminated plane-parallel model which treats consistently radiative, chemical and excitation processes of CO and its isotopes. A satisfactory agreement is achieved for a moderately high-density gas whose properties can be understood as resulting mainly from a higher r ate of photodissociation and of heating of CO through the combined eff ects of a higher UV radiation field and cosmic-ray flux, of a lower du st/gas ratio and of a lower abundance of carbon (compared to our Galax y). This confirms the qualitative results of a previous paper (Rubio e t al. 1993 = Paper III). In the Magellanic Clouds, contrary to the Gal axy, CO molecules can only survive in dense parts (''clumps'') of inte rstellar clouds at densities which depend on the radiation fields, whi le they are photodissociated in the less dense (''interclump'') medium . We also predict that hydrogen in this lower-density medium is mainly atomic rather than molecular. In two clouds submitted to a particular ly high UV radiation field, the line widths are significantly larger f or (CO)-C-12 than for (CO)-C-13, perhaps indicating an evaporation flo w. We also discuss the emission of far-infrared fine-structure lines b y the clouds, and discuss in particular the line of C+ at 158 mu m obs erved in some of them.