1994A&A...292..371L

We discuss recent molecular observations of CO in the Small Magellanic Cloud, made with the SEST telescope. For 6 resolved CO clouds the intensities of the ¹²CO(1-0), ¹²CO(2-1), ¹³CO(1-0) and ¹³CO(2-1) lines have been measured at the same angular resolution of 45 arc 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 rate of photodissociation and of heating of CO through the combined effects of a higher UV radiation field and cosmic-ray flux, of a lower dust/gas ratio and of a lower abundance of carbon (compared to our Galaxy). This confirms the qualitative results of a previous paper (Rubio et al. 1993 = Paper III). In the Magellanic Clouds, contrary to the Galaxy, CO molecules can only survive in dense parts ("clumps") of interstellar clouds at densities which depend on the radiation fields, while 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 particularly high UV radiation field, the line widths are significantly larger for ¹²CO than for ¹³CO, perhaps indicating an evaporation flow. We also discuss the emission of far-infrared fine-structure lines by the clouds, and discuss in particular the line of C⁺ at 158 µm observed in some of them.