Molecular Cloud Structure in the Magellanic Clouds: Effect of Metallicity

The chemical structure of neutral clouds in low metallicity environments isexamined with particular emphasis on the H to H_2 and C+ to CO transitions. Weobserved near-IR H_2 lines and the CO J=1-0 line from 30 Doradus and N159/N160in the Large Magellanic Cloud and from DEM S 16, DEM S 37, and LI-SMC 36 in theSmall Magellanic Cloud. We find that the H_2 emission is UV-excited and that(weak) CO emission always exists (in our surveyed regions) toward positionswhere H_2 and [CII] emission have been detected. Using a PDR code and aradiative transfer code, we simulate the emission of line radiation fromspherical clouds and from large planar clouds. Because the [CII] emission andH_2 emission arise on the surface of the cloud and the lines are opticallythin, these lines are not affected by changes in the relative sizes of theneutral cloud and the CO bearing core, while the optically thick CO emissioncan be strongly affected. The sizes of clouds are estimated by measuring thedeviation of CO emission strength from that predicted by a planar cloud modelof a given size. The average cloud column density and therefore size increasesas the metallicity decreases. Our result agrees with the photoionizationregulated star formation theory by Mc Kee (1989).Comment: 45 Pages including 15 figures. To be published in the ApJ May 10, 1998 issue, Vol. 49