MOLECULAR CLOUD STRUCTURE IN THE MAGELLANIC CLOUDS - EFFECT OF METALLICITY

The chemical structure of neutral clouds in low-metallicity environmen ts is examined, with particular emphasis on the H to H-2 and C+ to CO transitions. We observed near-IR H-2 (1, 0) S(1), (2, 1) S(1), and (5, 3) 0(3) lines and the (CO)-C-12 J = 1 --> 0 line from 30 Doradus and N159/N160 in the Large Magellanic Cloud and from DEM S 16, DEM S 37, a nd LI-SMC 36 in the Small Magellanic Cloud. We find that the H, emissi on is UV-excited and that tweak) CO emission always exists tin our sur veyed regions) toward positions where H-2 and [C II] emission have bee n detected. Using a PDR code and a radiative transfer code, we simulat e the emission of line radiation from spherical clouds and from large planar clouds. Because [C II] emission and H-2 emission arise on the s urface of the cloud and because the lines are optically thin, these li nes are not affected by changes in the relative sizes of the neutral c loud and the GO-bearing core, while the optically thick CO emission ca n be strongly affected. The sizes of clouds are estimated by measuring the deviation of CO emission strength from that predicted by a planar cloud model of a given size. The average cloud column density, and th erefore its size, increases as the metallicity decreases. Our result a grees with the photoionization-regulated star formation theory of McKe e.