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Session 51 - New Views of the Magellanic Clouds - I.
Oral session, Wednesday, June 12
Historical Society,
Using the Antarctic Submillimeter Telescope/Remote Observatory (AST/RO), in operation since January 1995, we have observed the ^3P_1\rightarrow^3P_0 492 GHz transition of neutral atomic carbon [CI] towards two positions in the Large Magellanic Cloud: the compact HII region N159 and the massive star formation region 30 Doradus. Neutral carbon, a product of the photodissociation of CO, traces the transition zone of the interstellar medium occurring at visual extinctions of a few (A_v\sim 1-3) where the presence of H_2 is not traced by CO. The extent of these so called photodissociation regions (PDRs) is enhanced in metal-poor systems, where low C and O abundances are coupled with low dust-to-gas ratios. Lower CO abundances result in diminished CO self-shielding, and lower dust-to-gas ratios decrease dust shielding. Consequently, the beam filling factor of the PDR is augmented, which leads to enhanced I_[CI]/I_CO intensity ratios. Our observation of N159 yields a ratio of I_[CI]/I_^12CO (1-0)\approx 0.2 when data is compared in the same beam. In the Galaxy this ratio is roughly 0.1. Towards 30 Doradus AST/RO has detected the faintest [CI] line ever seen, with a peak brightness of 28 mK. The ratio I_[CI]/I_^12CO (1-0)\approx 0.05 found for this region possibly reflects the extremely intense UV field to which the gas is exposed. This research was supported in part by the National Science Foundation under a cooperative agreement with the Center for Astrophysical Research in Antarctica (CARA), grant number NSF OPP 89-20223. CARA is a National Science Foundation Science and Technology Center.
