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Session 51 - New Views of the Magellanic Clouds - I.
Oral session, Wednesday, June 12
Historical Society,
We have mapped emission and absorption by the 21-cm line of H \tt I in the vicinity of 30 Dor using data from the Australia Telescope Compact Array and the Parkes Telescope. These maps have spatial resolution \sim30'' and velocity resolution 0.8 km s^-1. The total area of the three mosaiced fields is about 0.7 square degrees, which includes the northern end of the giant molecular cloud, the site of SN1987a, and the continuum sources N157b and N158a, in addition to 30 Dor.
The absorption toward 30 Dor shows strong velocity components at v_hel = 280 - 285 km s^-1 (the disk or ``D'' component) and at v_hel = 260 - 265 km s^-1 (the low velocity, ``L'', component). Thus both of these structures must be at least partly in front of 30 Dor. Comparing the absorption with interpolated emission spectra we find the characteristic signatures of various thermal components : a relatively widespread warm neutral medium with temperature greater than \sim700 K and several cool phase structures with temperatures of 50 to 150 K. The cool ``clouds'' all show spatial variations over the \sim50 pc extent of 30 Dor, but they show large scale continuity with the major emission structures in the surrounding region.
The brightest emission (T_b \simeq 180 K) is found in the direction of the giant molecular cloud at about 255 km s^-1. The spin temperature of this gas is apparently about 100 K, which probably represents an average over cloud gas which may be much cooler and warm gas which may be spatially associated with the cloud. The H \tt I column density must be at least 10^22 cm^-2 in this structure. It may be that this represents a threshold column density for shielding molecules in the LMC interstellar medium, in contrast to the corresponding threshold of about 3 to 5 x 10^20 cm^-2 in the Galactic ISM.
