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J. Tumlinson, J. M. Shull, B. L. Rachford, T. P. Snow (CASA, University of Colorado), K. R. Sembach (Johns Hopkins), E. B. Jenkins (Princeton), D. G. York (Chicago), FUSE Team
We present the results of an extensive survey of diffuse molecular hydrogen (H2) in the ISM of the Small and Large Magellanic Clouds with the Far Ultraviolet Spectroscopic Explorer (FUSE). These observations were obtained in late 1999 and October 2000 during two campaigns of LMC and SMC observing by FUSE. To date, roughly 60% of our ~90 sightlines to the LMC and SMC exhibit H2 absorption at LMC (+270 km s-1) or SMC (+130 km s-1) velocities. We examine the rotational populations, N(J), of these clouds and characterize the distributions of rotational temperature T01, excitation temperature T\rm ex, doppler b, and molecular fraction (f\rm H2).
We find a range of H2 column densities, N(H2) = 1014 -- 1020 cm-2, with no correlation between N(H2) and CO emission contours, IRAS 100 \mum flux, or color excess. We find tentative evidence for reduced f\rm H2 in the Clouds relative to the Milky Way. This result may indicate reduced formation rates at low metallicity and/or enhanced UV photodissociation. We use N(J) to constrain cloud models of the gas density and UV photodissociation rate. Our survey is a first step towards a direct CO/H2 measurement at low metallicity and a general picture of molecule formation and excitation that incorporates metallicity and radiation in detail.
This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support to U. S. participants has been provided by NASA contract NAS5-32985.