AFar Ultraviolet Spectroscopic ExplorerSurvey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution FUSE survey of H2 along 70 sight lines tothe Small and Large Magellanic Clouds, using hot stars as background sources.FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% ofSMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest columndensities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reducedH2 abundances in the Magellanic Clouds relative to the Milky Way, with averagemolecular fractions = 0.010 (+0.005, -0.002) for the SMC and =0.012 (+0.006, -0.003) for the LMC, compared with = 0.095 for theGalactic disk over a similar range of reddening. The dominant uncertainty inthis measurement results from the systematic differences between 21 cm radioemission and Lya in pencil-beam sight lines as measures of N(HI). These resultsimply that the diffuse H2 masses of the LMC and SMC are 8 x 10^6 Msun and 2 x10^6 Msun, respectively, 2% and 0.5% of the H I masses derived from 21 cmemission measurements. The LMC and SMC abundance patterns can be reproduced inensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x10^-18 cm^3 s^-1, and incident radiation fields ranging from 10 - 100 times theGalactic mean value. We find that these high-radiation, low-formation-ratemodels can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotationalexcitation ratios in the Clouds. We use H2 column densities in low rotationalstates (J = 0 and 1) to derive a mean kinetic and/or rotational temperature = 82 +/- 21 K for clouds with N(H2) >= 10^16 cm^-2, similar to Galacticgas. We discuss the implications of this work for theories of star formation inlow-metallicity environments. [Abstract abridged]