We describe a moderate-resolution FUSE survey of H _ { 2 } along 70 sight lines to the 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 % of SMC ) exhibit absorption lines from the H _ { 2 } Lyman and Werner bands between 912 and 1120 Å .
Our survey is sensitive to N ( H _ { 2 } ) \geq 10 ^ { 14 } cm ^ { -2 } ; the highest column densities are log N ( H _ { 2 } ) = 19.9 in the LMC and 20.6 in the SMC .
We find reduced H _ { 2 } abundances in the Magellanic Clouds relative to the Milky Way , with average molecular fractions \langle f _ { H 2 } \rangle = 0.010 ^ { +0.005 } _ { -0.002 } for the SMC and \langle f _ { H 2 } \rangle = 0.012 ^ { +0.006 } _ { -0.003 } for the LMC , compared with \langle f _ { H 2 } \rangle = 0.095 for the Galactic disk over a similar range of reddening .
The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Ly \alpha in pencil-beam sight lines as measures of N ( H I ) .
These results imply that the diffuse H _ { 2 } masses of the LMC and SMC are 8 \times 10 ^ { 6 } M _ { \odot } and 2 \times 10 ^ { 6 } M _ { \odot } , respectively , 2 % and 0.5 % of the H I masses derived from 21 cm emission measurements .
The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H _ { 2 } formation rate coefficient , R \sim 3 \times 10 ^ { -18 } cm ^ { 3 } s ^ { -1 } , and incident radiation fields ranging from 10 - 100 times the Galactic mean value .
We find that these high-radiation , low-formation-rate models can also explain the enhanced N ( 4 ) /N ( 2 ) and N ( 5 ) /N ( 3 ) rotational excitation ratios in the Clouds .
We use H _ { 2 } column densities in low rotational states ( J = 0 and 1 ) to derive kinetic and/or rotational temperatures of diffuse interstellar gas , and find that the distribution of rotational temperatures is similar to Galactic gas , with \langle T _ { 01 } \rangle = 82 \pm 21 K for clouds with N ( H _ { 2 } ) \geq 10 ^ { 16.5 } cm ^ { -2 } .
There is only a weak correlation between detected H _ { 2 } and far-infrared fluxes as determined by IRAS , perhaps due to differences in the survey techniques .
We find that the surface density of H _ { 2 } probed by our pencil-beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps .
We discuss the implications of this work for theories of star formation in low-metallicity environments .