FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight Toward HD 73882

摘要

We report the results of initial FUSE observations of molecular hydrogen (H2)in translucent clouds. These clouds have greater optical depth than any of thediffuse clouds previously observed for far-UV H2 absorption, and provide newinsights into the physics and chemistry of such regions. Our initial resultsinvolve observations of HD 73882, a well-studied southern hemisphere star lyingbehind substantial interstellar material (E(B-V) = 0.72; A_V = 2.44). We find atotal H2 column density, N(H2) = 1.2 x 10^{21} cm^{-2}, about three timeslarger than the values for diffuse clouds previously measured in the far-UV.The gas kinetic temperature indicated by the ratio N(J=1)/N(J=0) is 58 +/- 10K. With the aid of ground-based data to calculate an appropriatemulti-component curve of growth, we have determined column densities for allrotational levels up to J = 7. The J >= 2 states can be reasonably fitted witha rotational excitation temperature of 307 +/- 23 K. Both the kinetic androtational temperatures are similar to those found in previous investigationsof diffuse clouds. The ratios of carbonaceous molecules to hydrogen moleculesare also similar to ratios in diffuse clouds, suggesting a similar chemistryfor this line of sight.