MOLECULAR HYDROGEN EMISSION FROM THE BOUNDARIES OF THE TAURUS MOLECULAR CLOUD

We report Spitzer Space Telescope observations of the four lowest rotational transitions of H-2 in three portions of the boundary of the Taurus molecular cloud. Emission in the two lowest transitions, S(0) and S(1), was detected in almost all pointing directions, while the S(2) and S(3) lines were marginally detected only after further averaging of data. The widespread detection of lines coming from levels 510 K and 1016 K above the molecular ground state is indicative of gas at a temperature of at least 200 K containing column densities (1-5) x 10(18) cm(-2) of H-2. For the region with the simplest geometry, we have used the Meudon PDR code to model the chemistry, radiative transfer, and excitation of molecular hydrogen. We conclude that models with acceptable values of the UV interstellar radiation field can reproduce the amount of H-2 in the lowest excited state, but cannot account for the degree of excitation of the H-2. The unexpectedly high degree of excitation of the H-2 in the boundary layer of a molecular cloud, which cannot be explained by the presence of stellar sources, points to an enhanced heating rate which may be the result of, e. g., dissipation of turbulence. We have in one boundary region been able to obtain the ortho-to-para ratio (OPR) for H-2, which by modeling and possible detection of the S(2) and S(3) lines has a range 1.0 >= OPR >= 0.15, although this result must be treated with caution. The fact that the ortho-to-para ratio is lower than that expected for equilibrium at the gas kinetic temperature may be indicative of circulation of material from cold, purely molecular regions into the boundary layer, possibly due to turbulent diffusion. The explanation of these data may thus be suggestive of processes that are having a significant effect on the structure and evolution of molecular clouds and the star formation that takes place within them.