H2 in Molecular Clouds

Details H2 in Molecular Clouds H2 in Molecular Clouds

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aspc..417..177g&link_type=abstract

Submillimeter Astrophysics and Technology: a Symposium Honoring Thomas G. Phillips ASP Conference Series, Vol. 417, proceedings

Astronomy and Astrophysics

Astrophysics

Scientific paper

We report the detection of the S(0) and S(1) pure rotational transitions of H2 in emission from a boundary layer of the Taurus Molecular Cloud. These lines were observed with the Spitzer Infrared Spectrograph (IRS), and are very weak, with maximum specific intensity ≤0.4 MJy sr-1. The emission is strongest 8.5 arcmin (0.33 pc) outside the edge of the cloud defined by the 13CO. We see H2 emission towards the bulk of the cloud at a lower level, suggestive of limb brightening of an envelope around the purely molecular cloud. The peak column densities looking directly towards the cloud are N(J=3) = 1.3×1017 cm-2, and N(J=2) = 2.2×1018 cm-2. Towards the maximum of the rotationally excited H2H emission, N(J=3) = 5.2×1017 cm-2 and N(J=2) = 4.4×1018 cm-2. Interpreting the ratio is made difficult by the not well determined ortho- to para-H2 ratio (OPR), but for reasonable values of the OPR, we can constrain the characteristics of the region responsible for the H2 emission. For example, for OPR = 3 and n(H2) = 100 cm-3, TK must be greater than approximately 200 K. We have used the Meudon PDR code to model the H2 emission from the cloud edge, and find that while the column density in J = 2 can be reproduced with reasonable parameters for the region and its environment, the column density in J = 3 that is predicted falls more than a factor of 5 below that observed. This suggests that an additional source of H2 excitation is present. Further data and modeling are required to understand this issue, which will improve our understanding of cloud structure and the physics of H2 formation.

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