The life and death of H2 in a UV-rich environment - Towards a better understanding of H2 excitation and destruction

Grants and Contracts Details


Molecular hydrogen (H2) is the most abundant molecule in the universe. H2 is also a catalyst for the formation of other molecules like CO that are used to infer physical conditions in molecular gas. Therefore, it is crucial to understand H2 formation and destruction processes. Most existing H2 UV absorption studies sample regimes where H2 has already formed, and the level populations are thermalized. However, the Orion Veil, a thin layer immediately (~ 1 pc) in front of the Trapezium stars, offers a different environment. There, the fractional abundance of H2 is low, and UV radiation pumps H2 to high vibration and rotation levels. In this environment, comparisons between theory and observation provide stringent tests of models of H2 formation and destruction. For the Veil, we seek to compare STIS observations of vibrationally and rotationally excited H2 absorption lines with predictions of chemical models in the Cloudy code. Model calculations predict we will detect dozens of H2 lines from high vibration and rotational levels in the wavelength range 1133-1335A. We hope to address three important questions: (1) Why do chemical models fail to reproduce the observed spectrum of H2 in high rotation and vibration levels? (2) Why are observed PDR temperatures inconsistent with known processes of heating and cooling? (3) Is the deduced H2 destruction rate consistent with recent theories of formation on grain surfaces? Answers to these questions will significantly improve chemical models in Cloudy. Since Cloudy is publicly available, improvements in its chemical models will benefit other investigators studying the physics of molecular gas and PDRs.
Effective start/end date3/1/147/31/16


  • Space Telescope Science Institute: $6,869.00


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