Formation Pumping of Molecular Hydrogen in Dark Clouds

Abstract

Many theoretical and laboratory studies predict H2 to be formed in highly excited ro-vibrational states. The consequent relaxation of excited levels via a cascade of infrared transitions might be observable in emission from suitable interstellar regions. In this work, we model H2 formation pumping in standard dense clouds, taking into account the H/H2 transition zone, through an accurate description of chemistry and radiative transfer. The model includes recent laboratory data on H2 formation, as well as the effects of the interstellar UV field, predicting the populations of gas-phase H2 molecules and their IR emission spectra. Calculations suggest that some vibrationally excited states of H2 might be detectable towards lines of sight where significant destruction of H2 occurs, such as X-ray sources, and provide a possible explanation as to why observational attempts resulted in no detections reported to date.

32 pages, 8 figures, 5 tables

References in corpus (4)

• An Analysis of the Shapes of Interstellar Extinction Curves. V. The IR-Through-UV Curve Morphology
• Desorption From Interstellar Ices
• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
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Cited by in corpus (8)

• Grackle: a Chemistry and Cooling Library for Astrophysics
• Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes
• Surface chemistry in the interstellar medium - I - H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms
• H2 formation on interstellar grains and the fate of reaction energy
• Cosmic rays in molecular clouds probed by H${}_2$ rovibrational lines -- Perspectives for the James Webb Space Telescope
• Calculation of Electric Quadrupole Linestrengths for Diatomic Molecules: Application to the H2, CO, HF and O2 Molecules
• Cold Clouds as Cosmic-Ray Detectors
• Ortho-to-Para Ratio Studies of Shocked H2 Gas in the Two Supernova Remnants IC 443 and HB 21