Formation Pumping of Molecular Hydrogen in Dark Clouds

TL;DR

This study models the formation and infrared emission of molecular hydrogen in dark clouds, predicting observable signatures of excited H2 states and explaining past non-detections.

Contribution

It introduces a detailed model of H2 formation pumping in dense clouds, incorporating recent laboratory data and interstellar UV effects.

Findings

Some vibrationally excited H2 states may be detectable near H2 destruction zones.

The model explains why previous observations failed to detect H2 emission.

Predictions suggest specific conditions where IR emission from H2 can be observed.

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.