RED FLUORESCENT LINE EMISSION FROM HYDROGEN MOLECULES IN DIFFUSE MOLECULAR CLOUDS

We have modeled the fluorescent pumping of electronic and vibrational emissions of molecular hydrogen (H-2) within diffuse molecular clouds that are illuminated by ultraviolet continuum radiation. Fluorescent l ine intensities are predicted for transitions at ultraviolet, infrared , and red visible wavelengths as functions of the gas density, the vis ual extinction through the cloud, and the intensity of the incident UV continuum radiation. The observed intensity in each fluorescent trans ition is roughly proportional to the integrated rate of H-2 photodisso ciation along the line of sight. Although the most luminous fluorescen t emissions detectable from ground-based observatories lie at near-inf rared wavelengths, we argue that the lower sky brightness at visible w avelengths makes the red fluorescent transitions a particularly sensit ive probe. Fabry-Perot spectrographs of the type that have been design ed to observe very faint diffuse H alpha emissions are soon expected t o yield sensitivities that will be adequate to detect H-2 vibrational emissions from molecular clouds that are exposed to ultraviolet radiat ion no stronger than the mean radiation field within the Galaxy. Obser vations of red H-2 fluorescent emission together with cospatial 21 cm H I observations could serve as a valuable probe of the gas density in diffuse molecular clouds.