Near-Infrared Hydrogen Emission as a Probe of the Global Energetics and Morphology of Molecular Clouds.

Details Near-Infrared Hydrogen Emission as a Probe of the Global Energetics and Morphology of Molecular Clouds. Near-Infrared Hydrogen Emission as a Probe of the Global Energetics and Morphology of Molecular Clouds.

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........19l&link_type=abstract

Thesis (PH.D.)--THE UNIVERSITY OF TEXAS AT AUSTIN, 1995.Source: Dissertation Abstracts International, Volume: 56-10, Section: B,

Physics

Scientific paper

This thesis presents the results of a project to develop a new probe of the interstellar medium: large -scale observations of H_2 line emission in the near-infrared. When exposed to ultraviolet radiation from stars and/or shock waves, interstellar H_2 emits radiation at near-infrared wavelengths (1 -5 μm). By observing the near-infrared emission from H_2, one can examine the physical and excited state of molecular clouds and study the degree to which stars interact with their environs. Even though most of the molecular gas in the interstellar medium is H_2, the poor surface brightness sensitivity of most modern-day instruments generally limits direct detection of H_2 to highly energetic regions of no more than a few arc minutes in angular extent. Toward interstellar regions subtending several degrees or more, near-infrared H_2 emission is very faint and difficult to detect. In this thesis, I describe a new Fabry-Perot spectrometer constructed for this project and designed to detect diffuse Hz emission from the large-scale interstellar medium. The spectrometer is optimized for the detection of extended, low surface brightness line emission from 1.4 μm to 2.4 mum. With the spectrometer, I have observed the 1.601 mu m v = 6-4 Q(1), 2.121 μm v = 1-0 S(1), and 2.247 μm v = 2 -1 S(1) lines of H_2 from a variety of galactic interstellar environments displaying a wide range of physical and excitation conditions. I discuss the observational results, which include the following: (1) Near-infrared H_2 line emission is extremely widespread, detectable from the outlying regions of molecular clouds; (2) The distribution and strength of the H_2 lines demonstrate that thermal excitation cannot account for the large-scale emission and that ultraviolet-pumping (ultraviolet fluorescence) excites the H_2; (3) In the vicinity of dense cloud cores, both thermal and ultraviolet processes excite H_2, the relative contribution of each mechanism to the emergent line emission depending on the density and temperature of the gas; (4) Ultraviolet -excited H_2 emission can extend into regions where molecular tracers such as CO are dissociated; and (5) There is a significant correlation between ultraviolet -excited H_2 emission and the intensity of the far-infrared continuum, which implies an external stellar field both excites H_2 and heats the dust on the surfaces of molecular clouds.

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