Submillimeter and infrared spectroscopy of molecular clouds near O and B stars

Details Submillimeter and infrared spectroscopy of molecular clouds near O and B stars Submillimeter and infrared spectroscopy of molecular clouds near O and B stars

Aug 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt.........5h&link_type=abstract

Ph.D. Thesis Texas Univ., Austin.

Astronomy and Astrophysics

Astronomy

1

B Stars, Infrared Astronomy, Infrared Spectra, Interstellar Gas, Molecular Clouds, O Stars, Submillimeter Waves, Astronomical Spectroscopy, Emission Spectra, Excitation, Line Spectra, Molecular Gases, Molecular Spectra, Near Infrared Radiation, Photodissociation, Ultraviolet Radiation

Scientific paper

The structure and physical properties of molecular cloud boundaries illuminated by ultraviolet radiation from nearby or embedded O and B stars are investigated by observing atomic and molecular emission lines. The selected lines are sensitive to the local gas conditions and to the distribution of the UV field. Since the excitation of interstellar gas by UV photons extends to depths comparable to all but the highest column densities of gas in clouds illuminated by nearby OB stars, it is necessary to learn the physical conditions in the UV-illuminated gas if we are to understand the properties of the bulk of the mass of the interstellar medium. We mapped the spatial distribution of the 158 micron (C II) fine-structure line in the star-forming region W3, the ionization front NGC 1977, and the reflection nebula NGC 2023 to investigate the outer atomic layers of the UV-illuminated molecular gas. The (C II) emission arises from warm (100-300 K), dense (nH greater than 104 cm-3) photodissociation regions extending over parsec scales or greater. Clumpy or filamentary clouds with a density contrast of 100 or more can explain the distribution and intensity of the emission by allowing deep penetration of carbon-ionizing UV photons into the clouds. We observed near-IR H2 rovibrational lines in NGC 2023 to examine the properties of the outermost molecular gas layers. The density is high (nH2) approx. greater than 105 cm-3 right to the boundary of the molecular gas. The UV field dominates the heating and drives the temperature as high as approximately 500 K. At these temperatures and densities, collisional excitation becomes important in determining the H2 level populations. We mapped the distribution of emission from the J = 7 to 6 transition of CO in Orion to study the properties and structure of regions further into the UV-dominated molecular layers. We observe quiescent emission from gas with temperatures greater than 40 K extending over 2 pc. The temperature peaks to greater than 160 K at the position of Ori theta1C, more than twice the dust temperature. Multiple velocity components in the CO (7 to 6) lines suggest that the emission arises from photodissociation regions on the surfaces of several clumps or filaments embedded with the cloud.

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