Emission-absorption observations of OH in diffuse interstellar clouds

Details Emission-absorption observations of OH in diffuse interstellar clouds Emission-absorption observations of OH in diffuse interstellar clouds

May 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a....98..271d&link_type=abstract

Astronomy and Astrophysics, vol. 98, no. 2, May 1981, p. 271-285.

Astronomy and Astrophysics

Astronomy

51

Astronomical Spectroscopy, Hydrogen Clouds, Hydroxyl Emission, Interstellar Gas, Radio Sources (Astronomy), Absorption Spectra, Astronomical Catalogs, Centimeter Waves, Collision Parameters

Scientific paper

Results of a search for OH emission and absorption in diffuse interstellar clouds are presented. The Nancay radio telescope was used to search 58 regions showing H I absorption toward extragalactic background continuum sources at a wavelength of 18 cm. Sixteen of these regions are observed to exhibit OH absorption and/or emission, with low-latitude clouds more likely to show OH absorption than high-latitude clouds. The OH absorption lines are found to be narrower than the H I absorption lines, with little or no correlation between the linewidths in the two species, indicating that OH is spatially restricted to preferred regions. The emission and absorption observations are used to derive excitation temperatures for the OH main lines between 4 and 8 K, which, combined with measurements of H I spin temperatures, imply cloud densities of 30-100/cu cm and pressures of 4000 K/cu cm. An apparent correlation between the OH excitation temperature and the 21-cm opacity of the clouds is also noted.

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