Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...281..199s&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 281, no. 1, p. 199-208
Astronomy and Astrophysics
Astronomy
7
Gas Temperature, H I Regions, Infrared Cirrus (Astronomy), Interstellar Matter, Kinematics, Absorption Spectra, Astronomical Models, Emission Spectra, Radial Velocity, Spin Temperature, Very Large Array (Vla)
Scientific paper
We present the results of a combined absorption and emission study of the H I 21 cm line toward several cirrus clouds at high galactic latitude. The absorption measurements were obtained with the Very Large Array (VLA) synthesis telescope, the emission data with the Effelsberg telescope. The three cirrus complexes studied were selected from the Deul & Burton (1990) study. Deul & Burton had shown that these cirrus clouds have associated H I emission with radial velocities near 0 km/s as well as with more extreme velocities of v less than -20 km/s. Absorption against background continuum sources was detected in 40% of the lines of sight investigated. The H I line was always found to be optically thin, tau less than 0.6. The O km/s component was found in each direction where absorption was detected; in addition, three directions showed absorption at intermediate negative velocities. The derived apparent spin temperatures vary between 20 K and greater than 350 K. The emission profiles reveal much more widely distributed gas than is visible in absorption. Only the cool clumps in the cirrus show up in absorption. The observations suggest a collision of a high velocity cloud with the galactic disk gas, by which different parts of the cirrus are at present seen in different phases of shock completion. This model may explain the large observed differences in the H I emission and absorption profiles over small distances on the sky, as well as the variation of the far-infrared brightness ratio.
Burton William Butler
Dickey John M.
Stark Ronald
Wennmacher Alexander
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