HI Self-absorption in Dark Clouds

Details HI Self-absorption in Dark Clouds HI Self-absorption in Dark Clouds

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...195.7208l&link_type=abstract

American Astronomical Society, 195th AAS Meeting, #72.08; Bulletin of the American Astronomical Society, Vol. 31, p.1476

Astronomy and Astrophysics

Astronomy

1

Scientific paper

HI profiles in the Galaxy sometimes exhibit self-absorption. The low minimum temperatures (about 10K) of certain of these features suggests that this absorption is being produced by atomic hydrogen in ``dark'' molecular clouds. To verify this premise, profiles of both HI and molecular emission with high velocity resolution are needed. We have exploited the high sensitivity and broad frequency coverage of the Arecibo Gregorian system to obtain HI 21cm and OH (1665MHz, 1667MHz) profiles simultaneously toward a sample of nearby dark clouds, including TMC1, CB45, L1498, and B5. Our data show that OH emission and HI self-absorption profiles peak at the same velocity and have similar Gaussian shape. The correlation of molecular emission and HI self-absorption presents a way of determining the [HI]/[H2] ratio in dark clouds. In a region with high extinction and no embedded sources, such as TMC1, [HI]/[H2] is determined by the cosmic ray ionization rate, which is of considerable importance for heating of the gas and coupling to the magnetic field. Our HI map of TMC1 combined with a C18O map and the cloud kinetic temperature available from previous studies yields [HI]/[H2]=3x 10-3. This value is an order of magnitude lower than that given by Wilson and Minn (1977). The data as well as the significance of this result will be discussed.

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