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D. Li, P. F. Goldsmith (NAIC, Cornell University)
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]=3\times 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.
The author(s) of this abstract have provided an email address for comments about the abstract: dl42@cornell.edu