HI Narrow Line Absorption in Dark Clouds

We have used the Arecibo telescope to carry out an unbiased survey of 31 dark clouds in the Taurus/Perseus region for narrow absorption features in HI (λ 21cm) and OH (1667 and 1665 MHz) emission. We detected HI narrow line absorption (HINLA) in about 80% of the clouds that we observed. HINLA and OH emission, observed simultaneously are remarkably well correlated. Spectrally, they have the same nonthermal line width and the same peak velocity. Spatially, they both peak at the optically–selected central position of each cloud, and both fall off toward the cloud edges. Sources with clear HINLA feature have also been observed in transitions of CO, CO, CO, and CI. HINLA exhibits better correlation with molecular tracers than with CI. The line width of the absorption feature, together with analyses of the relevant radiative transfer provide upper limits to the kinetic temperature of the gas producing the HINLA. Some sources must have a temperature close to or lower than 10 K. The correlation of column densities and line widths of HINLA with those characteristics of molecular tracers suggest that a significant fraction of the atomic hydrogen is located in the cold, well–shielded portions of molecular clouds, and is mixed with the molecular gas. The average number density ratio [HI]/[H2] is 1.5 × 10−3. The inferred HI density appears consistent with but is slightly higher than the value expected in steady state equilibrium between formation of HI via cosmic ray destruction of H2 and destruction via formation of H2 on grain surfaces. The distribution and abundance of atomic hydrogen in molecular clouds could be critical tests of dark cloud chemistry and structure, including issues of grain surface rates, PDRs, circulation, and turbulent diffusion. The HINLA would appear to have great importance as well as a possible tracer of magnetic fields in dark clouds. Subject headings: ISM: atoms – individual (hydrogen) Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, [email protected] National Astronomy and Ionosphere Center, Department of Astronomy, Cornell University, Ithaca NY 14853