H I Narrow Self-Absorption in Dark Clouds

We have used the Arecibo telescope to carry out a survey of 31 dark clouds in the Taurus/Perseus region for narrow absorption features in H I (21 cm) and OH (1667 and 1665 MHz) emission. We detected H I narrow self-absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH emission, observed simultaneously, are remarkably well correlated. Spectrally, they have the same nonthermal line width and the same line centroid 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 HINSA features have also been observed in transitions of CO, ¹³CO, C¹⁸O, and C I. HINSA exhibits better correlation with molecular tracers than with C I. The line width of the absorption feature, together with analyses of the relevant radiative transfer, provides upper limits to the kinetic temperature of the gas producing the HINSA. Some sources must have a temperature close to or lower than 10 K. The correlation of column densities and line widths of HINSA with those characteristics of molecular tracers suggests 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 [H I]/[H₂] is 1.5×10^-3. The inferred H I density appears consistent with, but slightly higher than, the value expected in steady state equilibrium between formation of H I 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 are critical tests of dark cloud chemistry and structure, including the issues of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.