HiNarrow Self‐Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation

We present the results of a comparative study of H I narrow self-absorption (HINSA), OH, 13 CO, and C 18 O in five dark clouds. We find that the HINSA generally follows the distribution of the emission of the carbon monoxide isotopologs and has a characteristic size close to that of 13 CO. This confirms earlier work that determined that the HINSA is produced by cold H I that is well mixed with molecular gas in well-shielded regions. The OH and 13 CO column densities are essentially uncorrelated for the sources other than L1544. Our observations indicate that the central number densities of H I are between 2 and 6 cm -3 and that the ratio of the hydrogen density to total proton density for these sources is (5-27) × 10 -4 . Using cloud temperatures and the density of atomic hydrogen, we set an upper limit to the cosmic-ray ionization rate of 10 -16 s -1 . We present a model for H I to H 2 conversion in well-shielded regions that includes cosmic-ray destruction of H 2 and formation of this species on grain surfaces. We include the effect of a distribution of grain sizes, and we find that for an MRN distribution, the rate of H 2 formation is increased by a factor of 3.4 relative to that for a model with a single grain radius of 1700 Å. Comparison of observed and modeled fractional H I abundances indicates ages for these clouds, defined as the time since the initiation of H → H 2 conversion, to be 10 6.5 -10 7 yr. Several effects may make this time a lower limit, but the low values of n that we have determined make it certain that the timescale for evolution from a possibly less dense atomic phase to an almost entirely molecular phase must be a minimum of several million years. This clearly sets a lower limit to the overall timescale for the process of star formation and the lifetime of molecular clouds.