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“Cold Atomic Hydrogen, Narrow Self-Absorption, and the Age of Molecular Clouds” Dr. Paul Goldsmith JPL |
| We discuss a critical aspect of the formation and evolution of interstellar clouds the conversion of atomic to molecular hydrogen which marks the transition from an atomic to a molecular cloud. This is a first, essential step in the process of star formation. Observations have revealed that there remains a small amount of cold, atomic gas even in the cores of dense, essentially molecular clouds, and that this gas is well mixed with the 13CO and C18O which are generally used to trace relatively quiescent cloud cores. This gas shows up as very narrow HI self-absorption features against Galactic HI emission background, and we thus call it HINSA. Robust and relatively simple models can be used to follow the evolution of the HI to H2 conversion process. The observed HI/H2 ratio and HINSA spectra allow determination of the time elapsed since the initiation of the process, which is on the order of 107 yr. This is a valuable measure of the time scale for molecular cloud and star formation. |