The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds

We have investigated the time scale for formation of molecular clouds by examining the conversion of HI to H2 using a time–dependent model which includes H2 photodissociation with rate dependent on dust extinction and self shielding. H2 formation on dust grains and cosmic ray destruction are also included in one–dimensional model slab clouds which incorporate time–independent density and temperature distributions. We calculate 21cm spectral line profiles seen in absorption against a background provided by general Galactic HI emission, and compare the model spectra with HI Narrow Self–Absorption, or HINSA, profiles absorbed in a number of nearby molecular clouds. The time evolution of the HI and H2 densities is dramatic, with the atomic hydrogen disappearing in a wave propagating from the central, denser regions which have a shorter H2 formation time scale, to the edges, where the density is lower and the time scale for H2 formation longer. The model 21cm spectra are characterized by very strong absorption at early times, when the HI column density through the model clouds is extremely large. Excess emission produced by the warm edges of the cloud when the background temperature is relatively low can be highly confusing in terms of separating the effect of the foreground cloud from variations in the background spectrum. The minimum time required for a cloud to have evolved to its observed configuration, based on the model spectra, is set by the requirement that most of the HI in the outer portions of the cloud, which otherwise overwhelms the narrow absorption, be removed. The characteristic time that has elapsed since cloud compression and initiation of the HI → H2 conversion is a few × 10 14 s or ≃ 10 yr. This sets a minimum time for the age of these molecular clouds and thus for star formation that may take place within them. JPL, 4800 Oak Grove Drive, Pasadena CA 91109, [email protected] JPL, 4800 Oak Grove Drive, Pasadena CA 91109, [email protected] Department of Astronomy, Cornell University, Ithaca NY 14853 [email protected]