Chemical rates on small grains and PAHs: c+ recombination and H2Formation.
WOLFIRE M.G., TIELENS A.G.G.M., HOLLENBACH D. and KAUFMAN M.J.
Abstract (from CDS):
We use observations of the C I, C II, H I, and H2 column densities along lines of sight in the Galactic plane to determine the formation rate of H2 on grains and to determine chemical reaction rates with polycyclic aromatic hydrocarbons (PAHs). Photodissociation region models are used to find the best-fit parameters to the observed columns. We find the H2formation rate on grains has a low rate (R∼1x10–17 cm3/s) along lines of sight with low column density (AV≲0.25) and low molecular fraction (fH2≲10–4). At higher column densities (0.25≤AV≤2.13), we find a rate of R∼3.5x10–17 cm3/s. The lower rate at low column densities could be the result of grain processing by interstellar shocks, which may deplete the grain surface area or process the sites of H+H formation, thereby inhibiting H2production. Alternatively, the formation rate may be normal, and the low molecular fraction may be the result of lines of sight that graze larger clouds. Such lines of sight would have a reduced H2self-shielding compared to the line-of-sight column. We find the reaction C++PAH–⟶C+PAH0 is best fit with a rate 2.4x10–7ΦPAHT–0.52cm3/s with T2=T/100 K, and the reaction C++PAH0⟶C+PAH+ is best fit with a rate 8.8x10–9ΦPAHcm3/s. In high-column-density gas, we find ΦPAH∼0.4. In low-column-density gas, ΦPAH is less well constrained, with ΦPAH∼0.2-0.4.