Chemical Rates on Small Grains and PAHs: C^+ Recombination and H_2 Formation

Abstract

We use observations of the CI, CII, HI, and H_2 column densities along lines of sight in the Galactic plane to determine the formation rate of H_2 on grains and to determine chemical reaction rates with Polycyclic Aromatic Hydrocarbons. Photodissociation region models are used to find the best fit parameters to the observed columns. We find the H_2 formation rate on grains has a low rate (R ~ 1 x 10^(-17) cm^(3) s^(-1)) along lines of sight with low column density (A_V < 0.25) and low molecular fraction (f_(H_2) < 10^(-4)). At higher column densities (0.25 < A_V <2.13), we find a rate of R ~ 3.5x10^(-17) cm^(3) s^(-1). 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 H_2 production. Alternatively, the formation rate may be normal, and the low molecular fraction may be the result of lines of sight which graze larger clouds. Such lines of sight would have a reduced H_2 self-shielding compared to the line-of-sight column. We find the reaction C^+ +PAH^- --> C + PAH^0 is best fit with a rate 2.4 x 10^(-7) Φ_PAH T_2^(-0.5) cm^(3) s^(-1) with T_2= T/100 K and the reaction C^+ + PAH^0 --> C + PAH^+ is best fit with a rate 8.8x 10^(-9)Φ_PAH cm^(3) s^(-1). In high column density gas we find Φ_PAH ~ 0.4. In low column density gas, Φ_PAH is less well constrained with Φ_PAH ~ 0.2 - 0.4.