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

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) Phi_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)Phi_PAH cm^(3) s^(-1). In high column density gas we find Phi_PAH ~ 0.4. In low column density gas, Phi_PAH is less well constrained with Phi_PAH ~ 0.2 - 0.4.

Variations of the Mid-IR Aromatic Features Inside and Among Galaxies

We present the results of a systematic study of mid-IR spectra of Galactic regions, Magellanic HII regions, and galaxies of various types (dwarf, spiral, starburst), observed by the satellites ISO and Spitzer. We study the relative variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially resolved objects (such as M82, M51, 30 Doradus, M17 and the Orion Bar), as well as among 90 integrated spectra of 50 objects. Our main results are that the 6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios between these bands and the 11.3 micron band varies by one order of magnitude. This implies that the properties of the PAHs are remarkably universal throughout our sample, and that the relative variations of the band ratios are mainly controled by the fraction of ionized PAHs. In particular, we show that we can rule out both the modification of the PAH size distribution, and the mid-infrared extinction, as an explanation of these variations. Using a few well-studied Galactic regions (including the spectral image of the Orion Bar), we give an empirical relation between the I(6.2)/I(11.3) ratio and the ionization/recombination ratio G0/ne.Tgas^0.5, therefore providing a useful quantitative diagnostic tool of the physical conditions in the regions where the PAH emission originates. Finally, we discuss the physical interpretation of the I(6.2)/I(11.3) ratio, on galactic size scales.