The Effects of Polycyclic Aromatic Hydrocarbons on the Chemistry of Photodissociation Regions

Abstract

We have investigated the effects of including polycylic aromatic hydrocarbons (PAHs) on the abundance of neutral atoms and molecules for two typical photodissociation regions (PDRs): a high-density case (the Orion complex) and a low-density case. PAHs provide a large surface area for chemistry between themselves and the gas phase and influence this chemistry directly through their charge state. The PAH charge is dominated by the balance of the photoelectric ejection of electrons and direct PAH-electron recombination. This means that the incident UV flux and gas density determine the PAH charge balance. We have investigated a wide parameter space for the incident far-ultraviolet flux (measured in units of G0 = 1.6 × 10-3 ergs s-1 cm-2) varying as 10 < G0 < 105 and gas density, nH, where 102 cm-3 < nH < 105 cm-3. Because PAHs are highly efficient charge exchange intermediaries, our results show that their presence in the PDR strongly influences the gas phase abundance of neutral atomic carbon, sulfur, silicon, and metals throughout the PDR by mutual neutralization. We have determined the effects of PAHs on the column density and line intensity of the chemical species in the PDR, and we present these results as functions of depth into the PDR, G0, and nH. We discuss the PDR model in detail for the Orion PDR. Typically, the presence of PAHs in a PDR increases the peak C I line intensities by a factor of 2. Our model for the Orion PDR produces a [C I] 610 μm line intensity of 1.3 × 10-5 ergs cm-2 s-1, which compares well with the observed peak intensity toward this source (10-5 ergs cm-2 s-1). We conclude that PAHs play a central role in the gas phase chemistry and that they are crucial to our understanding of the structure and evolution of star-forming regions.