15min:

P. B. RIMMER, Department of Physics, Ohio State University, Columbus, OH 43210; O. MORATA, Institute of Astronomy and Astrophysics, Academia Sinica, Tapei 11677, Taiwan; E. ROUEFF, Observatoire de Paris, LUTH and Université Denis Diderot, Place J. Janssen 92190 Meudon, France; E. HERBST, Departments of Astronomy, Physics, and Chemistry, Ohio State University, Columbus, OH 43210.

The molecules CCH, C₄H, HCO⁺, and HC₃N have been observed at the edge of the Horsehead Nebula in abundances far higher than those predicted by Photodissociation Region (PDR) models. Using a column-dependent cosmic ray ionization rate ( ), we model the edge of the Horsehead Nebula as a one-dimensional nearly-``edge-on'' heterogeneous PDR with temperature ranging from 15-250 K, number density from 10³ - 5 × 10⁵ cm^-3, and = 10^-16 - 10^-14 s^-1. The resulting abundances for the molecules listed above are much closer to the observed abundances. In this talk, we will discuss this method, its results, and the usefulness of incorporating a column-dependent in astrochemical PDR models, especially in the advent of ALMA.