Knowledge Bank

The molecules CCH, C$4$H, HCO$+$, and HC$3$N have been observed at the edge of the Horsehead Nebula in abundances far higher than those predicted by Photodissociation Region (PDR) models, 885 (2005)}. Using a column-dependent cosmic ray ionization rate ($\zeta$), 619 (2009)}, 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 $103$ - $5\times 105$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-3</mrow>}$, and $\zeta =10-16$ - $10-14$ s$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. 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 $\zeta$ in astrochemical PDR models, especially in the advent of ALMA.