2012A&A...537A...7R

Observations of small carbon-bearing molecules such as CCH, C₄H, c-C₃H₂, and HCO in the Horsehead nebula have shown these species to have higher abundances towards the edge of the source than towards the center. Given the determination of a wide range of values for ζ (s^–1), the total ionization rate of hydrogen atoms, and the proposal of a column-dependent ζ(N_H), where N_H is the total column of hydrogen nuclei, we desire to determine if the effects of ζ(N_H) in a single object with spatial variation can be observable. We chose the Horsehead nebula because of its geometry and high density. We model the Horsehead nebula as a near edge-on photon dominated region (PDR), using several choices for ζ, both constant and as a function of column. The column-dependent ζ functions are determined by a Monte Carlo model of cosmic ray penetration, using a steep power-law spectrum and accounting for ionization and magnetic field effects. We consider a case with low-metal elemental abundances as well as a sulfur-rich case. We show that use of a column-dependent ζ(N_H) of 5x10^–15s^–1 at the surface and 7.5x10^–16s^–1 at A_V=10 on balance improves agreement between measured and theoretical molecular abundances, compared with constant values of ζ.