Observing a column-dependentζin dense interstellar sources: the case of the Horsehead nebula

Abstract

Context. 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.Aims. 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.Methods. 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. Results. We show that use of a column-dependent ζ(N H) of 5 × 10-15 s-1 at the surface and 7.5 × 10-16 s-1 at A V = 10 on balance improves agreement between measured and theoretical molecular abundances, compared with constant values of ζ.