Observing a column-dependent ζ in dense interstellar sources: the case of the Horsehead nebula

¹ Department of PhysicsThe Ohio State University, Columbus, OH 43210, USA
e-mail: pbrimmer@mps.ohio-state.edu
² Departments of Chemistry, Astronomy, & Physics, University of Virginia, Charlottesville, VA 22904, USA
³ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan
⁴ Observatoire de Paris-Meudon, LUTH, 5 place Jules Janssen, 92190 Meudon, France

Received: 7 April 2011
Accepted: 7 October 2011

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.