2011/10/11 by P. B. Rimmer, E. Herbst, O. Morata +1 · 1 citation
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.1051/0004-6361/201117048
12 pages, 6 figures, 5 tables, accepted in A&A
arxiv created 2011/10/11 · arxiv updated 2015/05/29
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 zeta (s-1), the total ionization rate of hydrogen atoms, and the proposal of a column-dependent zeta(NH), where NH is the total column of hydrogen nuclei, we desire to determine if the effects of zeta(NH) in a single object with spatial variation can be observable. We chose the Horsehead Nebula because of its geometry and high density. Method: We model the Horsehead Nebula as a near edge-on photon dominated region (PDR), using several choices for zeta, both constant and as a function of column. The column-dependent zeta 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 zeta(NH) of 5(-15) s-1 at the surface and 7.5(-16) s-1 at Av = 10 on balance improves agreement between measured and theoretical molecular abundances, compared with constant values of zeta.