Volume 537, January 2012
|Number of page(s)||12|
|Section||Interstellar and circumstellar matter|
|Published online||20 December 2011|
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
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
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(NH), where NH is the total column of hydrogen nuclei, we desire to determine if the effects of ζ(NH) 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(NH) of 5 × 10-15 s-1 at the surface and 7.5 × 10-16 s-1 at AV = 10 on balance improves agreement between measured and theoretical molecular abundances, compared with constant values of ζ.
Key words: astrochemistry / molecular processes / ISM: abundances / ISM: molecules / ISM: individual objects: Horsehead nebula
© ESO, 2012
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