Introducing the KOSMA-$\tau$ 3D PDR code: Simulations of the Orion Bar
- Mrs. Silke ANDREE-LABSCH
- Mrs. Silke ANDREE-LABSCH (1. Physikalisches Institut, Universität zu Köln)
The components of the interstellar medium (ISM) are continuously heated due to energy input from different sources, one being the radiation of young and massive stars. In photon-dominated regions (PDRs) the interstellar far-ultraviolet (FUV; 6-13.6 eV) radiation field determines the energy balance and the chemistry of the ISM. Cooling of the gas in PDRs is dominated by fine structure line emission by atoms and ions, as well as rotational and vibrational line emission by molecules.
The KOSMA-$\tau$ PDR model simulates the chemical and the physical structure and the line emission of spherical clouds ("clumps") in the ISM. Furthermore, it has been shown that a superposition of spherical clumps, having a specific mass-spectrum and a specific mass-size relation, can be used to mimic the fractal structure of the ISM. Here, we introduce an extension of the KOSMA-$\tau$ code, denoted KOSMA-$\tau$ 3D, which can be used to model star forming regions with arbitrary 3D geometry. In this model, a 3D compound made of voxels ("3D pixels"), containing ensembles of clumps with discrete mass distributions, is generated. The characteristics defining the ensembles can vary between different voxels. A probabilistic approach is used to calculate the averaged FUV extinction caused by the clumps within each voxel. To analyse each individual clump the new code is combined with the KOSMA-$\tau$ PDR model. Line emissivities and optical depths of individual clumps are used to calculate the distribution of voxel-averaged emissivities and optical depths, and the radiative transfer through the compound yields full spectral cubes. Thereby, the new code accounts for the intrinsic linewidths of single clumps and additionally for a velocity dispersion of the clump ensemble.
The Orion Bar PDR, a well-known and bright star forming region with an interesting edge-on geometry, is used as a test-case for the new 3D code. New HIFI/Herschel data from the HEXOS guaranteed-time key program and complementary data from the Caltech Submillimeter Observatory (CSO) are fitted. Furthermore, we present simulation results, based on the clumpy edge-on cavity wall suggested by Hogerheijde et al. (1995) and on a cylindrical filament geometry. Simulations and observations are compared in terms of the layered positions of the emission peaks, the "chemical stratification" and on the line integrated intensities at the peak positions.
REFERENCE: Hogerheijde, M. R., Jansen, D. J., and van Dishoeck, E. F. (1995) A&A, 294, 792