# KOSMA-tau - The Cologne PDR code

**KOSMA-tau** is a numerical code to compute the physical and chemical structure of a spherical molecular cloud, a so called photo-dissociation region (PDR). This code has been developed from an earlier PDR code, written by A. Sternberg from Tel Aviv University in Israel (Sternberg & Dalgarno 1989; Sternberg & Dalgarno 1995). His original code uses a plane-parallel geometry and was updated to employ spherical geometry (Gierens, Stutzki and Winnewisser 1992; Köster et al. 1994; Störzer, Stutzki and Sternberg 1996; Zielinsky, Stutzki & Störzer 2000).

The figure to the right shows the numerical solution scheme, i.e. the main blocks of the problem to be solved:

**Chemistry:**Solving the chemical problem is equivalent to solving a nonlinear system of rate equations, one equation for eacht chemical species that is includedin the model. The rate equation included all formation and destruction reactions of this particular species.**Energy balance:**Solving the energy balance means balancing all heating and cooling processes to derive the local kinetic gas (and dust) temperature.**Level population:**Solving the excitation problem, i.e. compute the energy level population of species that are relevant for the energy balance and for the emission of the model clump. This is again a system of nonlinear rate equations where all ppopulating and de-populating processes, such as collisions and radiative decays, are included (one rate equation per energy level).**Radiative transfer:**The radiative transfer computes the emission and absorption processes along a line-of-sight through the model cloud. This brings geometry into play and couples remote cloud volumes with each other, i.e. couples the physical and chemical conditions of different parts of the cloud together.

The different 'blocks' in the mentioned scheme depend on each other which makes an iterative solution necessary. The result is a the chemical and physical structure of the model cloud as a function of radius.

## Model features

- spherical geometry
- modular chemistry including carbon and oxygen isotopes
- applicable to clumpy environments
- full continuum radiative transfer
- complex dust properties (e.g. Weingartner & Draine 2001 size distribution, full description in Röllig et al. 2012)
- optimized for large parameter space surveys
- fully benchmarked in the PDR Comparison Benchmark