Numerical models of photodissociation regions (PDRs) are an essential tool to quantitatively understand observations of massive star forming regions through simulations. Few mature PDR models are available and the Cologne KOSMA- τ PDR model is the only sophisticated model that uses a spherical cloud geometry, thereby allowing us to simulate clumpy PDRs. We present the current status of the code as a reference for modelers and for observers that plan to apply KOSMA- τ to interpret their data. For the numerical solution of the chemical problem, we present a superior Newton-Raphson stepping algorithm and discuss strategies to numerically stabilize the problem and speed up the iterations. The chemistry in KOSMA- τ is upgraded to include the full surface chemistry in an up-to-date formulation and we discuss a novel computation of branching ratios in chemical desorption reactions. The high dust temperature in PDRs leads to a selective freeze-out of oxygen-bearing ice species due to their higher condensation temperatures and we study changes in the ice mantle structures depending on the PDR parameters, in particular the impinging ultraviolet field. Selective freeze-out can produce enhanced C abundances and higher gas temperatures, resulting in a fine-structure line emission of atomic carbon [C i ] enhanced by up to 50% if surface reactions are considered. We show how recent Atacama Large Millimeter Array (ALMA) observations of HCO + emission in the Orion Bar with high spatial resolution on the scale of individual clumps can be interpreted in the context of nonstationary, clumpy PDR ensembles. Additionally, we introduce WL-PDR, a simple plane-parallel PDR model written in Mathematica to act as a numerical testing environment of PDR modeling aspects.

M. Röllig

V. Ossenkopf-Okada

Astronomy and Astrophysics

ASTRONOMY & ASTROPHYSICS 是一本国际期刊，独立于用于获得结果的技术发表关于天文学和天体物理学（理论、观测和仪器）各个方面的论文。

Astronomy and Astrophysics is an international journal that focuses on various aspects of astronomy and astrophysics, including theory, observation, and instrumentation. The journal publishes high-quality academic papers independently of the techniques used to obtain research results.

《Astronomy and Astrophysics》是一本国际性期刊，专注于天文学和天体物理学的各个方面，包括理论、观测和仪器技术。该期刊独立于用于获得研究结果的技术，发表高质量的学术论文。

Category	Quartile
物理与天体物理	2
物理与天体物理, 天文与天体物理	2

The KOSMA-τPDR model

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