My research mostly concentrates on studying the physics and chemistry of the interstellar medium (ISM) in the context of star formation. I am mostly doing modeling but am also involved in observational projects. I observe regularly at the NANTEN2 telescope (Chile) and am Co-I and affiliated to a number of Herschel Key projects (WADI, HexGal, HERM33ES, Hexos).
PDR modeling is my main focus. I am responsible for supporting and developing the KOSMA-τ PDR code and its applications. KOSMA-τ features a spherical geometry with isotropic FUV illumination and was part of the large PDR Comparison Benchmark (Röllig et al. 2007). The code is a further development of the plane-parallel PDR code from A. Sternberg, Tel Aviv, and has originally been built to study the C+ - C - CO stratification in PDRs. We constantly update the code and expand its capabilities. Present development projects include the inclusion of a much more refined dust treatment by coupling KOSMA-τ with a full dust radiative transfer code by R. Szczerba, Torun. This will provide the full continuum properties of the model clouds including observable SED's given any dust composition. Additionally, we will also include the stochastic photo-heating of the very small grains (VSG's), which is bealived to be an important contribution to the total energy balance of the cloud.
The spherical model geometry is perfectly suited to be applied in modeling clumpy ensembles of many individual clumps (Cubick et al. 2008). This technique is particularly useful in the light of the recent Herschel observations, as, for instance, demonstrated in our recent study of DR21 (Ossenkopf, Röllig, et al. 2010).
Recent Herschel/HIFI observations provide the full range of far infrared (FIR) cooling lines in DR21. For DR 21 two ensembles with different properties have to be superimposed, a hot component, close to the inner HII region with strong FUV illumination, but only a small fraction of the total mass (orange clumps in the Figure), and a cooler component that provides the bulk of the material (beige clumps in Figure).
This two ensemble fit is able to reproduce all observed lines. We find no evidence for shock-heated material, in agreement with Lane et al. (1990). This seems to be in contradiction with the line profiles that show excited outflow material.
Figure: Examplary realisation of a two-ensemble model configuration. All dimensions are plotted true to scale. The position of the central OB cluster is indicated by a blue sphere. The edge of the surrounding HII region is shown by the red wireframe sphere. The hot component clumps are shown as orange spheres. They populate the inner shell. The cool component clumps are shown in beige populating the outer shell. All clumps are randomly positioned and assumedly embedded in a diffuse inter-clump gas.