Young massive stars emit large amounts of ultraviolet radiation, which ionize surrounding HII regions. An HII region is characterized by both its hot ionized and rarefied interior and by its sharp edges, where the temperature and ionization fraction can drop by as much as three orders of magnitude within a relatively thin zone known as a photodissociation region (PDR), a zone where molecular hydrogen is being dissociated into atomic hydrogen. PDRs are responsible for many emission characteristics of the interstellar medium: they dominate the far-infrared/submm (FIR/submm) spectra of star formation regions and they are the key interface regions between ionized and molecular gas. Therefore, elucidating the physical and chemical structure of PDRs can help provide new insights into star formation processes.

3D-PDR (Bisbas et al. 2012) is a three-dimensional astrochemistry code which is able to treat PDRs with arbitrary geometries and density distributions. It uses a HEALPix (Gorski et al. 2006) ray-tracing scheme in which rays emanate from every computational element within the cloud.  This allows one to evaluate the attenuation of the far-ultraviolet (FUV) radiation in the PDR and the propagation of FIR/submm emission lines out of the PDR. The 3D-PDR code solves the chemistry and the thermal balance self-consistently within a given three-dimensional cloud. It calculates the total heating and cooling functions at any point by adopting an escape probability method (Sobolev 1960). The code uses the chemical modelling features of the fully benchmarked one-dimensional code UCL_PDR (Bell et al. 2006). In particular, it includes a comprehensive treatment of gas heating mechanisms (photoelectric heating from grains and PAHs, H2* vibrational de-excitation, etc.), along with the emission from major cooling lines ([CII], [CI], [OI] and CO), which are calculated at each computational element within the cloud. The code comes with the UMIST2012 reduced chemical network containing 33 species and 330 reactions, as well as the UMIST2012 (McElroy et al. 2013) full chemical network containing 128 species and more than 2200 reactions, and can handle X-ray reactions to simulate X-ray dominated regions (XDR). It is also possible for users to set up their own chemical network.

3D-PDR can post-process density distributions obtained from hydrodynamical codes such as Smoothed Particle Hydrodynamics (SPH) and Adaptive Mesh Refinement (AMR). 3D-PDR has been used for various applications (Bisbas et al. 2012; Offner et al. 2013). The image shows an example of a post-processed PDR calculation of a fractal cloud. The cloud has been initially simulated using the SEREN SPH code (Hubber et al. 2011) which accounted for hydrodynamics, gravity, and the transport of ionizing radiation (Bisbas et al. 2009). For a full discussion of the overall dynamical evolution of this fractal cloud see Walch et al. (2012, 2013).


The image shows an RGB composition of three different emission-line maps.  Red shows the [CII] 158um line emission, while green shows emission in the [CI] 609um line. Blue shows the [OI] 63um line. Simulated emission line images such as this can be directly compared with observed line maps to deduce the global physical parameters of the emitting regions.



Bisbas et al. 2012

Gorski et al. 2006

Sobolev 1960

Bell et al. 2006

McElroy et al. 2013

Offner et al. 2013

Hubber et al. 2011

Bisbas et al. 2009

Walch et al. 2012 

Walch et al. 2013

For more information about 3D-PDR, please contact Thomas Bisbas (tb AT star.ucl.ac.uk)

Page last modified on 12 aug 13 12:46 by Alexandra N D Fanghanel