<scp>3d-pdr</scp>: a new three-dimensional astrochemistry code for treating photodissociation regions

Abstract

Photodissociation regions (PDRs) define the transition zone between an\nionized and a dark molecular region. They consist of neutral gas which\ninteracts with far-ultraviolet radiation and are characterized by strong\ninfrared line emission. Various numerical codes treating one-dimensional PDRs\nhave been developed in the past, simulating the complexity of chemical\nreactions occurring and providing a better understanding of the structure of a\nPDR. In this paper we present the three-dimensional code, 3D-PDR, which can\ntreat PDRs of arbitrary density distribution. The code solves the chemistry and\nthe thermal balance self-consistently within a given three-dimensional cloud.\nIt calculates the total heating and cooling functions at any point in a given\nPDR by adopting an escape probability method. It uses a HEALPix-based\nray-tracing scheme to evaluate the attenuation of the far-ultraviolet radiation\nin the PDR and the propagation of the far-infrared/submm line emission out of\nthe PDR. We present benchmarking results and apply 3D-PDR to i) a\nuniform-density spherical cloud interacting with a plane-parallel external\nradiation field, ii) a uniform-density spherical cloud interacting with a\ntwo-component external radiation field, and iii) a cometary globule interacting\nwith a plane-parallel external radiation field. We find that the code is able\nto reproduce the benchmarking results of various other one-dimensional\nnumerical codes treating PDRs. We also find that the accurate treatment of the\nradiation field in the fully three-dimensional treatment of PDRs can in some\ncases leads to different results when compared to a standard one-dimensional\ntreatment.\n