An accelerated Monte Carlo method to solve two-dimensional radiative transfer and molecular.pdfVIP

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An accelerated Monte Carlo method to solve two-dimensional radiative transfer and molecular.pdf

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An accelerated Monte Carlo method to solve two-dimensional radiative transfer and molecular

a r X i v : a s t r o - p h / 0 0 0 8 1 6 9 v 1 1 0 A u g 2 0 0 0 AA manuscript no. (will be inserted by hand later) Your thesaurus codes are: 08 (02.12.1; 02.18.7; 03.13.4; 08.06.2; 09.13.2) ASTRONOMY AND ASTROPHYSICS An accelerated Monte Carlo method to solve two- dimensional radiative transfer and molecular excitation With applications to axisymmetric models of star formation Michiel R. Hogerheijde1 and Floris F. S. van der Tak2 1 Radio Astronomy Laboratory, University of California at Berkeley, Astronomy Department, 601 Campbell Hall # 3411, Berkeley, CA 94720-3411, USA e-mail: michiel@ 2 Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands e-mail: vdtak@strw.leidenuniv.nl Received / Accepted Abstract. We present a numerical method and computer code to calculate the radiative transfer and excitation of molecular lines. Formulating the Monte Carlo method from the viewpoint of cells rather than photons allows us to separate local and external contributions to the radia- tion field. This separation is critical to accurate and fast performance at high optical depths (τ ～ 100). The ran- dom nature of the Monte Carlo method serves to verify the independence of the solution to the angular, spatial, and frequency sampling of the radiation field. These features allow use of our method in a wide variety of astrophysi- cal problems without specific adaptations: in any axially symmetric source model and for all atoms or molecules for which collisional rate coefficients are available. Continuum emission and absorption by dust is explicitly taken into account but scattering is neglected. We illustrate these features in calculations of (i) the HCO+ J=1–0 and 3–2 emission from a flattened protostellar envelope with infall and rotation, (ii) the CO, HCO+, CN and HCN emission from a protoplanetary disk and (iii) HCN emission from a high-mass young stellar object, where infrared pump- ing is important. The program can be used for optical depths up to 103 ? 104, d