Fast approximation of angle-dependent partial redistribution in moving atmospheres

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
e-mail: jorritl@astro.uio.no
² Utrecht University, Postbus 80 000, 3508 TA Utrecht, The Netherlands
³ NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴ Lockheed Martin ATC, Solar & Astrophysics Lab, Org. H1–12, Bldg. 252, 3251 Hanover Street, Palo Alto, CA 94304–1187, USA
⁵ NSO/Sacramento Peak, PO Box 62, Sunspot, NM 88349–0062, USA

Received: 12 April 2012
Accepted: 23 May 2012

Abstract

Aims. Radiative transfer modeling of spectral lines including partial redistribution (PRD) effects requires the evaluation of the ratio of the emission to the absorption profile. This quantity requires a large amount of computational work if one employs the angle-dependent redistribution function, which prohibits its use in 3D radiative transfer computations with model atmospheres containing velocity fields. We aim to provide a method to compute the emission to absorption profile ratio that requires less computational work but retains the effect of angle-dependent scattering in the resulting line profiles.

Methods. We present a method to compute the profile ratio that employs the angle-averaged redistribution function and wavelength transforms to and from the rest frame of the scattering particles. We compare the emergent line profiles of the Mg II k and Lyα lines computed with angle-dependent PRD, angle-averaged PRD and our new method in two representative test atmospheres.

Results. The new method yields a good approximation of true angle-dependent profile ratio and the resulting emergent line profiles while keeping the computational speed and simplicity of angle-averaged PRD theory.