A new nonlocal thermodynamical equilibrium radiative transfer method for cool stars

Method and numerical implementation

¹ Lund Observatory, Box 43, 221 00 Lund, Sweden
e-mail: julien.lambert@astro.lu.se
² Laboratoire Univers et Particules de Montpellier (LUPM), UMR 5299, CNRS, Université Montpellier 2 − CC72 Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
³ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5109, Université Joseph Fourier, CNRS, OSUG, 38041 Grenoble Cedex 9, France

Received: 15 October 2013
Accepted: 2 April 2015

Abstract

Context. The solution of the nonlocal thermodynamical equilibrium (non-LTE) radiative transfer equation usually relies on stationary iterative methods, which may falsely converge in some cases. Furthermore, these methods are often unable to handle large-scale systems, such as molecular spectra emerging from, for example, cool stellar atmospheres.

Aims. Our objective is to develop a new method, which aims to circumvent these problems, using nonstationary numerical techniques and taking advantage of parallel computers.

Methods. The technique we develop may be seen as a generalization of the coupled escape probability method. It solves the statistical equilibrium equations in all layers of a discretized model simultaneously. The numerical scheme adopted is based on the generalized minimum residual method.

Results. The code has already been applied to the special case of the water spectrum in a red supergiant stellar atmosphere. This demonstrates the fast convergence of this method, and opens the way to a wide variety of astrophysical problems.