Opacity for realistic 3D MHD simulations of cool stellar atmospheres

¹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
e-mail: andrperdomo@gmail.com
² Departamento de Astrofísica de la Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
³ Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de La Palma, Canary Islands, Spain
⁴ Steward Observatory, University of Arizona, Tucson, USA

Received: 7 March 2023
Accepted: 5 June 2023

Abstract

Context. Realistic three-dimensional time-dependent simulations of stellar near-surface convection employ the opacity binning method for the efficient and accurate computation of the radiative energy exchange. The method provides several orders of magnitude of speedup, but its implementation includes a number of free parameters.

Aims. Our aim is to evaluate the accuracy of the opacity binning method as a function of the choice of these free parameters.

Methods. The monochromatic opacities computed with the SYNSPEC code were used to construct opacity distribution function (ODF) that was then verified through detailed comparison with the results of the ATLAS code. The opacity binning method was implemented with the SYNSPEC opacities for four representative cool main-sequence stellar spectral types (F3V, G2V, K0V, and M2V).

Results. The ODFs from SYNSPEC and ATLAS show consistent results for the opacity and bolometric radiative energy exchange rate Q in the case of the F-, G-, and K-type stars. Significant differences, coming mainly from the molecular line lists, are found for the M-type star. It is possible to optimise a small number of bins to reduce the deviation of the results coming from the opacity grouping with respect to the ODF for the F-, G-, and K-type stars. In the case of the M-type star, the inclusion of splitting in wavelength is needed in the grouping to get similar results, with a subsequent increase in computing time. In the limit of a large number of bins, the deviation for all the binning configurations tested saturates and the results do not converge to the ODF solution. Due to this saturation, the Q rate cannot be improved by increasing the number of bins to more than about 20 bins. The more effective strategy is to select the optimal location of fewer bins.