Opacity for realistic 3D MHD simulations of cool stellar atmospheres

Abstract

Context. Realistic 3D time-dependent simulations of stellar near-surface convection employ the opacity binning method for efficient and accurate computation of the radiative energy exchange. The method provides several orders of magnitude of speed-up, 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 are used to construct opacity distribution function (ODF) that is then verified through detailed comparison with the results of the ATLAS code. The opacity binning method is 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 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.

arXiv comment26 pages, 29 figures. Accepted for publication in section 15 (Numerical methods and codes) of Astronomy and Astrophysics

References in corpus (16)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The photospheric solar oxygen project: I. Abundance analysis of atomic lines and influence of atmospheric models
• The Stagger-grid: A Grid of 3D Stellar Atmosphere Models - I. Methods and General Properties
• POLLUX : a database of synthetic stellar spectra
• A Grid of 3D Stellar Atmosphere Models of Solar Metallicity: I. General Properties, Granulation and Atmospheric Expansion
• Magneto-acoustic waves in sunspots: first results from a new 3D nonlinear magnetohydrodynamic code
• How realistic are solar model atmospheres?
• Three-dimensional simulations of solar magneto-convection including effects of partial ionization
• Three-dimensional simulations of near-surface convection in main-sequence stars - I. Overall structure
• NLTE for APOGEE: Simultaneous Multi-Element NLTE Radiative Transfer
• Non-LTE Model Atmospheres for Late-Type Stars I. A Collection of Atomic Data for Light Neutral and Singly-Ionized Atoms
• The benchmark halo giant HD 122563: CNO abundances revisited with three-dimensional hydrodynamic model stellar atmospheres
• Numerical simulations of quiet Sun magnetic fields seeded by Biermann battery
• MPS-ATLAS: A fast all-in-one code for synthesising stellar spectra
• Non-LTE Model Atmospheres for Late-Type Stars II. Restricted NLTE Calculations for a Solar-Like Atmosphere
• Opacity distribution functions for stellar spectra synthesis

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