Stellar models with self-consistent Rosseland opacities

A Hui-Bon-Hoa

doi:10.1051/0004-6361/202040095

Abstract

Context. The building of a stellar structure requires knowing the Rosseland mean opacity at each layer of the model. This mean opacity is very often interpolated in pre-computed tables due to the overwhelming time to compute it from monochromatic cross sections. The main drawback to using tables is that the opacities can be inconsistent with the actual local chemical composition, for instance in the regions of the star where nucleosynthesis occurs. Aims. We study the effects of self-consistent Rosseland mean opacity calculations on the stellar structure and evolution, in comparison with models where the metal mixture remains equal to the initial one. Methods. We developed a strategy that allows very fast calculations of Rosseland opacities from monochromatic cross sections. We are then able to compute evolutionary tracks with models whose Rosseland opacities are fully consistent with the chemical mix everywhere in the star. This method has been implemented in the Toulouse-Geneva evolution code. Results. Our self-consistent models show very small structural differences compared to models where the Rosseland opacity is computed with a fixed metal mixture. As a consequence, the main-sequence evolutionary tracks are almost the same for models of mass ranging from 2 to 8 M⊙. At a given surface gravity the relative difference in age is lower than 2% and generally below 1% between the two kinds of calculations, the self-consistent model being younger most of the time. Unless such a precision in age is sought out, the use of tabulated Rosseland opacities with a metal content defined globally is still acceptable, at least in main-sequence stars where the chemical mix changes only through nucleosynthesis.

Highlights

The Rosseland mean opacity (RMO) is a major ingredient in the building of stellar structure since it describes how the radiative field carries energy inside the star
The difference in opacity between the Z- and SC-models is most important in the core where nucleosynthesis has already changed the chemical composition during the pre-main sequence (PMS)
Having the ability to compute the Rosseland opacities from monochromatic data in a reasonable time, we show that the evolutions of model stars between 2 and 8 M are very similar whether we take into account the details of the metal content in the stellar core or not, the evolutionary tracks being superimposed during almost all the main sequence (MS)

Summary

Introduction

The Rosseland mean opacity (RMO) is a major ingredient in the building of stellar structure since it describes how the radiative field carries energy inside the star. A self-consistent stellar model requires computing RMOs according to the local abundance in each part of the star. This computation makes use of the monochromatic cross sections of each element, weighted by its abundance. Several evolution codes can compute RMOs from monochromatic data,. In this study we evaluate the changes in the stellar evolution between models computed with self-consistent RMOs and those calculated with fixed metal abundance ratios, mimicking the use of opacity tables specified only by the mass fraction of H, He, and that of the metals. 3 we show the evolution of models computed with self-consistent Rosseland opacities compared to models whose metal content ratios are fixed, and discuss the differences induced by the computation method on the stellar parameters. Is equal to the sum of the monochromatic cross sections σA,ν of each chemical element weighted by its number fraction NA:

Stellar evolution code

Computational strategy for the Rosseland opacity

Hui-Bon-Hoa

Results and discussion

Conclusion