Low temperature Rosseland opacities with varied abundances of carbon and nitrogen

TL;DR

This paper provides detailed low-temperature Rosseland mean opacities considering varied carbon and nitrogen abundances, crucial for stellar models, highlighting significant changes especially in carbon-rich regimes due to molecular chemistry effects.

Contribution

It introduces a comprehensive dataset of Rosseland opacities accounting for different C and N abundances across various metallicities, emphasizing the impact of molecular chemistry on opacity.

Findings

Opacity varies significantly with carbon abundance, especially in carbon-rich regimes.

Nitrogen abundance changes have a smaller but notable effect on opacity.

Molecular data and chemistry critically influence low-temperature opacity calculations.

Abstract

We attempt to produce low temperature opacity data incorporating the effects of varied abundances of the elements carbon and nitrogen. For our temperature range of interest, molecules represent the dominant opacity source. Our dataset covers a wide metallicity range and is meant to provide important input data for stellar evolution models and other applications. We conduct chemical equilibrium calculations to evaluate the partial pressures of neutral atoms, ions, and molecules. Based on a large dataset containing atomic line and continuum data and, most importantly, a plethora of molecular lines, we calculate Rosseland mean opacity coefficients not only for a number of different metallicities, but also for varied abundances of the isotopes ^{12}C and ^{14}N at each metallicity. The molecular data comprise the main opacity sources for either an oxygen-rich or carbon-rich chemistry. We tabulate the opacity coefficients as a function of temperature and, basically, density. Due to the special role of the CO molecule, within a certain chemistry regime an alteration to the carbon abundance causes considerable changes in the Rosseland opacity. The transition from a scaled solar (i. e. oxygen-rich) mixture to a carbon-rich regime results in opacities that can, at low temperatures, differ by orders of magnitude from to the initial situation. The reason is that the mean opacity in either case is due to different molecular absorbers. Variations in the abundance of nitrogen have less pronounced effects but, nevertheless, cannot be neglected. [abridged]