Korg: fitting, model atmosphere interpolation, and Brackett lines

TL;DR

Korg is an updated spectral synthesis package for FGKM stars that improves atmosphere modeling, reduces abundance scatter, and enhances infrared hydrogen line synthesis, broadening its applicability to cooler stars.

Contribution

The paper introduces new features in Korg, including improved atmosphere interpolation, extended molecular chemistry, and better infrared hydrogen line modeling, advancing spectral analysis capabilities.

Findings

Reduced line-to-line abundance scatter in analysis of 18 Sco.

Atmosphere interpolation introduces negligible error for stars with T_eff > 4000 K.

Brackett line profiles match observations more closely than previous methods.

Abstract

We describe several updates to Korg, a package for 1D LTE spectral synthesis of FGKM stars. Built-in functions to fit observed spectra via synthesis or equivalent widths make it easy to take advantage of Korg's automatic differentiation. Comparison to a past analysis of 18 Sco shows that we obtain significantly reduced line-to-line abundance scatter with Korg. Fitting and synthesis are facilitated by a rigorously-tested model atmosphere interpolation method, which introduces negligible error to synthesized spectra for stars with $T_\mathrm{eff} \gtrsim 4000\,\mathrm{K}$. For cooler stars, atmosphere interpolation is complicated by the presence of molecules, though we demonstrate an adequate method for cool dwarfs. The chemical equilibrium solver has been extended to include polyatomic and charged molecules, extending Korg's regime of applicability to M stars. We also discuss a common oversight regarding the synthesis of hydrogen lines in the infrared, and show that Korg's Brackett line profiles are a much closer match to observations than others available. Documentation, installation instructions, and tutorials are available at https://github.com/ajwheeler/Korg.jl.