New Fe I level energies and line identifications from stellar spectra

TL;DR

This paper improves the understanding of Fe I atomic energy levels by deriving 66 new levels from stellar spectra, many of which are higher than laboratory limits, enabling new line identifications crucial for astrophysics.

Contribution

It introduces a novel method to determine high-lying Fe I energy levels using stellar spectra, expanding the known atomic data beyond laboratory capabilities.

Findings

Derived 66 new Fe I energy levels, many above laboratory reach.

Identified over two thousand new Fe I spectral lines.

Provided new line data with gf-values for astrophysical applications.

Abstract

The spectrum of the Fe I atom is critical to many areas of astrophysics and beyond. Measurements of the energies of its high-lying levels remain woefully incomplete, however, despite extensive analysis of ultraviolet laboratory iron absorption spectra, optical laboratory iron emission spectra, and the solar infrared spectrum. In this work we use as sources the high-resolution archival absorption-line ultraviolet and optical spectra of stars, whose warm temperatures favor moderate Fe I excitation. We derive the energy for a particular upper level in Kurucz's semiempirical calculations by adopting a trial value that yields the same wavelength for a given line predicted to be about as strong as that of a strong unidentified spectral line observed in the stellar spectra, then checking the new wavelengths of other strong predicted transitions that share the same upper level for coincidence with other strong observed unidentified lines. To date this analysis has provided the upper energies of 66 Fe I levels. Many new level energies are higher than those accessible to laboratory experiments; several exceed the Fe I ionization energy. These levels provide new identifications for over two thousand potentially detectable lines. Almost all of the new levels of odd parity include UV lines that were detected but unclassified in laboratory Fe I absorption spectra, providing an external check on the energy values. We motivate and present the procedure, provide the resulting new level energies and their uncertainties, list all the potentially detectable UV and optical new Fe I line identifications and their gf-values, point out new lines of astrophysical interest, and discuss the prospects for additional Fe I energy-level determinations in the near future.