Identification of numerous unidentified photospheric lines in GSC and UCAC4

Identify the atomic transitions responsible for the unidentified photospheric lines listed under “Unidentified photospheric lines” for the PG1159 star GSC and the CO-sdO star UCAC4, including precise species and ionization stages, to improve spectral modeling and abundance analyses of these stars.

Background

The spectra of the PG1159 star GSC and the CO-sdO star UCAC4 show many carbon and oxygen features due to their high C and O abundances, but also numerous lines that could not be identified despite extensive use of atomic databases. The authors suspect many of these features arise from highly excited states of C III, O IV, and O V, indicating gaps in currently available atomic line data for these ions at high excitation.

Resolving these identifications would refine atmospheric models, address blending issues, and support more accurate determinations of stellar parameters and abundances.

References

These two objects also exhibit many lines that we are unable to identify. We suspect that they are mostly unknown lines from highly excited states in #1{C}{3}, #1{O}{4}, and #1{O}{5}.

— Three new hot hydrogen-deficient pre-white dwarfs  (2412.15984 - Werner et al., 2024) in Section 2.4 (Unidentified photospheric lines); Table 'Unidentified photospheric lines'

The species involved are only partially identified, but include O and N. These unusual features are poorly understood, but may arise from shocks in magnetically-confined winds \citep{Reindl2021}, and are often photometrically variable \citep{Reindl2023}.

— The DESI Data Release 1 white dwarf catalogue  (2609.04314 - Swan et al., 3 Sep 2026) in Section 5.8.1, Ultra-high-excitation lines