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Cause of anomalously high Si abundances from specific Si I UV lines

Establish the cause of the anomalously high silicon abundances derived from the Si I lines at 2438.77 Å and 2443.36 Å in the UV spectrum of 2MASS J00512646-1053170 relative to other Si I and Si II lines, despite their similar excitation potentials.

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Background

In the silicon analysis, most Si lines yield consistent abundances between Si I and Si II, but two UV Si I lines produce values ~0.4 dex higher.

The authors exclude these lines from the final abundance and caution their use, noting the discrepancy cannot be explained simply by excitation potential differences.

References

It is unclear why this is the case, since the excitation potential of these lines, 0.00 and 0.01 eV, respectively, are only slightly lower than that of the other \ion{Si}{I} lines investigated, which have excitation potentials in the range of 0.78 - 1.91 eV.

— The $R$-Process Alliance: Detailed Composition of an $R$-Process Enhanced Star with UV and Optical Spectroscopy (2401.12311 - Shah et al., 22 Jan 2024) in Section 6 (Chemical Abundances), Subsection “α-Elements: O, Mg, Si, S, and Ca”