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Cause of the required wavelength shift for the Au I λ2675.95 line

Determine the physical cause of the ≈0.04 Å blueward shift required for the Au I λ2675.95 center-of-gravity wavelength relative to the NIST value when modeling metal-poor r-process-enhanced stars.

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Background

To fit the Au I λ2675.95 line, the authors apply a 0.04 Å blueward shift, consistent with a similar adjustment reported previously for another r-process-enhanced star (HD 222925).

While this empirical shift yields acceptable fits, its underlying physical origin is not known.

References

The need for a similar wavelength shift was discussed in for HD 222925, but the cause is unclear.

— 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 “Third R-Process Peak: Os, Pt, and Au”