Establish whether spectral blends bias high oxygen-isotopic ratios in NGC 6791

Determine whether contamination by atomic or molecular spectral blends causes systematic underestimation of the 16O/17O ratios derived from weak C17O features in metal-rich RGB stars with intrinsically high oxygen isotopic ratios, particularly the NGC 6791 stars V715 Lyr and 3012.

Background

The two RGB stars in NGC 6791 have high inferred 16O/17O ratios and unusually large dispersions among measurements from different C17O lines. The paper attributes the problem potentially to inaccurate stellar parameters, inaccurate oscillator strengths, or contamination by spectral blends, but the available low signal-to-noise spectra do not distinguish among these explanations.

At high intrinsic 16O/17O ratios, C17O lines become very weak and less sensitive to 17O abundance, while blends may become relatively dominant. The issue may be more severe at the super-solar metallicity of NGC 6791, potentially causing some lines to yield underestimated oxygen isotopic ratios and thereby biasing the inferred initial masses and ages.

References

Finally, we cannot exclude that this large dispersion may be due to blends with atomic or molecular lines whose contribution becomes dominant in some of the selected C${17}$O spectral features when the 16 ratio is intrinsically large ($\gtrsim 1300$ or so).

— Oxygen isotopic ratio in open cluster RGB stars: A new method for accurate mass and age estimations  (2610.11404 - Straniero et al., 8 Oct 2026) in Section 4, subsection 4.4, “NGC 6791”