Determine the origin of the discrepant carbon-isotope measurement for NGC 7789 star 415

Determine the origin of the discrepancy between the carbon isotopic ratio measured for star 415 in NGC 7789 and the substantially different value reported by Takeda et al., including whether it results from the differing spectral diagnostics or other methodological factors.

Background

The paper reports a carbon isotopic ratio of 12C/13C = 22 ± 5 for star 415 in NGC 7789, consistent within the uncertainties with the value 19 ± 8 reported by Lebzelter et al. However, this result disagrees with the value 9 ± 3 reported by Takeda et al.

The authors note that Takeda et al. used weak CN lines near 8000 Å, outside the near-infrared spectral range used in the present analysis, but they do not resolve the cause of the discrepancy. Establishing its origin is important because carbon isotopic ratios are used to diagnose first dredge-up and possible extra-mixing processes in RGB stars.