NGC 6752 AGB stars revisited

Abstract

Context.A recent study reported a strong apparent depression of Fe i, relative to Fe ii, in the AGB stars of NGC 6752. This depression is much greater than that expected from the neglect of non-local thermodynamic equilibrium effects, in particular the dominant effect of overionisation. The iron abundances derived from Fe i were then used to scale all other neutral species in the study. Aims. Here we attempt to reproduce the apparent Fe discrepancy, and investigate differences in reported sodium abundances. Methods. We compare in detail the methods and results of the recent study with those of an earlier study of NGC 6752 AGB stars. Iron and sodium abundances are derived using Fe i, Fe ii, and Na i lines. We explore various uncertainties to test the robustness of our abundance determinations. Results. We reproduce the large Fe i depression found by the recent study, using different observational data and computational tools. Further investigation shows that the degree of the apparent Fe i depression is strongly dependent on the adopted stellar effective temperature. To minimise uncertainties in Fe i we derive temperatures for each star individually using the infrared flux method (IRFM). We find that the T eff scales used by both the previous studies are cooler, by up to 100 K; such underestimated temperatures amplify the apparent Fe i depression. Our IRFM temperatures result in negligible apparent depression, consistent with theory. We also re-derived sodium abundances and, remarkably, found them to be unaffected by the new temperature scale. [Na/H] in the AGB stars is consistent between all studies. Since Fe is constant, it follows that [Na/Fe] is also consistent between studies, apart from any systematic offsets in Fe. Conclusions. We recommend the use of (V − K) relations for AGB stars, based on comparisons with our individually-derived IRFM temperatures, and their inherently low uncertainties. We plan to investigate the effect of the improved temperature scale on other elements, and re-evaluate the subpopulation distributions on the AGB, in the next paper of this series.

Multiple Stellar Populations of Globular Clusters from Homogeneous Ca–CN Photometry. III. NGC 6752*

“…On the other hand, nine AGB stars classified as an intermediate population from the sodium abundance by Lapenna et al (2016) do not appear to be the CN-s population from our photometry. The discrepancy in the elemental abundances is beyond the scope of the current work and we decline to discuss this matter further (see, for example, Lee 2010Lee , 2016Campbell et al 2017). Our main purpose here is to show that our cn JWL,cor is still useful to study multiple AGB populations in GCs.…”

The GALAH Survey: Chemically tagging the Fimbulthul stream to the globular cluster ω Centauri

“…As noted in Section 3, 6...376 is an asymptotic giant branch (AGB) star. There has been much discussion in the literature about differences in the abundances of stars on the AGB versus those on the RGB in globular clusters (Norris et al 1981;Campbell et al 2013Campbell et al , 2017MacLean et al 2016MacLean et al , 2018, in particular with respect to apparent differences in their derived metallicities and light element abundances (Lapenna et al 2016;Mucciarelli et al 2019). There are two related effects that are causing these discrepancies.…”

The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XIV. Multiple stellar populations within M 15 and their radial distribution★

“…These diagrams demonstrate that the AGB of M 15 hosts more than one stellar population. Spectroscopy is mandatory to connect multiple populations along the AGB and the RGB and to understand whether the RGB stars with extreme chemical composition ascend the AGB phase or avoid this evolutionary phase (Campbell et al 2013;Cassisi et al 2014;Lapenna et al 2016;Campbell et al 2017;Marino et al 2017).…”