NGC 6752 AGB stars revisited

I. Improved AGB temperatures remove apparent overionisation of Fe i

¹ Max-Planck-Institut für Astrophysik (MPA), Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
e-mail: scampbell@mpa-garching.mpg.de
² School of Physics and Astronomy, Monash University, 3800 Clayton, Victoria, Australia
³ Monash Centre for Astrophysics (MoCA), Monash University, 3800 Clayton, Victoria, Australia
⁴ INAF-Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁶ Australian Astronomical Observatory, 105 Delhi Rd, North Ryde, NSW 2113, Australia
⁷ Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
⁸ Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, 8140 Christchurch, New Zealand

Received: 4 May 2017
Accepted: 4 July 2017

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.