TY - JOUR
T1 - NGC 6752 AGB stars revisited
T2 - I. Improved AGB temperatures remove apparent overionisation of Fe I
AU - Campbell, S. W.
AU - Maclean, B. T.
AU - D'Orazi, V.
AU - Casagrande, L.
AU - De Silva, G. M.
AU - Yong, D.
AU - Cottrell, P. L.
AU - Lattanzio, J. C.
PY - 2017/9/1
Y1 - 2017/9/1
N2 - 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 Teff 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.
AB - 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 Teff 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.
KW - Globular clusters: general
KW - Stars: abundances
KW - Stars: AGB and post-AGB
KW - Stars: evolution
KW - Techniques: spectroscopic
UR - http://www.scopus.com/inward/record.url?scp=85029695926&partnerID=8YFLogxK
U2 - 10.1051/0004-6361/201731101
DO - 10.1051/0004-6361/201731101
M3 - Article
VL - 605
JO - Astronomy and Astrophysics
JF - Astronomy and Astrophysics
SN - 0004-6361
M1 - A98
ER -