TY - JOUR
T1 - On the AGB stars of M4
T2 - A robust disagreement between spectroscopic observations and theory
AU - MacLean, B. T.
AU - Campbell, S. W.
AU - Amarsi, A. M.
AU - Nordlander, T.
AU - Cottrell, P. L.
AU - De Silva, G. M.
AU - Lattanzio, J.
AU - Constantino, T.
AU - D'Orazi, V.
AU - Casagrande, L.
PY - 2018/11/21
Y1 - 2018/11/21
N2 - Several recent spectroscopic investigations have presented conflicting results on the existence of Na-rich asymptotic giant branch (AGB) stars in the Galactic globular cluster M4 (NGC 6121). The studies disagree on whether or not Na-rich red giant branch (RGB) stars evolve to the AGB. For a sample of previously published HERMES/AAT AGB and RGB stellar spectra, we present a re-analysis of O, Na, and Fe abundances, and a new analysis of Mg and Al abundances; we also present CN band strengths for this sample, derived from low-resolution AAOmega spectra. Following a detailed literature comparison, we find that the AGB samples of all studies consistently show lower abundances of Na and Al, and are weaker in CN, than RGB stars in the cluster. This is similar to recent observations of AGB stars in NGC6752 and M62. In an attempt to explain this result, we present new theoretical stellar evolutionary models for M4; however, these predict that all stars, including Na-rich RGB stars, evolve on to the AGB. We test the robustness of our abundance results using a variety of atmospheric models and spectroscopic methods; however, we do not find evidence that systematic modelling uncertainties can explain the apparent lack of Na-rich AGB stars in M4. We conclude that an unexplained, but robust, discordance between observations and theory remains for the AGB stars in M4.
AB - Several recent spectroscopic investigations have presented conflicting results on the existence of Na-rich asymptotic giant branch (AGB) stars in the Galactic globular cluster M4 (NGC 6121). The studies disagree on whether or not Na-rich red giant branch (RGB) stars evolve to the AGB. For a sample of previously published HERMES/AAT AGB and RGB stellar spectra, we present a re-analysis of O, Na, and Fe abundances, and a new analysis of Mg and Al abundances; we also present CN band strengths for this sample, derived from low-resolution AAOmega spectra. Following a detailed literature comparison, we find that the AGB samples of all studies consistently show lower abundances of Na and Al, and are weaker in CN, than RGB stars in the cluster. This is similar to recent observations of AGB stars in NGC6752 and M62. In an attempt to explain this result, we present new theoretical stellar evolutionary models for M4; however, these predict that all stars, including Na-rich RGB stars, evolve on to the AGB. We test the robustness of our abundance results using a variety of atmospheric models and spectroscopic methods; however, we do not find evidence that systematic modelling uncertainties can explain the apparent lack of Na-rich AGB stars in M4. We conclude that an unexplained, but robust, discordance between observations and theory remains for the AGB stars in M4.
KW - Galaxy: abundances
KW - Galaxy: formation
KW - Galaxy: globular clusters: general
KW - Stars: abundances
KW - Stars: AGB and post-AGB
UR - http://www.scopus.com/inward/record.url?scp=85054079651&partnerID=8YFLogxK
U2 - 10.1093/mnras/sty2297
DO - 10.1093/mnras/sty2297
M3 - Article
AN - SCOPUS:85054079651
SN - 0035-8711
VL - 481
SP - 373
EP - 395
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 1
ER -