On the AGB stars of M4

A robust disagreement between spectroscopic observations and theory

B. T. MacLean, S. W. Campbell, A. M. Amarsi, T. Nordlander, P. L. Cottrell, G. M. De Silva, J. Lattanzio, T. Constantino, V. D'Orazi, L. Casagrande

Research output: Contribution to journalArticleResearchpeer-review

3 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)373-395
Number of pages23
JournalMonthly Notices of the Royal Astronomical Society
Volume481
Issue number1
DOIs
Publication statusPublished - 21 Nov 2018

Keywords

  • Galaxy: abundances
  • Galaxy: formation
  • Galaxy: globular clusters: general
  • Stars: abundances
  • Stars: AGB and post-AGB

Cite this

MacLean, B. T. ; Campbell, S. W. ; Amarsi, A. M. ; Nordlander, T. ; Cottrell, P. L. ; De Silva, G. M. ; Lattanzio, J. ; Constantino, T. ; D'Orazi, V. ; Casagrande, L. / On the AGB stars of M4 : A robust disagreement between spectroscopic observations and theory. In: Monthly Notices of the Royal Astronomical Society. 2018 ; Vol. 481, No. 1. pp. 373-395.
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abstract = "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.",
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MacLean, BT, Campbell, SW, Amarsi, AM, Nordlander, T, Cottrell, PL, De Silva, GM, Lattanzio, J, Constantino, T, D'Orazi, V & Casagrande, L 2018, 'On the AGB stars of M4: A robust disagreement between spectroscopic observations and theory', Monthly Notices of the Royal Astronomical Society, vol. 481, no. 1, pp. 373-395. https://doi.org/10.1093/mnras/sty2297

On the AGB stars of M4 : A robust disagreement between spectroscopic observations and theory. / MacLean, B. T.; Campbell, S. W.; Amarsi, A. M.; Nordlander, T.; Cottrell, P. L.; De Silva, G. M.; Lattanzio, J.; Constantino, T.; D'Orazi, V.; Casagrande, L.

In: Monthly Notices of the Royal Astronomical Society, Vol. 481, No. 1, 21.11.2018, p. 373-395.

Research output: Contribution to journalArticleResearchpeer-review

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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.

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KW - Galaxy: abundances

KW - Galaxy: formation

KW - Galaxy: globular clusters: general

KW - Stars: abundances

KW - Stars: AGB and post-AGB

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