AGB sodium abundances in the globular cluster 47 Tucanae (NGC 104)

Christian I. Johnson, Iain McDonald, Catherine A. Pilachowski, Mario Mateo, John I. Bailey, Maria J. Cordero, Albert A. Zijlstra, Jeffrey D. Crane, Edward Olszewski, Stephen A. Shectman, Ian Thompson

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40 Scopus citations

Abstract

A recent analysis comparing the [Na/Fe] distributions of red giant branch (RGB) and asymptotic giant branch (AGB) stars in the Galactic globular cluster NGC 6752 found that the ratio of Na-poor to Na-rich stars changes from 30:70 on the RGB to 100:0 on the AGB. The surprising paucity of Na-rich stars on the AGB in NGC 6752 warrants additional investigations to determine if the failure of a significant fraction of stars to ascend the AGB is an attribute common to all globular clusters. Therefore, we present radial velocities, [Fe/H], and [Na/Fe] abundances for 35 AGB stars in the Galactic globular cluster 47 Tucanae (47 Tuc; NGC 104), and compare the AGB [Na/Fe] distribution with a similar RGB sample published previously. The abundances and velocities were derived from high-resolution spectra obtained with the Michigan/Magellan Fiber System and MSpec spectrograph on the Magellan-Clay 6.5 m telescope. We find the average heliocentric radial velocity and [Fe/H] values to be 〈RVhelio.〉 = -18.56 km s-1 (σ = 10.21 km s-1) and 〈[Fe H]〉 = -0.68 (σ = 0.08), respectively, in agreement with previous literature estimates. The average [Na/Fe] abundance is 0.12 dex lower in the 47 Tuc AGB sample compared to the RGB sample, and the ratio of Na-poor to Na-rich stars is 63:37 on the AGB and 45:55 on the RGB. However, in contrast to NGC 6752, the two 47 Tuc populations have nearly identical [Na/Fe] dispersion and interquartile range values. The data presented here suggest that only a small fraction (≲20%) of Na-rich stars in 47 Tuc may fail to ascend the AGB, which is a similar result to that observed in M13. Regardless of the cause for the lower average [Na/Fe] abundance in AGB stars, we find that Na-poor stars and at least some Na-rich stars in 47 Tuc evolve through the early AGB phase. The contrasting behavior of Na-rich stars in 47 Tuc and NGC 6752 suggests that the RGB [Na/Fe] abundance alone is insufficient for predicting if a star will ascend the AGB.

Original languageEnglish (US)
Article number71
JournalAstronomical Journal
Volume149
Issue number2
DOIs
StatePublished - Feb 1 2015

Keywords

  • Globular clusters: general
  • Globular clusters: individual (47 Tucanae, NGC 104)
  • Stars: abundances

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Johnson, C. I., McDonald, I., Pilachowski, C. A., Mateo, M., Bailey, J. I., Cordero, M. J., Zijlstra, A. A., Crane, J. D., Olszewski, E., Shectman, S. A., & Thompson, I. (2015). AGB sodium abundances in the globular cluster 47 Tucanae (NGC 104). Astronomical Journal, 149(2), [71]. https://doi.org/10.1088/0004-6256/149/2/71