Atomic diffusion and mixing in old stars: III. Analysis of NGC 6397 stars under new constraints
2012 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 753, no 1, 48- p.Article in journal (Refereed) Published
We have previously reported on chemical abundance trends with evolutionary state in the globular cluster NGC 6397 discovered in analyses of spectra taken with FLAMES at the Very Large Telescope. Here, we reinvestigate the FLAMES-UVES sample of 18 stars, ranging from just above the turnoff point to the red giant branch below the bump. Inspired by new calibrations of the infrared flux method, we adopt a set of hotter temperature scales. Chemical abundances are determined for six elements (Li, Mg, Ca, Ti, Cr, and Fe). Signatures of cluster-internal pollution are identified and corrected for in the analysis of Mg. On the modified temperature scales, evolutionary trends in the abundances of Mg and Fe are found to be significant at the 2 sigma and 3 sigma levels, respectively. The detailed evolution of abundances for all six elements agrees with theoretical isochrones, calculated with effects of atomic diffusion and a weak to moderately strong efficiency of turbulent mixing. The age of these models is compatible with the external determination from the white dwarf cooling sequence. We find that the abundance analysis cannot be reconciled with the strong turbulent-mixing efficiency inferred elsewhere for halo field stars. A weak mixing efficiency reproduces observations best, indicating a diffusion-corrected primordial lithium abundance of log epsilon(Li) = 2.57 +/- 0.10. At 1.2 sigma, this value agrees well with Wilkinson Microwave Anisotropy Probe calibrated big bang nucleosynthesis predictions.
Place, publisher, year, edition, pages
2012. Vol. 753, no 1, 48- p.
diffusion, globular clusters: individual (NGC 6397), stars: abundances, stars: atmospheres, stars: evolution, stars: fundamental parameters, stars: Population II
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:uu:diva-177847DOI: 10.1088/0004-637X/753/1/48ISI: 000305632500048OAI: oai:DiVA.org:uu-177847DiVA: diva2:541609