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A Chemical Trompe-L'ceil: No Iron Spread In The Globular Cluster M22

Mucciarelli, A and Lapenna, E and Massari, D and Pancino, E and Stetson, PB and Ferraro, FR and Lanzoni, B and Lardo, C (2015) A Chemical Trompe-L'ceil: No Iron Spread In The Globular Cluster M22. Astrophysical Journal, 809 (2). ISSN 0004-637X

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Abstract

We present the analysis of high-resolution spectra obtained with UVES and UVES-FLAMES at the Very Large Telescope of 17 giants in the globular cluster M22, a stellar system suspected to have an intrinsic spread in the iron abundance. We find that when surface gravities are derived spectroscopically (by imposing to obtain the same iron abundance from FeI and FeII lines) the [Fe/H] distribution spans _0.5 dex, according to previous analyses. However, the gravities obtained in this way correspond to unrealistic low stellar masses (0.1-0.5 M⊙) for most of the surveyed giants. Instead, when photometric gravities are adopted, the [FeII/H] distribution shows no evidence of spread at variance with the [FeI/H] distribution. This difference has been recently observed in other clusters and could be due to non-local thermodynamical equilibrium effects driven by over- ionization mechanisms, that mainly affect the neutral species (thus providing lower [FeI/H]) but leave [FeII/H] unaltered. We confirm that the s-process elements show significant star-to-star variations and their abundances appear to be correlated with the difference between [FeI/H] and [FeII/H]. This puzzling finding suggests that the peculiar chemical composition of some cluster stars may be related to effects able to spuriously decrease [FeI/H]. We conclude that M22 is a globular cluster with no evidence of intrinsic iron spread, ruling out that it has retained the supernovae ejecta in its gravitational potential well.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences, 0305 Organic Chemistry, 0306 Physical Chemistry (Incl. Structural)
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society; IOP Publishing
Related URLs:
Date Deposited: 22 Nov 2016 12:37
Last Modified: 07 Sep 2017 02:49
DOI or Identification number: 10.1088/0004-637X/809/2/128
URI: http://researchonline.ljmu.ac.uk/id/eprint/4841

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