A Chemical Trompe-L'oeil: No Iron Spread in the Globular Cluster M22
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 (GC) M22, a stellar system suspected to have an intrinsic spread in iron abundance. We find that when surface gravities are derived spectroscopically (in order to obtain the same iron abundance from Fe i and Fe ii lines) the [Fe/H] distribution spans ∼0.5 dex, according to previous analyses. However, the gravities obtained in this way correspond to unrealistically low stellar masses (0.1-0.5 {M}⊙ ) for most of the surveyed giants. Instead, when photometric gravities are adopted, the [Fe ii/H] distribution shows no evidence of spread, in variance with the [Fe i/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 [Fe i/H]), but leave [Fe ii/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 [Fe i/H] and [Fe ii/H]. This puzzling finding suggests that the peculiar chemical composition of some cluster stars may be related to effects able to spuriously decrease [Fe i/H]. We conclude that M22 is a GC with no evidence of intrinsic iron spread, ruling out that it has retained the supernovae ejecta in its gravitational potential well.
Based on observations collected at the ESO-VLT under programs 068.D-0332, 071.D-0217 and 073.D-0211.- Publication:
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The Astrophysical Journal
- Pub Date:
- August 2015
- DOI:
- 10.1088/0004-637X/809/2/128
- arXiv:
- arXiv:1507.01596
- Bibcode:
- 2015ApJ...809..128M
- Keywords:
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- stars: abundances;
- techniques: spectroscopic;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication to ApJ