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arXiv · 1804.04158

Metallicity variations in the Type II globular cluster NGC6934

A. F. Marino·D. Yong·A. P. Milone·G. Piotto·M. Lundquist·L. R. Bedin·A. -N. Chene'·G. Da Costa·M. Asplund·H. Jerjen
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Abstract

The Hubble Space Telescope photometric survey of Galactic globular clusters (GCs) has revealed a peculiar "chromosome map" for NGC6934. Besides a typical sequence, similar to that observed in Type I GCs, NGC 6934 displays additional stars on the red side, analogous to the anomalous, Type II GCs, as defined in our previous work. We present a chemical abundance analysis of four red giants in this GC. Two stars are located on the chromosome map sequence common to all GCs, and another two on the additional sequence. We find: (i) star-to-star Fe variations, with the two anomalous stars being enriched by ~0.2 dex. Due to our small-size sample, this difference is at the ~2.5 sigma level; (ii) no evidence for variations in the slow neutron-capture abundances over Fe, at odds with what is often observed in anomalous Type II GCs, e.g. M 22 and Omega Centauri; (iii) no large variations in light elements C, O and Na, compatible with the targets location on the lower part of the chromosome map where such variations are not expected. Since the analyzed stars are homogeneous in light elements, the only way to reproduce the photometric splits on the sub-giant (SGB) and the red-giant (RGB) branches is to assume that red-RGB/faint-SGB stars are enhanced in [Fe/H] by ~0.2. This fact corroborates the spectroscopic evidence of a metallicity variation in NGC6934. The observed chemical pattern resembles only partially the other Type II GCs, suggesting that NGC6934 might belong either to a third class of GCs, or be a link between normal Type I and anomalous Type II GCs.

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A. F. Marino, D. Yong, A. P. Milone, G. Piotto, M. Lundquist, L. R. Bedin, A. -N. Chene', G. Da Costa, M. Asplund, H. Jerjen. 2018-04-11. Metallicity variations in the Type II globular cluster NGC6934. https://doi.org/10.3847/1538-4357%2Faabdea

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