The discovery of an anomalous RGB in M 2.

Lardo, C.; Pancino, E.; Mucciarelli, A.; Milone, A. P.
Bibliographical reference

Memorie della Societa Astronomica Italiana, v.84, p.228 (2013)

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Using UV images taken with the Telescopio Nazionale Galileo, we discovered an anomalous sequence in the color-magnitude diagram of M 2. This feature appears as a narrow poor-populated red giant branch, which extends down to the sub giant branch region. We speculate that this new feature could be the extension of the faint component of the split sub giant branch recently discovered by Piotto et al. We identified in our U,V images two CH stars detected in previous studies. These stars, which are both cluster members, fall on this redder sequence, suggesting indeed that the anomalous RGB should have a peculiar chemical pattern. Unfortunately, no additional spectra were obtained for stars in this previously unknown substructure.