The discovery of an anomalous RGB in M 2

TL;DR

This study reports the discovery of an unusual red giant branch sequence in the globular cluster M 2 using UV imaging, potentially indicating a unique chemical composition and extending our understanding of stellar populations.

Contribution

The paper presents the first detection of an anomalous RGB in M 2, linking it to chemical peculiarities and previous subgiant branch discoveries.

Findings

Identified a narrow, poorly populated red giant branch in M 2.

Associated the anomalous RGB with stars showing peculiar chemical patterns.

Suggested the anomalous RGB extends the split subgiant branch previously observed.

Abstract

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 UV 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