The double Subgiant Branch of NGC 1851: the role of the CNO abundance

TL;DR

This paper investigates whether the split in NGC 1851's Subgiant branch is caused by two stellar populations with different CNO abundances, suggesting they are nearly the same age but with different chemical compositions.

Contribution

It proposes a hypothesis that the Subgiant branch split results from two populations with distinct CNO abundances and tests this against empirical constraints.

Findings

Two populations are likely coeval with similar initial He-content.

A significant increase in CNO abundance can explain the split.

Further spectroscopic data are needed to confirm the scenario.

Abstract

We explore the possibility that the anomalous split in the Subgiant branch of the galactic globular cluster NGC 1851 is due to the presence of two distinct stellar populations with very different initial metal mixtures: a normal alpha-enhanced component, and one characterized by strong anticorrelations among the CNONa abundances, with a total CNO abundance increased by a factor of two. We test this hypothesis taking into account various empirical constraints, and conclude that the two populations should be approximately coeval, with the same initial He-content. More high-resolution spectroscopical measurements of heavy elements -- and in particular of the CNO sum -- for this cluster are necessary to prove (or disprove) this scenario.