The Presence of Two Distinct Red Giant Branches in the Globular Cluster NGC 1851

TL;DR

This study reveals two distinct red giant branches in NGC 1851, indicating multiple stellar populations with different heavy element abundances, suggesting complex chemical enrichment processes.

Contribution

First detection of split RGB in NGC 1851 in U - I color, linking it to heavy element variation and multiple stellar populations.

Findings

RGB split observed in U - I color but not in V - I.

Heavy element abundance variation confirmed via Calcium measurements.

Bimodal horizontal-branch distribution explained by helium enhancement in second-generation stars.

Abstract

There is a growing body of evidence for the presence of multiple stellar populations in some globular clusters, including NGC 1851. For most of these peculiar globular clusters, however, the evidence for the multiple red giant-branches (RGBs) having different heavy elemental abundances as observed in Omega Centauri is hitherto lacking, although spreads in some lighter elements are reported. It is therefore not clear whether they also share the suggested dwarf galaxy origin of Omega Cen or not. Here we show from the CTIO 4m UVI photometry of the globular cluster NGC 1851 that its RGB is clearly split into two in the U - I color. The two distinct RGB populations are also clearly separated in the abundance of heavy elements as traced by Calcium, suggesting that the type II supernovae enrichment is also responsible, in addition to the pollutions of lighter elements by intermediate mass asymptotic giant branch stars or fast-rotating massive stars. The RGB split, however, is not shown in the V - I color, as indicated by previous observations. Our stellar population models show that this and the presence of bimodal horizontal-branch distribution in NGC 1851 can be naturally reproduced if the metal-rich second generation stars are also enhanced in helium.