The radial distributions of the two main-sequence components in the young massive star cluster NGC 1856

TL;DR

The study investigates the spatial distribution of two main-sequence components in the young star cluster NGC 1856, finding they are homogeneously distributed, and suggests rapid stellar rotation as the likely cause of the observed bifurcation.

Contribution

This paper provides the first analysis of the radial distribution of multiple stellar populations in NGC 1856, favoring stellar rotation over multiple generations as the explanation for the main-sequence split.

Findings

Both components are homogeneously distributed across the cluster.

Rapid stellar rotation can explain the main-sequence bifurcation.

Multiple stellar populations model is less favored due to spatial homogeneity.

Abstract

The recent discovery of double main sequences in the young, massive star cluster NGC 1856 has caught significant attention. The observations can be explained by invoking two stellar generations with different ages and metallicities or by a single generation of stars composed of two populations characterized by different rotation rates. We analyzed the number ratios of stars belonging to both main-sequence components in NGC 1856 as a function of radius. We found that their number ratios remain approximately unchanged from the cluster's central region to its periphery, indicating that both components are homogeneously distributed in space. Through a comparison of the loci of the best-fitting isochrones with the ridge lines of both stellar components, we found that both multiple stellar populations and rapid stellar rotation can potentially explain the observed main-sequence bifurcation in NGC 1856. However, if NGC1856 were a young representative of the old globular clusters, then the multiple stellar populations model would not be able to explain the observed homogeneity in the spatial distributions of these two components, since all relevant scenarios would predict that the second stellar generation should be formed in a more compact configuration than that of the first stellar generation, while NGC 1856 is too young for both stellar generations to have been fully mixed dynamically. We speculate that the rapid stellar rotation scenario would be the favored explanation of the observed multiple stellar sequences in NGC 1856.