Radial Dependence of the Proto-Globular Cluster Contribution to the Milky Way Formation

TL;DR

This study investigates how second-generation, He-rich stars from globular clusters contribute to the Milky Way's bulge and inner halo, revealing a radial dependence in their distribution supported by observational proxies and simulations.

Contribution

It introduces a radial analysis of He-rich second-generation star contribution from globular clusters to the Milky Way's inner regions, supported by observational proxies and dynamical simulations.

Findings

Fraction of He-rich stars increases towards the Galactic center.

Simulations show tidal forces strip more stars from cluster centers closer to the core.

He-enhanced stars likely play a significant role in the MW bulge and inner halo formation.

Abstract

Recent interpretation of the color$-$magnitude diagrams of the Milky Way (MW) bulge has suggested that the observed double red-clump feature can be a natural consequence of He-enhanced stellar populations in the MW bulge. This implies that globular clusters (GCs), where the He-enhanced second-generation (SG) stars can be efficiently created, are the most likely candidate contributors of He-rich stars to the MW bulge. We extend this idea to the Galactic inner halo and investigate the fraction of the SG stars as a function of the Galactocentric distance. We use bluer blue-horizontal branch (bBHB) stars, which are assumed to be originated from He-rich SG populations, as proxies of SG stars, and find that the fraction of bBHB stars increases with decreasing Galactocentric distance. Simulations of the GC evolution in the MW tidal field qualitatively support the observed trend of bBHB enhancement in the inner halo. In these simulations, the increasing tidal force with decreasing Galactocentric distance leads to stripping of stars not only from the outskirts but also from the central regions of GCs, where SG stars are more abundant. We discuss the implication and prospect of our findings concerning the formation history of the bulge and inner halo of the MW.