Secondary star formation within massive star clusters: Origin of multiple stellar populations in globular clusters

TL;DR

This study uses 3D hydrodynamical simulations to explore how gaseous ejecta from AGB stars can form second-generation stars within massive star clusters, explaining multiple stellar populations in globular clusters.

Contribution

It demonstrates that star clusters above a threshold mass can efficiently convert AGB ejecta into new stars, providing a physical mechanism for multiple populations in GCs.

Findings

Gaseous ejecta retention is efficient in clusters >10^6 M_sun.

High star formation efficiencies lead to very compact second-generation clusters.

Deeper gravitational potential in nuclear clusters enhances ejecta retention and star formation.

Abstract

We numerically investigate whether and how gaseous ejecta from AGB stars can be converted into new stars within originally massive star clusters (MSCs) in order to understand the origin of multiple stellar populations in globular clusters (GCs). We adopt a scenario in which (i) MSCs with masses of M_s can be formed from high-mass, high-density giant molecular clouds (GMCs) in their host galactic building blocks embedded in dark matter halos at high redshifts and (ii) their evolution therefore can be significantly influenced by M_s, their initial locations, and physical properties of their hosts. Our 3D hydrodynamical simulations show that gaseous ejecta from AGB stars can be retained within MSCs and consequently converted into new stars very efficiently in the central regions of MSCs, only if M_s exceed a threshold mass (M_th) of ~10^6 M_sun. The new stars can correspond to the ``second generation (SG)'' of stars with higher Na and lower O abundances observed in GCs. Star formation efficiencies during the formation of SG stars within MSCs with M_s > M_th can be rather high (0.3-0.9) so that very compact new clusters within original MSCs can be formed. M_s should be as large as 10^6-10^7 M_sun to explain the observed large fraction of SG stars in the present ordinary Galactic GCs, because new stars can consist of only 1-4% among all stars for the standard IMF. Nuclear MSCs are found to retain much more effectively the AGB ejecta and convert more efficiently the gas into new stars owing to much deeper gravitational potential of their hosts. We suggest that both M_s and their locations within their hosts can determine whether abundance spread can be seen only in light elements or even in heavy ones.