Physical and Chemical Properties of Galactic Global Clusters with Various Origins Identified from the Gaia DR2 Data

TL;DR

This study investigates the physical and chemical properties of galactic globular clusters with various origins using Gaia DR2 data, revealing differences linked to their formation sites and evolutionary histories.

Contribution

It provides new insights into the origins and evolution of globular clusters by analyzing their chemical abundances and orbital parameters in relation to their formation environments.

Findings

Metal-poor clusters have high alpha-element abundances.

Clusters of low energy are related to a single protogalactic cloud.

Accreted clusters with large initial mass show complex stellar populations.

Abstract

The differences in the relationships between the physical parameters and the chemical-element abundances in accreted globular star clusters and those formed inside the Galaxy have been investigated. The information on the supposed formation sites of the clusters based on the Gaia~DR2 data is borrowed from the literature. Those sources estimate the probability of belonging to the Galactic bulge and disk, as well as to six known events of the merger of dwarf satellite galaxies with the Milky Way, for 151~globular clusters. It is shown that all metal-poor ($\rm{[Fe/H]}<-1.0$) genetically related globular clusters have high relative abundances of $\alpha$-elements. According to modern views, since type II supernovae release more $\alpha$-elements into the interstellar medium with increasing mass, it has been suggested that masses of type II supernovae in the Galaxy were greater than in the accreted galaxies. It is proved that the clusters of the low-energy group, which were considered accreted, are genetically related to a single protogalactic cloud, same as the unstratified clusters UKS~1 and Liller~1, which most likely belong to the bulge. It is shown that not only the lower but also the upper limits of the clusters' masses decrease with an increase in the average radius of their orbits. The latter fact is explained by a decrease in the masses of emerging clusters with a decrease in the masses of their host galaxies. It is demonstrated that an extremely multicomponent stellar population is observed only in accreted globular clusters with an initial mass $>10^{6}M_{\odot}$. It has been suggested that these clusters retained all the matter ejected by their evolved stars, from which new generations of stars formed due to long evolution far from our Galaxy.