The formation and evolution of dark star clusters II: The impact of primordial mass segregation

TL;DR

This study uses N-body simulations to explore how primordial mass segregation influences the formation, evolution, and observable properties of dark star clusters, highlighting its effects on cluster lifespan, black hole populations, and gravitational wave potential.

Contribution

It provides new insights into the role of primordial mass segregation in dark star cluster development, including effects on their timing, structure, and black hole dynamics.

Findings

Clusters with higher primordial mass segregation reach the dark phase earlier.

Primordial mass segregation extends the maximum Galactocentric distance for dark phase entry.

Higher segregation increases binary black hole formation rates by 2.5 times.

Abstract

We investigate the impact of primordial mass segregation on the formation and evolution of dark star clusters (DSCs). Considering a wide range of initial conditions, we conducted $N$-body simulations of globular clusters (GCs) around the Milky Way. In particular, we assume a canonical IMF for all GCs without natal kicks for supernovae remnants, namely neutron stars or black holes. Our results demonstrate that clusters with larger degrees of primordial mass segregation reach their DSC phase earlier and spend a larger fraction of their dissolution time in such a phase, compared to clusters without mass segregation. In primordially segregated clusters, the maximum Galactocentric distance that the clusters can have to enter the DSC phase is almost twice that of the clusters without primordial mass segregation. Primordially segregated clusters evolve with a higher number of stellar mass black holes, accelerating energy creation in their central regions and consequently increasing evaporation rates and cluster sizes during dark phases. The simulations reveal that aggregating heavy components at the centre doubles the time spent in the dark phase. Additionally, the study identifies potential links between simulated dark clusters and initial conditions of Milky Way globular clusters, suggesting some may transition to dark phases before dissolution. Higher primordial mass segregation coefficients amplify the average binary black hole formation rate by 2.5 times, raising higher expectations for gravitational wave emissions.