Evidence for primordial mass segregation in globular clusters

TL;DR

This study investigates how primordial mass segregation influences the evolution and current mass functions of globular clusters, suggesting initial mass segregation explains observed low-mass star depletion and cluster properties.

Contribution

It provides evidence that primordial mass segregation can account for observed mass function slopes and cluster concentration correlations in globular clusters.

Findings

Globular clusters closer to dissolution are more depleted in low-mass stars.

Primordial mass segregation explains the strong low-mass star depletion in some clusters.

A new algorithm for initializing mass segregated clusters with arbitrary profiles is introduced.

Abstract

We have studied the dissolution of initially mass segregated and unsegregated star clusters due to two-body relaxation in external tidal fields, using Aarseth's collisional N-body code NBODY4 on GRAPE6 special-purpose computers. When extrapolating results of initially not mass segregated models to globular clusters, we obtain a correlation between the time until destruction and the slope of the mass function, in the sense that globular clusters which are closer to dissolution are more strongly depleted in low-mass stars. This correlation fits observed mass functions of most globular clusters. The mass functions of several globular clusters are however more strongly depleted in low-mass stars than suggested by these models. Such strongly depleted mass functions can be explained if globular clusters started initially mass segregated. Primordial mass segregation also explains the correlation between the slope of the stellar mass function and the cluster concentration which was recently discovered by De Marchi et al. (2007). In this case, it is possible that all globular clusters started with a mass function similar to that seen in young open clusters in the present-day universe, at least for stars below m=0.8 Msun. This argues for a near universality of the mass function for different star formation environments and metallicities in the range -2 < [Fe/H] < 0. We finally describe a novel algorithm which can initialise stationary mass segregated clusters with arbitrary density profile and amount of mass segregation.