Spatial mixing of binary stars in multiple-population globular clusters

Abstract

We present the results of a study aimed at investigating the effects of dynamical evolution on the spatial distribution and mixing of primordial binary stars in multiple-population globular clusters. Multiple stellar population formation models predict that second-generation (SG) stars form segregated in the inner regions of a more extended first-generation (FG) cluster. Our study, based on the results of a survey of N -body simulations, shows that the spatial mixing process for binary stars is more complex than that of single stars since additional processes such as binary ionization, recoil and ejection following binary-single and binarybinary interactions play a key role in determining the spatial distribution of the population of surviving binaries. The efficiency and relative importance of these additional effects depends on the binary binding energy and determines the timescale of the spatial mixing of FG and SG binaries. Our simulations illustrate the role of ionization, recoil and ejection combined with the effects of mass segregation driven by two-body relaxation and show that the complex interplay of all these processes results in a significant extension of the time needed for the complete spatial mixing of FG and SG binaries compared to that of single stars. Clusters in which FG and SG single stars have already reached complete spatial mixing might be characterized by a significant radial gradient in the ratio of the FG-to-SG binary fraction. The implications of the delayed mixing of FG and SG binaries for the differences between the kinematics of the two populations are discussed.

Unexpected kinematics of multiple populations in globular clusters

“…To estimate the potential effect of such different binary fractions on the derived velocity dispersion profiles, we have studied the evolution of the line-of-sight velocity dispersions of FP and SP stars in a set of N -body simulations (Vesperini et al 2018;Hong et al 2019) also including binaries and compared them to the observations. Models are not tailored to fit in detail the dynamics of NGC6362, but rather to gather some fundamental insight into the dynamical ingredients necessary to explain the observational results.…”

The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters – XXI. Binaries among multiple stellar populations

“…The evolution of the ratio of the 1G to the 2G binary incidence is driven by the initial differences between the structural properties of the 1G and the 2G populations and depends on the cluster's dynamical age as well as on the binary properties (see e.g. Hong et al 2015Hong et al , 2016Hong et al , 2019.…”

“…Calura et al 2019). Since the rates of binary disruption and evolution of the parameters of surviving binaries strongly depend on the stellar density, the incidence of binaries in first-and secondgeneration (hereafter 1G and 2G) stars of GCs can provide information and constraints on their formation environment and their long-term evolution (Vesperini et al 2011;Hong et al 2015Hong et al , 2016Hong et al , 2019.…”

“…In the interpretation of observations covering a specific range of radial distances from a cluster's center, it is necessary to consider that local values of the 1G and 2G binary incidences (i.e. values of the binary incidence measured at a given distance from a cluster's center) are determined by a combination of dynamical effects on binary evolution and disruption and the extent of spatial mixing reached by a cluster at any given time during its dynamical evolution (Hong et al 2019).…”

Dynamical effects on the stellar mass function of multiple stellar populations in globular clusters

“…The differences between the structural properties of 1G and 2G stars have been also shown to result in significant differences in the evolution and survival of 1G and 2G binary stars and their spatial distribution (see e.g. Vesperini et al 2011;Lucatello et al 2015;Hong et al 2015Hong et al , 2016Hong et al , 2019Dalessandro et al 2018). Finally a number of theoretical investigations have been devoted to the study of the evolution of the 1G and 2G structural properties and the spatial mixing of the two populations (see e.g.…”