The Binary Populations of Stellar Streams are Set by Cluster Dynamics

Abstract

We present a suite of direct N-body simulations of low mass ($<10^4~M_{\odot}$) globular cluster streams initialized with observationally-motivated binary demographics in order to understand the effect of in-cluster dynamical processing on the stream binary population. The models are initialized with a range of stellar densities and cluster orbits, and Poisson variation in the number of massive and short-lived stars. Wide binaries are disrupted on short timescales by internal tides and on long timescales by two-body encounters. Tides are most important prior to impulsive mass loss-driven cluster expansion. Close binaries ($P_{\rm orb}<10^2~\rm yr$) are most abundant at the stream center due to cluster mass segregation. The wide binary fraction and the degree of binary segregation in the resulting stream are sensitive to the initial cluster density and massive star fraction. In mock radial velocity surveys of the simulated streams, undetectable binaries have velocity amplitudes of $\sim$$0.5$-$1~\rm km~s^{-1}$, adding $\sim0.1~\rm km\ s^{-1}$ of velocity dispersion to the streams, and are dynamically depleted by $\sim10$-$60\%$ compared to the initial binary population. Custom N-body models of Milky Way streams with binaries will allow a holistic understanding of their dynamical structures in advance of upcoming multi-epoch spectroscopic surveys.

Figures (16)

• Figure 1: Schematic demonstration of the effects of cluster mass loss on its structure. The black line shows the cluster mass loss history, including impulsive mass loss (which occurs at $t=2~t_{\rm dyn}$ in this example) and adiabatic mass loss (which begins at $t=3~t_{\rm dyn}$). After impulsive mass loss, the cluster radius increases and its density decreases on a dynamical time. During adiabatic mass loss, the radius and density change more gradually. The transition points depend on the stellar population and dynamical state of the cluster. The annotations at the top of the panel note relevant disruption mechanisms for wide binaries in the low mass clusters studied in this work.
• Figure 2: The two samplings of the kroupa2001 initial mass function used in this work. Poisson variation leads to more high-mass stars in the hi-OB draw than in the lo-OB draw.
• Figure 3: Summary of COSMIC binary demographics used in our initial conditions. We show the distributions in orbital period $P$, eccentricity $e$, and mass ratio $q$ (as probability density functions) and the binary fraction as a function of primary mass for a $N=10^6$ stellar population in small panels. The large panel shows the two-dimensional distribution in eccentricity versus period for a subset of 2000 binaries. We show the total demographic distributions, along with the distributions for the primaries in several mass ranges separately, motivated by the piecewise prescriptions in moe2017. The eccentricity distribution includes a peak of circularized systems at short periods, and the mass ratio distribution includes a peak of equal mass systems. The dashed black curve in the bottom left panel shows the underlying binary fraction as a function of primary mass from the COSMIC sampler, and the colored curve shows the resulting sample in bins of primary mass.
• Figure 4: Snapshots over the course of the evolution of each lo-OB cluster with $R_{\rm vir,0}=0.75$ pc. The first, second, and third rows of panels display clusters on circular, GD-1, and Pal 5-like orbits, respectively. Successive columns display the stream at later stages of the progenitor cluster's dissolution. The circular orbit is shown in the $x\text{-}y$ plane, while the eccentric orbits are shown in the meridional plane ($z$ vs $R=\sqrt{x^2 + y^2}$) to best showcase the orbital structure.
• Figure 5: Dissolution time of each simulated cluster listed in Table \ref{['tab:simulation_grid']} as a function of initial virial radius for lo-OB (left panel) and hi-OB (right panel) simulations. Marker shapes correspond to the progenitor orbit.