Research

Abstract

It is currently unknown to what extent binary stars affect the velocity dispersion of stellar streams because the binary fraction in stellar streams, as well as the fraction of binaries detectable by Gaia, remains poorly constrained. It is also unclear how the population densities of binary stars and compact objects in globular clusters change over time as clusters are tidally stripped by internal dynamics, especially in initially high-mass globular clusters. Researchers use the FIRE-1 and FIRE-2 simulations together with the Cluster Monte Carlo code to evolve globular clusters at star-by-star resolution. We plot positions along the orbit, orbital-period histograms, and proper-motion histograms for binary stars at the final recorded simulation time of 13.7 Gyr. The period histogram indicates that approximately 98% of binaries can be detected by Gaia through proper motion, while approximately 20% can be detected through orbital periods. Scatter plots of positions along the orbit show that, in low-density streams, binaries tend to occupy the thinner parts of the stream and often coincide with overdensities of single stars. Because binary systems are generally more massive than single stars, this distribution may indicate mass segregation in some cases. However, this inference is poorly constrained and does not account for variation with stream density or internal evolution. In high-density streams, stars exhibit a broader distribution along the stream than in low-density streams, although this observation is imprecise. Overall, binary stars do not appear to scatter more widely than single stars.