AGB-star Mass Transfer Products in Stellar Streams as Tracers of Density in Dissolved Star Clusters

Abstract

The least dense globular clusters (GCs) appear to host the highest fractions of stars with chemical signatures of mass transfer from asymptotic giant branch (AGB) companions. Recent observations suggest the fraction may be even higher in stellar streams that are debris of tidally disrupted (preferentially low-density) GCs. We explore whether this is attributable to lower encounter rates in less-dense GCs allowing more of the relevant binaries to survive dynamical hardening, or outright disruption, long enough to undergo AGB mass transfer. We run a small suite of GC simulations varying initial density using the code CMC with its latest observationally motivated binary prescriptions. Using CMC's built-in COSMIC code for binary stellar evolution, we first identify primordial binaries that would, if left unperturbed, likely undergo AGB mass transfer via either standard or wind Roche-lobe overflow. We then check how many of those binaries survive GC dynamics in CMC long enough to experience AGB mass transfer. We find that wind Roche-lobe overflow is necessary to reproduce the fraction of (sub)giant stars in Galactic GCs that are likely products of AGB mass transfer (~1%) and that binary dynamics plausibly explain why this fraction appears lower in dense, intact GCs than stellar streams. The abundance of mass-transfer products (also collision products and binaries) in upcoming surveys of stellar streams without known progenitors will thus probe the densities of dissolved GCs---difficult to determine otherwise but key to improved stream modeling and predicted rates of many astrophysical transients.