Binary Star Evolution Modules in REBOUNDx

Abstract

Close-binary evolution couples Roche-lobe overflow (RLOF), common-envelope (CE) drag, stellar winds, magnetic braking, and gravitational-wave losses, exchanging mass and angular momentum while reshaping orbits and spins. We present interoperable effects in the REBOUNDx extension to REBOUND that embed these processes within high-accuracy N-body dynamics. The suite includes: a momentum-conserving RLOF operator with conservative and systemic channels and configurable specific-j loss; a CE drag model based on Mach-dependent dynamical friction with kick limiting; isotropic Reimers winds, Parker-type thermal winds, and Eddington-limited outflows powered by a parametric stellar-evolution module supplying mass-dependent R and L; magnetic braking via the Verbunt-Zwaan/Kawaler torque with a saturation-aware closed-form spin update; and post-Newtonian corrections 2PN point-mass and spin-spin; 2.5PN radiation reaction. Linear momentum is conserved for conservative transfer, a minimal corrective torque enforces angular-momentum consistency, and adaptive sub-stepping stabilizes evolution near contact. Inter-module flags coordinate wind/RLOF/CE activity. The unit-agnostic framework enables self-consistent, time-resolved studies of close binaries in isolated or dynamically rich settings. Multiple examples and comparisons against other codes are provided in the Appendix. The code is available at https://github.com/malidib/ReboundS .