Angular Momentum Changes Due to Direct Impact Accretion in a Simplified Binary System

TL;DR

This study models angular momentum exchange in binary star systems during direct impact mass transfer, revealing that such impact can sometimes increase orbital angular momentum rather than decrease it, challenging previous assumptions.

Contribution

The paper introduces a simplified ballistic model to analyze angular momentum transfer, showing that direct impact can transfer angular momentum from stellar spins to the orbit, contrary to prior beliefs.

Findings

Direct impact can increase orbital angular momentum.

Angular momentum transfer depends on system parameters.

Simulation results challenge previous assumptions.

Abstract

We model a circular mass-transferring binary system to calculate the exchange of angular momentum between stellar spins and the orbit due to direct impact of the mass transfer stream onto the surface of the accretor. We simulate mass transfer by calculating the ballistic motion of a point mass ejected from the $L_1$ point of the donor star, conserving the total linear and angular momentum of the system, and treating the stars as uniform density spheres with main sequence radii determined by their masses. We show that, contrary to previous assumptions in the literature, direct impact does not always act as a sink of orbital angular momentum and may in fact increase it by facilitating the transfer of angular momentum from the spin of the donor to the orbit. Here, we show an example of the exchange of angular momentum, as well as a measure of the orbital angular momentum changes for a variety of binary star systems with main sequence components.