# How Drag Force Evolves in Global Common Envelope Simulations

## TL;DR

This study investigates how drag forces evolve during common envelope phases in binary star systems through global simulations, revealing when traditional models apply and explaining late-stage drag reduction.

## Contribution

It provides detailed global simulation data on drag force evolution in common envelope events, highlighting discrepancies with analytic models and the impact of flow symmetry.

## Key findings

- Drag force matches analytic predictions during initial in-spiral
- Drag force significantly weaker after first periastron passage
- Flow symmetry reduces net drag at late stages

## Abstract

We compute the forces, torque and rate of work on the companion-core binary due to drag in global simulations of common envelope (CE) evolution for three different companion masses. Our simulations help to delineate regimes when conventional analytic drag force approximations are applicable. During and just prior to the first periastron passage of the in-spiral phase, the drag force is reasonably approximated by conventional analytic theory and peaks at values proportional to the companion mass. Good agreement between global and local 3D "wind tunnel" simulations, including similar net drag force and flow pattern, is obtained for comparable regions of parameter space. However, subsequent to the first periastron passage, the drag force is up to an order of magnitude smaller than theoretical predictions, quasi-steady, and depends only weakly on companion mass. The discrepancy is exacerbated for larger companion mass and when the inter-particle separation reduces to the Bondi-Hoyle-Lyttleton accretion radius, creating a turbulent thermalized region. Greater flow symmetry during this phase leads to near balance of opposing gravitational forces in front of and behind the companion, hence a small net drag. The reduced drag force at late times helps explain why companion-core separations necessary for envelope ejection are not reached by the end of limited duration CE simulations.