Common envelope events with low-mass giants: understanding the transition to the slow spiral-in

TL;DR

This study uses 3D simulations to analyze common envelope events with low-mass giants, revealing different ejection processes and highlighting limitations of 1D models in capturing the self-regulated phase of CEEs.

Contribution

The paper introduces tools to compare 3D and 1D models of CEEs, identifying key physical processes and conditions influencing outcomes, especially in the slow spiral-in phase.

Findings

Multiple ejection processes identified, including recombination-driven outflows.

Significant mass loss occurs even in mergers, affecting the self-regulated phase.

Differences between 1D and 3D models impact the understanding of envelope ejection.

Abstract

We present a three-dimensional (3D) study of common envelope events (CEEs) to provide a foundation for future one-dimensional (1D) methods to model the self-regulated phase of a CEE. The considered CEEs with a low-mass red giant end with one of three different outcomes -- merger, slow spiral-in, or prompt formation of a binary. To understand which physical processes determine different outcomes, and to evaluate how well 1D simulations model the self-regulated phase of a CEE, we introduce tools that map our 3D models to 1D profiles. We discuss the differences in the angular momentum and energy redistribution in 1D and 3D codes. We identified four types of ejection processes: the pre-plunge-in ejection, the outflow during the plunge-in, the outflow driven by recombination, and the ejection triggered by a contraction of the circumbinary envelope. Significant mass is lost in all cases, including the mergers. Therefore a self-regulated spiral-in can start only with a strongly reduced envelope mass. We derive the condition to start a recombination outflow, which can proceed either as a runaway or a stationary outflow. We show that the way the energy of the inspiraling companion is added to the envelope in 1D studies intensifies the envelope's entropy increase, alters the start of the recombination outflow, and leads to different outcomes in 1D and 3D studies. The steady recombination outflow may dispel most of the envelope in all slow spiral-in cases, making the existence of a long-term self-regulated phase debatable, at least for low-mass giant donors.