Stars with Plumbing Issues: The Multi-component Ejecta of Common Envelopes

Abstract

Common-envelope evolution (CEE) is one of the major open problems in binary stellar evolution, even though it is the primary channel for forming close binaries with at least one compact object. While CEE is often assumed to lack direct observational constraints, recent connections between CEE and the transient class, luminous red novae (LRNe), as well as the post-AGB systems known as Water Fountains (WFs), provide new opportunities to understand how this event unfolds. Collectively, these observations suggest that envelope ejection during CEE is neither spherically symmetric nor homologous, but rather produces complex ejecta, providing constraints for simulations of CEE. In this work, we present a series of three-dimensional hydrodynamical (non-MHD) simulations of CEE for a 2 Msun red-giant-branch (RGB) star and show that the event naturally produces multi-component ejecta including an extended radially expanding shell, a thick circumbinary disk (CBD), and collimated bipolar outflows. Consistent with previous works in the literature, the bipolar outflows in our simulations arise naturally through geometrical constraints imposed by the disk and do not require magnetic fields. While we find that the formation of the collimated outflows can be sensitive to the choice of Riemann solver, they appear across all resolutions and mass ratios tested in this work. We show that the multi-component ejecta formed during CEE simulations naturally reproduce some of the key properties of CEE observations, including the classical bipolar shape seen in some LRNe, WFs, and planetary nebulae.

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