The Shapes of AGB Envelopes as Probes of Binary Companions

TL;DR

This paper presents a method to infer unseen binary companions around AGB stars by analyzing the large-scale shapes of their circumstellar envelopes, linking envelope geometry to binary parameters.

Contribution

It develops a prescription based on simulations to interpret envelope shapes, aiding in detecting binary companions and understanding planetary nebula symmetry.

Findings

Envelope shapes can indicate the presence of binary companions.

Low-mass companions may be undetectable until they influence mass loss.

Spherical halos around nebulae limit the mass of close companions.

Abstract

We describe how the large scale geometry of the circumstellar envelopes of asymptotic giant branch stars can be used to probe the presence of unseen stellar companions. A nearby companion modifies the mass loss by gravitationally focusing the wind towards the orbital plane, and thereby determines the shape of the envelope at large distances from the star. Using available simulations, we develop a prescription for the observed shapes of envelopes in terms of the binary parameters, envelope orientation, and type of observation. The prescription provides a tool for the analysis of envelope images at optical, infrared, and millimetre wavelengths, which can be used to constrain the presence of companions in well observed cases. We illustrate this approach by examining the possible role of binary companions in triggering the onset of axi-symmetry in planetary nebula formation. If interaction with the primary leads to axi-symmetry, the spherical halos widely seen around newly formed nebulae set limits on the companion mass. Only low mass objects may orbit close to the primary without observable shaping effects: they remain invisible until the interaction causes a sudden change in the mass loss geometry.