A Phenomenological Model for the Extended Zone Above AGB Stars

TL;DR

This paper proposes a phenomenological model of an extended 'effervescent zone' above AGB stars, where bound gas parcels coexist with wind, influencing planetary nebula morphology through enhanced accretion onto binary companions.

Contribution

It introduces the effervescent zone concept and characterizes its density profile, providing a new framework for understanding mass transfer in binary systems with AGB stars.

Findings

Density of bound gas falls as r^{-5/2}

Effervescent zone extends to ~100AU

Enhanced accretion efficiency onto companions

Abstract

I suggest the existence of an extended zone above the surface of asymptotic giant branch (AGB), as well as similar stars experiencing high mass loss rates. In addition to the escaping wind, in this zone there are parcels of gas that do not reach the escape velocity. These parcels of dense gas rise slowly and then fall back. The wind and bound gas exist simultaneously to distances of ~100AU. I term this region the effervescent zone. In this phenomenological study I find that the density of the bound material in the effervescent zone falls as ~r^{-5/2}, not much faster than the wind density. The main motivation to propose the effervescent model is to allow wide binary companions to influence the morphology of the descendant planetary nebulae (PN) by accreting mass from the effervescent zone. Accretion from the effervescent zone is more efficient than accretion from the wind in forming an accretion disk around the companion. The companion might then blow two jets that will shape the descendant PN.