The Creation of AGB Fallback Shells

TL;DR

This paper investigates the dynamics of fallback shells in AGB stars, combining simulations and analytic models to identify conditions under which ejected material falls back or escapes, with implications for post-AGB star formation.

Contribution

It introduces a combined hydrodynamic and analytic approach to characterize fallback shells in AGB stars, identifying a critical velocity threshold for fallback versus escape.

Findings

Identified a critical shell velocity for fallback

Developed a simplified analytic model matching simulations

Discussed implications for post-AGB dusty disk formation

Abstract

The possibility that mass ejected during Asymptotic Giant Branch (AGB) stellar evolution phases falls back towards the star has been suggested in applications ranging from the formation of accretion disks to the powering of late-thermal pulses. In this paper, we seek to explicate the properties of fallback flow trajectories from mass-loss events. We focus on a transient phase of mass ejection with sub-escape speeds, followed by a phase of a typical AGB wind. We solve the problem using both hydrodynamic simulations and a simplified one-dimensional analytic model that matches the simulations. For a given set of initial wind characteristics, we find a critical shell velocity that distinguishes between "shell fallback" and "shell escape". We discuss the relevance of our results for both single and binary AGB stars. In particular, we discuss how our results help to frame further studies of fallback as a mechanism for forming the substantial population of observed post-AGB stars with dusty disks.