Fallback in bipolar Planetary Nebulae ?

TL;DR

This paper proposes that fallback accretion from earlier evolutionary stages energizes bipolar planetary nebulae structures, explaining their unique features through hydrodynamic shockwave interactions and low-power outflows.

Contribution

It introduces an alternative accretion-based scenario for bipolar PN structures, supported by literature evidence, differing from standard models.

Findings

Fallback accretion can energize bipolar outflows.

Shockwaves guide material into equatorial regions.

Accretion rates may drive low-power outflows.

Abstract

The subclass of bipolar Planetary Nebulae (PN) exhibits well-defined low-power outflows and some show shock-related equatorial spiderweb structures and hourglass structures surrounding these outflows. These structures are distinctly different from the phenomena associated with spherical and elliptical PN and suggest a non-standard way to simultaneously energise both kind of structures. This paper presents evidence from the published literature on bipolar PN Hb\,12 and other sources in support of an alternative scenario for energizing these structures by means of accretion from material shells deposited during earlier post-AGB and pre-PN evolutionary stages. In addition to energizing the bipolar outflow, a sub-Eddington accretion scenario could hydrodynamically explain the spiderweb and outer hourglass structures as oblique shockwaves for guiding the accreting material into the equatorial region of the source. Estimates of the accretion rate resulting from fallback-related spherical accretion could indeed help to drive a low-power outflow and contribute to the total luminosity of these sources.