Photodissociation in proto-planetary nebulae. Hydrodynamical simulations and solutions for low-velocity multi-lobes

TL;DR

This study uses hydrodynamical simulations to show how photodissociation influences the formation of multi-lobe structures in proto-planetary nebulae, highlighting the role of dissociation fronts and instabilities.

Contribution

It introduces two-dimensional gasdynamical models demonstrating the impact of photodissociation on nebula morphology and the development of instabilities in low-velocity multi-lobe nebulae.

Findings

Photodissociation occurs in stars hotter than 7,000 K, affecting circumstellar gas.

Early expansion of dissociation fronts influences nebula lobe formation.

Cooling enhances thin-shell Vishniac instability, leading to shell fragmentation.

Abstract

We explore the effects of photodissociation at the stages of post-asymptotic giant branch stars to find a mechanism able to produce multi-polar shapes. We perform two-dimensional gasdynamical simulations to model the effects of photodissociation in proto-planetary nebulae. We find that post-asymptotic giant branch stars with 7,000 K or hotter are able to photodissociate a large amount of the circumstellar gas. We compute several solutions for nebulae with low-velocity multi-lobes. We find that the early expansion of a dissociation front is crucial to understand the number of lobes in proto-planetary nebulae. A dynamical instability appears when cooling is included in the swept-up molecular shell. This instability is similar to the one found in photoionization fronts, and it is associated with the thin-shell Vishniac instability. The dissociation front exacerbates the growth of the thin-shell instability, creating a fast fragmentation in shells expanding into media with power-law density distributions such as r^-2.