Carbon chemistry in Galactic Bulge Planetary Nebulae

TL;DR

This study investigates the widespread mixed chemistry in Galactic Bulge Planetary Nebulae, revealing it is linked to dense tori and hydrocarbon formation in UV-irradiated regions, rather than stellar dredge-up or ISM interaction.

Contribution

The paper demonstrates that mixed chemistry in these nebulae results from hydrocarbon formation in dense, UV-irradiated tori, a novel explanation differing from previous theories.

Findings

Mixed chemistry is common in 75% of the sample nebulae.

Hydrocarbon chains form in dense tori through gas-phase reactions.

A chemical model predicts layered hydrocarbon formation at different depths.

Abstract

Galactic Bulge Planetary Nebulae show evidence of mixed chemistry with emission from both silicate dust and PAHs. This mixed chemistry is unlikely to be related to carbon dredge up, as third dredge-up is not expected to occur in the low mass Bulge stars. We show that the phenomenon is widespread, and is seen in 30 nebulae out of 40 of our sample, selected on the basis of their infrared flux. HST images and UVES spectra show that the mixed chemistry is not related to the presence of emission-line stars, as it is in the Galactic disk population. We also rule out interaction with the ISM as origin of the PAHs. Instead, a strong correlation is found with morphology, and the presence of a dense torus. A chemical model is presented which shows that hydrocarbon chains can form within oxygen-rich gas through gas-phase chemical reactions. The model predicts two layers, one at $A_V\sim 1.5$ where small hydrocarbons form from reactions with C$^+$, and one at $A_V\sim 4$, where larger chains (and by implication, PAHs) form from reactions with neutral, atomic carbon. These reactions take place in a mini-PDR. We conclude that the mixed chemistry phenomenon occurring in the Galactic Bulge Planetary Nebulae is best explained through hydrocarbon chemistry in an UV-irradiated, dense torus.