Talk abstract details
Carbon chemistry in Galactic Bulge Planetary Nebulae
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 $A_Vsim 1.5$ where small hydrocarbons
form from reactions with C$^+$, and one at $A_Vsim 4$, 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.
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 $A_Vsim 1.5$ where small hydrocarbons
form from reactions with C$^+$, and one at $A_Vsim 4$, 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.