Gas and Dust in Galactic Planetary Nebulae at Sub-Solar Metallicity

Pagomenos, George (2019). Gas and Dust in Galactic Planetary Nebulae at Sub-Solar Metallicity. PhD thesis The Open University.




Planetary nebulae are the final phases of evolution for low- to intermediate-mass stars (∼ 0.8– 8 M⊙). They consist of a central star surrounded predominantly by a circumstellar envelope containing the products of nuclear processing, and a photodissociation region containing dust and molecules. These include silicates and large organic molecules such as polycyclic aromatic hydrocarbons (PAHs), which are ubiquitous throughout the Universe, and fullerenes, which are the largest molecules to be firmly detected in space. All of this material coasts away from the central star, causing enrichment of the interstellar medium.

Although there have been significant advances in studies of this circumstellar material, we still do not fully understand how the metallicity (i.e. the abundance of elements heavier than helium) of the local environment affects the dust composition around these stars, or the processes that govern the formation and evolution of these large organic molecules.

This thesis presents a series of studies in which the abundances and dust composition around planetary nebulae are characterised in the low metallicity regions of the Milky Way, with use of data from the Spitzer Space Telescope and SOFIA. These include investigations into the metallicity and dust content of planetary nebulae in the outer thin disk and the halo of the Galaxy, and the physical conditions in which large organic molecules form.

I find that the outer regions of the Galactic disk have a lower metallicity than the solar neighbourhood, and that regions of low metallicity favour carbon-rich dust production over oxygen-rich dust (i.e. silicates), except for within the Galactic halo. These regions show a greater diversity of carbonaceous material than observed towards the Galactic bulge and in the solar neighbourhood. Fullerenes are preferentially formed in environments with low hydrogen density.

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