Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006phdt.........6r&link_type=abstract
PhD Thesis
Astronomy and Astrophysics
Astronomy
2
Planetary Nebulae: Diameters, Flux, Ism: Clouds, Dust, Extinction, Molecules, Metal-Poor, Galaxy: Bulge, Centre, Edge, Clouds: Individual (Edge Cloud 1, Edge Cloud 2), Astrochemistry: Models, Radio Lines: Ism
Scientific paper
I examine the detected effects of dust grains in two dissimilar regions at opposite ends of the Milky Way: firstly in explaining anomalous extinction values towards the Galactic bulge through observations of Planetary Nebulae (PNe); and secondly in explaining the apparent photon-dominated regions (PDRs) observed in metal-poor molecular clouds at the Galactic edge. Angular diameters, fluxes and extinction of compact PNe are derived from observations, providing evidence for steeper extinction towards the Bulge. I suggest that for the inner Galaxy the low-density warm ionized medium is the site of the anomalous extinction, and that low values of extinction can also be derived using dust models with a turnover radius of 0.08 microns. I then go on to examine the chemistry of Edge Clouds 1 and 2 (EC1 & EC2), molecular clouds with the largest galactocentric distances in the Milky Way. I present observations of both clouds and these are used to determine their physical characteristics. Chemical models are used to reproduce the abundances in EC2 and they indicate: heavy elements may be reduced by a factor of five relative to the solar neighbourhood; very low extinction due to a high gas to dust ratio; an enhanced cosmic ray ionisation rate; and a high UV field compared to local interstellar values. Observed high abundances of the radicals C2H and CN are typical of PDRs, but at the Galactic edge, metal abundances are expected to be much reduced. In addition, although EC2 does contain young stars, there is no evidence of the late-type stars which produce dust grains, thereby justifying the assumption of a high ratio of UV flux to grain surface area. Finally, I note that shocks from an old supernova remnant may be the source of the structure and dynamics observed in EC2.
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