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Gas-Grain Chemical Models: Inclusion of a Grain Size Distribution and a Study of Young Stellar Objects in the Magellanic Clouds

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dc.contributor.authorPauly, Tyler Andrew
dc.contributor.chairGarrod, Robin T.
dc.contributor.committeeMemberCordes, James Martin
dc.contributor.committeeMemberDavis, Harry Floyd
dc.contributor.committeeMemberLunine, Jonathan I.
dc.date.accessioned2018-10-03T19:27:14Z
dc.date.available2018-10-03T19:27:14Z
dc.date.issued2017-12-30
dc.description.abstractComputational models of interstellar gas-grain chemistry have aided in our understanding of star-forming regions. Chemical kinetics models rely on a network of chemical reactions and a set of physical conditions in which atomic and molecular species are allowed to form and react. We replace the canonical single grain-size in our chemical model MAGICKAL with a grain size distribution and analyze the effects on the chemical composition of the gas and grain surface in quiescent and collapsing dark cloud models. We find that a grain size distribution coupled with a temperature distribution across grain sizes can significantly affect the bulk ice composition when dust temperatures fall near critical values related to the surface binding energies of common interstellar chemical species. We then apply the updated model to a study of ice formation in the cold envelopes surrounding massive young stellar objects in the Magellanic Clouds. The Magellanic Clouds are local satellite galaxies of the Milky Way, and they provide nearby environments to study star formation at low metallicity. We expand the model calculation of dust temperature to include a treatment for increased interstellar radiation field intensity; we vary the radiation field to model the elevated dust temperatures observed in the Magellanic Clouds. We also adjust the initial elemental abundances used in the model, guided by observations of Magellanic Cloud HII regions. We are able to reproduce the relative ice fractions observed, indicating that metal depletion and elevated grain temperature are important drivers of the envelope ice composition. The observed shortfall in CO in Small Magellanic Cloud sources can be explained by a combination of reduced carbon abundance and increased grain temperatures. The models indicate that a large variation in radiation field strength is required to match the range of observed LMC abundances. CH$_3$OH abundance is found to be enhanced (relative to total carbon abundance) in low-metallicity models, providing seed material for complex organic molecule formation. We conclude with a preliminary study of the recently discovered hot core in the Large Magellanic Cloud; we create a grid of models to simulate hot core formation in Magellanic Cloud environments, comparing them to models and observations of well-characterized galactic counterparts.
dc.identifier.doihttps://doi.org/10.7298/X49K48D4
dc.identifier.otherPauly_cornellgrad_0058F_10561
dc.identifier.otherhttp://dissertations.umi.com/cornellgrad:10561
dc.identifier.otherbibid: 10474135
dc.identifier.urihttps://hdl.handle.net/1813/59032
dc.language.isoen_US
dc.subjectAstrochemistry
dc.subjectISM
dc.subjectStar Formation
dc.subjectAstronomy
dc.subjectChemistry
dc.titleGas-Grain Chemical Models: Inclusion of a Grain Size Distribution and a Study of Young Stellar Objects in the Magellanic Clouds
dc.typedissertation or thesis
dcterms.licensehttps://hdl.handle.net/1813/59810
thesis.degree.disciplineAstronomy and Space Sciences
thesis.degree.grantorCornell University
thesis.degree.levelDoctor of Philosophy
thesis.degree.namePh. D., Astronomy and Space Sciences

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