Organic Chemistry During Low-Mass Star-Formation: The Role of Reaction Barriers and Ice Desorption
Graninger, Dawn Melissa
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AbstractOrganic molecules are present throughout all stages of star-formation, potentially being incorporated into the planetesimals that are later formed. These species serve as the molecular backbone of prebiotic chemistry, and thus understanding their chemistry is of great importance for understanding the origins of life. This thesis aims to deepen our understanding of two elements regulating the organic chemistry present in star-forming regions: reaction barriers and desorption.
The first half of this thesis address the role of reaction barriers for the isomer pair, HNC and HCN. The unstable HNC is found to have the same abundance as the stable HCN molecule at 10 K. At temperatures above 10K, HNC/HCN decreases with increasing temperature. This temperature dependence is proposed to be driven by the reaction of H+HNC, which possesses a reaction barrier. In Chapter 2 of this thesis, kinetic chemical modeling is used to constrain the reaction barrier H+HNC via observations. The predictions derived from this modeling motivate the observations of HNC presented in Chapter 3. In Chapter 3, the first spatially resolved image of HNC in a protoplanetary disk are presented. These observations verify the reaction barrier determined in Chapter 2 through observations of the HNC/HCN ratio in a source with a known temperature profile.
The second half of this thesis investigates the influence and properties of CH4 desorption. Chapter 4 presents observations of carbon chain molecules, which form through gas-phase reactions between C+ and sublimated CH4, and CH3OH, which is known to form on grain surfaces. Carbon chains are found to be correlated with CH3OH, suggesting that the gas-phase molecular inventory is tied to the composition of the ices. These observations provide the first observational link between CH4 ice and gas-phase carbon chain abundances. To fully understand the importance of CH4 ice sublimation, constraints on the processes which drive desorption must be derived. Chapter 5 presents such laboratory experiments which aim to constrain both the thermal and non-thermal desorption properties of CH4 ice on astrophysically relevant surfaces. These experiments indicate that in some regions, both thermal and non-thermal desorption processes are important for obtaining a complete picture of CH4 ice desorption.
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