Abundance Variation of Dense Gas Tracers in the Gould Belt

Abstract

Molecular abundance ratios have been proposed to trace the chemical and evolutionary state of star forming molecular clouds because the formation and destruction of such molecules is dependent on the local environmental conditions, like the density and radiation field that both evolve with time. In this thesis, we use the Green Bank Ammonia Survey (K-Band) ammonia (NH3) and rare-carbon chain (C2S and HC5N) emission maps to make controlled measurements of how the abundances of key molecular tracers vary across different star forming regions. We extend prior work on molecular abundance ratios by applying it across a range of density and temperature regimes in NGC 1333 and Orion A by spectrally stacking NH3, C2S, and HC5N emission in spatially relevant bins determined by Voronoi tessellation of the underlying H2 density. The resultant molecular abundances are used to test the NH3/C2S abundance ratio that has been previously shown to be enhanced in star forming gas. Using the VISION YSO catalogue, we classify the evolutionary stage of the regions in Orion A and find that NH3/C2S abundance ratio is indeed enhanced in the star forming regions of Orion A. In addition to this analysis, we compare molecular abundances to the strength of the radiation fields in NGC 1333 and Orion A and find that C2S and HC5N abundances are higher where the radiation field is stronger. We also find that NH3 abundances increase with decreasing radiation field strength, suggesting that the chemical conditions for NH3 formation are different than C2S and HC5N.