Master's Thesis Defense by Giulia Perotti – Niels Bohr Institute - University of Copenhagen

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Master's Thesis Defense by Giulia Perotti

Modelling the CO chemistry of star-forming regions

A simple path to complex organic molecules


Carbon monoxide (CO) is an important molecule in the interstellar medium. It is the cornerstone for synthesizing complex molecules in the Universe, starting with methanol (CH3OH) and ultimately leading to prebiotic molecules, e.g. amino acids.

To model the solid and gas-phase CO and CH3OH abundances in star-forming regions, I developed a Simplified CO Network (SCON). The great advantage of this reduced chemical network is that it is computationally cheap. Furthermore, it is easy to explore the effects of varying physical and chemical parameters. SCON is coupled to two physical models of protostellar envelopes: first to a 1D model, and second to a sophisticated 3D simulation. In the cold outer parts of the envelope, most of the gas is frozen out onto dust grains. As matter falls toward the star, the temperature increases and these molecules sublimate. In the 1D model, the collapse is radial and all CH3OH sublimates into the gas phase. In the 3D simulation, results show that only a small fraction of gas reaches a high enough temperature for CH3OH to sublimate. Hence, the total amount of gas-phase CH3OH is lower in the 3D model compared to the 1D model.

To test the model results, these are compared to observations of both ice and gas-phase CO and CH3OH. I find that the 1D model is sufficient for reproducing observations of the young protostar Serpens SMM4, even though this model is not specifically tuned to this source. The non-detection of gas-phase CH3OH toward more evolved protostars in Ophiuchus is not easily explained, however. The 3D simulation provides a natural solution, though: only a small amount of CH3OH sublimates into the gas phase, and this is not observable. The results of this study provide highly valuable limits for the CO and CH3OH freeze-out and sublimation zones in star-forming regions.

Supervisors: Jes K. Jørgensen and Lars E. Kristensen