PhD Defense by Charl van der Walt

Title: Nature versus Nurture: What sets the chemical complexity in star-forming regions?

By studying the molecules detected in and near star-forming regions, we can gain an understanding of the physical processes involved in the evolution of newly-formed stars. Different molecules trace different regions in and around young stars, which allows us to study its physical environment. Simple 2- to 3-atom molecules trace the larger structure of the envelopes and outflows, while Complex Organic Molecules (COMs: molecules with 6 or more atoms, and at least one carbon atom) trace the warm, inner regions where the temperature rises above 100 K. From molecular spectral lines we can determine physical parameters such as the density, temperature and excitation conditions of the gas surrounding the young star, as well as the velocity of the gas, from a comparison between the rest velocity of a molecular transition and the observed velocity. It is therefore important to obtain broad-band spectra from newly formed stars, covering a large frequency range, and which contain molecular lines of as many molecules and their transitions as possible. Furthermore, it is important to obtain high-angular resolution observations in order to resolve the observed regions and determine the origin of the observed molecular emission. Then, by studying a sample of several sources, one can compare results and determine questions such as how does the environment in which stars form affect the chemistry that we observe, and is this large-scale environment, from which the star inherits its parental material, more important in setting the chemical complexity in newly-formed stars, or is the local conditions more important?

This PhD thesis utilises the capabilities of the Submillimeter Array to obtain broad frequency coverage in a single setting, at high-angular resolution, to obtain observations of 10 intermediate- to high-mass star-forming regions in the same molecular cloud, the Cygnus X region. These sources are all located, at various distances from an association of bright OB stars, a source of strong UV radiation in the region, which allows us to study the influence of this external environment on the chemistry of the newly formed stars directly. By studying the spectra from these 10 sources we identify the molecular lines and make molecular emission maps in order to determine the origin of the emission. We then derive column densities and excitation temperatures for detected molecules, and compare our results in order to answer the question of Nature versus Nurture: What sets the chemical complexity in star-forming regions.

Our results from the comparison of the sources suggest that the local conditions play a larger role in setting the chemistry of young stars, while the large-scale environment plays only a minor role. From the study of the morphology of the sample sources we further conclude that the origin of COM emission is from a combination of processes, including thermal processes from the young stars, shocks caused by outflows and jets, as well as accretion of material from the envelope onto the newly formed disc and star, and finally UV-irradiation of the outflow-cavity walls. Our results further demonstrate the power and importance of high-angular resolution observations over a broad frequency range.

Supervisor: Lars Egstrøm Kristensen

Assessment committee:
Troels Haugbølle (chair), Associate Professor, Niels Bohr Institute, University of Copenhagen
Marta Sewilo, Research Scientist, NASA Goddard Space Flight Center & University of Maryland, USA
Maria (Maite) Beltran, Senior Researcher, Osservatorio Astrofisico di Arcetri, Italy

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