Microwave observations and modeling of the molecular coma in comets

This thesis consists of the results from observations of comets 73P-C/Schwassmann-Wachmann 3, 17P/Holmes, and 8P/Tuttle, which I observed at millimeter wavelengths between May 2006 and March 2008, using the Submillimeter and the Kitt Peak telescopes of the Arizona Radio Observatory. I also investigated prospects for in-situ observations of comet 67P/Churyumov-Gerasimenko with the Microwave Instrument for the Rosetta Orbiter (MIRO). Observations of the first comet, analyzed with a simple model, revealed that the nucleus rotation period might be unusually short. This tentatively suggests a rotational break-up of the parent body, and sets a meaningful upper limit on the bulk tensile strength of the nucleus. Observations of the second comet provided a detailed portrait of the evolution of molecular environment during its spectacular outburst. I concluded that the gas cloud was anisotropic, and presumably resulted from several competing sources of activity. The explosion itself appears as a sudden, impulsive event, and only 1.5 months later the nucleus was back to (or close to) its expected non-explosive activity. The last comet exhibited short-term variability of the HCN line profile, which I interpreted as being caused by jets emanating from a rotating nucleus. Therefore, I also developed a new model of molecular line emission in microwaves. The model is the first fully time-dependent anisotropic construction of that kind. Applied to the data of comet Tuttle, it provided the spin axis orientation, and the distribution of activity over the nucleus – the properties, which are normally very difficult to determine from ground. The simulations of the MIRO observations of comet 67P/Churyumov-Gerasimenko were based on the most advanced models of the comet’s coma available to date, and the radiativetransfer code that had been developed in house. They show that water vapor can already be detected at the arrival of Rosetta, and that the most favorable observing direction will be nadir, whereas zenith should be avoided. I also demonstrate that the lines will be optically thick, thus the full radiative transfer treatment is the only plausible approach. Overall, my Ph. D. research provides characterization of three objects, supports the preparation for science with Rosetta, and contributes to the methodology of observational astrophysics of comets. It is also a baseline for future studies, an example of which is presented in the outlook.