Exploring the formation and evolutionary pathways of young stars and planetary systems at high precision

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2022-08-12

Authors

Krolikowski, Daniel Milker

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The star and planet formation process is one continuous event on scales from a giant molecular cloud to an individual circumstellar disk. There are a multitude of competing theories for the dominant physical mechanisms acting at each stage of this process, ranging from cloud conditions to how planet characteristics change over time. The properties of young stellar associations, the stars within them, and their planetary systems are crucial direct tests of these competing models. However, finding and characterizing isolated young systems is hard. Our best opportunity is to measure the ensemble age of a coeval group of stars, and observe systems within them to study the inherently intertwined star and planet formation and evolution process.

In my dissertation, I use precision observations of young stars and planetary systems to explore their formation and evolutionary pathways. I first present a comprehensive census of the Taurus star forming region to reconstruct its complex star forming history. I use Gaia astrometry to reveal its highly substructured nature, identifying subgroups with age spreads indicating a prolonged star forming event, and kinematics indicating a typical turbulent environment and early dynamical evolution. Taurus is likely connected to a long-lasting, larger-scale galactic star forming event that can only be uncovered in the Gaia era. I then discuss exoplanet-related projects using high precision NIR spectroscopy from the Habitable-zone Planet Finder. Young stars are highly active, which introduces significant noise in spectroscopic observations. In a sample of young transiting planet hosts, I characterize the NIR helium spectral feature, which is an important probe of atmospheric mass loss and conditions in the stellar chromosphere. Stellar helium variability decreases with age, reflecting the higher activity levels in youth, but the line strength is constant beyond 100 Myr implying similar line formation conditions across the sample. Stellar variability should not preclude detection of mass loss at young ages. With this same data set, I search for giant planets exterior to the known transiting planets to measure their occurrence rate and constrain the typical dynamical history of a planetary system. I find three candidate signals of long-period companions, although the occurrence rate remains largely unconstrained. My dissertation exemplifies the power that precision observations of young stars has to improve our understanding of the complicated and interrelated processes of star and planet formation and evolution.

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