Cosmic fossils : the spectroscopic study of stars and comets




Nelson, Tyler William

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Chemical Fossils, i.e. objects which preserve their initial elemental composition, provide vital constraints on the nature and evolution of the Universe. The study of these objects is therefore fundamental for astronomy, from the scales of the Solar System, to the Galaxy and beyond. The unique focus of this dissertation is to advance our knowledge of and apply chemical fossils across a wide range of scales (cometary fossils in the Solar System and stellar fossils on a Galactic scale) to better understand the formation and assembly of our Solar System and the Galaxy in which it resides. The first half of this dissertation considers the evolution of the Universe at the scale of a single star, the Sun, and its Solar System, using comets. I concentrate on characterizing the emission spectrum from the diatomic carbon (C₂) Swan bands. This fragment species is ubiquitous in comets. It is commonly used as a proxy to measure production rates of gas as well as a taxonomic classification tool. However, its parent species and the details of its emission are not well understood. A bimodal rotational temperature has been found in the Swan bands for comet 1P/Halley (Lambert et al., 1990). The following models have been proposed to explain this phenomenon: C₂ inheriting excited states from the parent species (Jackson et al., 1996), properties inherent to C₂ through intercombinational/satellite transitions (Lambert et al., 1990), and multiple populations of C₂ present in the photochemical environment (Lambert et al., 1990). Leveraging a unique library of high resolution, high signal-to-noise optical spectra, collected at McDonald Observatory, I investigate the proposed mechanisms for this rotational temperature bimodality for comets 122P/de Vico, 153P/Ikeya-Zhang, and C/1995 O1 (Hale-Bopp). I find bimodal temperatures in all spectra studied and supersolar temperatures in C/1995 O1, which is incompatible with the models from the literature. I suggest the supersolar temperatures for Hale-Bopp are a consequence of heating from the Solar wind for material outside the cometopause. The second half of this dissertation considers cosmic fossils at the scale of the Galaxy. Photospheric abundances of stars are mostly conserved over their lifetimes, and therefore stars can act as chemical fossils for the Galaxy. I focus on the use of chemical tagging within the Milky Way. Chemical tagging of stars is one of the pillars of Galactic Archaeology, motivating numerous large scale surveys. It has dramatically reshaped our knowledge of the Galaxy over the last two decades. Chemical tagging relies upon stars which are born together, i.e. co-natal, sharing a common chemical composition. I find observational evidence for an untapped reservoir of co-natal, co-moving pairs of stars, through the application of chemical tagging. Co-natal stars provide an excellent laboratory for numerous areas of astronomy, from stellar physics, to survey calibration. A common application of chemical tagging is relating a wayward star to a possible birthplace. Hyper- velocity stars (HVSs) are gravitationally unbound to the Milky Way. However the physical mechanisms that give rise to the large velocities of late-type HVSs are poorly understood. To solve this problem, I applied chemo-dynamic tagging to a sample of HVS candidates identified in Gaia data. Since these production mechanisms are connected to specific locations or chemical environments within the Galactic neighborhood, chemical tagging can distinguish which production pathways could create these enigmatic fast stars. I present work on the chemo-dynamic tagging, i.e. using both chemical and kinematic tagging, of late-type candidate HVSs. I find conclusive evidence of one unbound late-type HVS and two marginally unbound HVSs. These stars appear to originate in the ‘in situ’ stellar halo based on their chemical composition and orbit properties. These stars are produced by some of the most extreme astrophysical phenomena in the Galaxy. The origins of these late-type HVS constrain their production mechanisms and hence the importance of these energetic processes within the Galaxy. This knowledge can then be applied to models of the Milky Way’s evolution. Furthermore, expanding the number of HVSs is useful for studying the dark matter halo of the Galaxy.



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