Browsing by Subject "Galactic archaeology"

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Cosmic fossils : the spectroscopic study of stars and comets
(2023-08-17) Nelson, Tyler William; Hawkins, Keith A.; Cochran, Anita Light, 1954-; Ting, Yuan-Sen; Offner, Stella; Morley, Caroline
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.
The analysis of stellar populations in the Milky Way and beyond
(2021-08-13) Carrillo, Andreia Jessica Arabani; Hawkins, Keith A.; Drory, Niv; Boylan-Kolchin, Michael; Gebhardt, Karl; Wyse, Rosemary
No other astronomical object can unlock the mysteries of the Universe more than stars. Studying the crowded, unresolved stellar populations of nearby galaxies aids in understanding what brings about their observed properties, morphology, activity, and assembly history. Studying the Milky Way’s resolved stellar populations, specifically their detailed chemical abundances and kinematics, provides us with an unparalleled, zoomed-in view of galaxy formation. Additionally, understanding Milky Way stellar populations can supplement our knowledge in other fields in Astronomy, such as exoplanet populations, as the gas that forms a star also forms the planets around it. This is the focus of my dissertation: analyzing stellar populations--both resolved in the Milky Way (observed and simulated) and unresolved in a nearby galaxy--to holistically understand galaxy formation, the Milky Way assembly, and the Galactic context of exoplanet demographics. This Thesis is structured going from the largest and farthest of scales e.g. external galaxy, to the smallest of scales e.g. planet-hosting stars. With unresolved, ensemble stellar populations, I investigate the assembly of the different components: the bulge, bar, and disc, of the nearby galaxy NGC 2903, using the VIRUS-P Exploration of Nearby Galaxies (VENGA) Integral Field Spectroscopy (IFS) survey. This work benefited from high signal-to-noise, spatially-resolved spectroscopic data that enabled me to construct a more comprehensive picture of NGC 2903’s formation history by understanding the growth of its different components. Moving closer to the Milky Way and unveiling the history of its halo, I present a detailed chemical study for stars from one of Milky Way’s most significant mergers dubbed Gaia-Enceladus-Sausage (GES), aided by high-resolution optical spectra from McDonald Observatory and Magellan Telescope. I contrast these stars’ abundance trends to those found in the Milky Way and its surviving satellites to understand how its chemical signatures compare to other stellar populations and what this tells us about its star formation history. As emphasized in this Thesis, it is important to investigate galaxy assembly through the lenses of different galaxy components and in an interdisciplinary way. Therefore, I also aim to understand the formation of the Milky Way disc. I do this by turning to a zoom-in cosmological simulation of a Milky Way mass galaxy from the FIRE-2 suite and where I determine how the ages, metallicities, and detailed chemical abundances of stars relate to each other and to their current and birth locations. Specifically, I investigate if the stars in the simulations exhibit a tight age-abundance trend, similar to what is found in observations. Further, I explore how the dispersion found around this trend, at different metallicities and locations in the galaxy, relates to the star formation history of the simulated Milky Way. Lastly, taking advantage of the power of Milky Way large surveys, I kinematically and chemically characterized targets from the Transiting Exoplanet Survey Satellite (TESS) to understand the Galactic context of planet-hosting stars. This is especially important as we find more exoplanets in different parts of the Galaxy, enabling us to understand if and how planet formation and demographics are different for different Milky Way stellar populations. The accomplishments of this Thesis have contributed to a broad range of fields in Astronomy, but all tied together by the analysis of stellar populations in the Milky Way and beyond.