Matching Items (57)
Description
Study of the early Universe is filled with many unknowns, one of which is the nature of the very first generation of stars, otherwise designated as "Population III stars". The early Universe was composed almost entirely of cold hydrogen and helium, with only trace amounts of any heavier elements. As such, these stars would have compositions very different from the stars we are able to observe today, which would in turn change how these stars functioned, as well as their lifespans. Population III stars are so old that the light they emitted has not yet reached us here on Earth. Yet we know they have to have existed, so how do we go about studying objects that we have not yet observed? And more importantly, is there a metallicity threshold at which stars begin to behave like the stars we observe today? These areas are where stellar modelling programs such as TYCHO8 and the Spanish Virtual Observatory's Theoretical Spectra Web Server (TSWS) come in. These programs allow astronomers to model the physics of Pop III stars. We can get a pretty good understanding of how these stars behaved, how long they lived, and the visual spectra they would have emitted. Such information is crucial to astronomers being able to search for remnants of these stars, and one day, the stars themselves.
ContributorsMena, Julian (Author) / Young, Patrick (Thesis director) / Bowman, Judd (Committee member) / Barrett, The Honors College (Contributor) / School of Earth and Space Exploration (Contributor)
Created2022-05
Description
Direct imaging is a powerful tool in revealing the architectures of young planetary systems, clearly showing the structure of circumstellar disks. Circumstellar disks, similar to the asteroid belt, are critical elements of any planetary system, and the study of them is important to understanding planet formation. Disks around several main sequence stars have already been observed directly interacting with exoplanets in their respective systems. Imaging can help answer many of the key questions of how disks interact in their respective systems. The Gemini Planet Imager is a high contrast imaging instrument that has spatially resolved several circumstellar disks for the first time, many exhibiting tracers of ongoing planet formation or the presence of a perturbing exoplanet. With this new sample, population analyses of characteristics of disks can now be explored and compared to information at other wavelengths. Direct imaging is also a uniquely accessible tool in engaging students and the community in astronomy. In combination with a course-based undergraduate research experience, direct imaging has the ability to engage students in the process of doing research in a very accessible manner. In Chapter 1, I introduce the concepts related to circumstellar debris disks, further focusing on the sub-field of direct imaging and its value in understanding these systems and engaging students in astronomy. In Chapter 2, I present four images of newly-resolved debris disks in the Scorpius-Centaurus association, comparing their characteristics with many other spatially-resolved circumstellar disks within the moving group. In Chapter 3, I present a uniform analysis of debris disk structure using a consistent and empirically-informed modeling approach. In Chapter 4, I present my findings and experiences in developing and teaching a course-based undergraduate research experience for students in the country’s first online astronomy degree program centered on the direct imaging of brown dwarfs. In Chapter 5, I present my conclusions on the topics I have investigated and discuss future work within the field of direct imaging and its role in driving astronomy research and education forward.
ContributorsHom, Justin (Author) / Patience, Jennifer (Thesis advisor) / Knierman, Karen (Committee member) / Scowen, Paul (Committee member) / Simon, Molly (Committee member) / Young, Patrick (Committee member) / Arizona State University (Publisher)
Created2023
Description
Studying natural variations in the isotopic composition of oxygen-sensitive elements in ancient marine sediments is a powerful way to study the geochemical evolution of Earth’s environments in the past. My dissertation focused on two broad aspects of isotope geochemistry: 1) the development of rhenium (Re) isotopes as a paleoredox and nuclear forensics tool, and 2) the application of mercury (Hg) isotopes as a tool to trace Hg mobility in the environment and what this movement means for isotopic changes in sedimentary rocks used to study Earth’s past. Chapter 2 is the first examination of Re isotopes in sedimentary rocks that formed ~2.5 billion years ago during a period of ocean and atmospheric oxygenation prior to the Great Oxidation Event. The data show variations in Re isotope ratios coincide with evidence for changes in oceanic and atmospheric oxygenation, supporting the use of Re isotopes as a tool to track paleoredox conditions throughout Earth's history. Another application of rhenium isotopes is explored in the third chapter on nuclear forensics. Rhenium isotopes in uranium ore concentrates (UOC) from known production locations revealed more than double the range of isotope fractionation previously reported for any natural geologic samples so far. These first Re isotope ratio data indicate that Re is a promising new tool for provenance assessment of UOCs. Chapter 4 focuses on geochemical applications of Hg isotopes. Mercury isotopes in shales are a geochemical tool that can be utilized to study the prevalence of global volcanism and detect oxygen-depleted conditions in the photic zone of ancient oceans. I measured Hg isotope ratio data from a Devonian shale bed in a road cut with varying degrees of weathering that has been well characterized for variations in elemental concentrations and other isotopic ratios. I found significant variation in mass-dependent and mass-independent Hg isotope fractionation in weathered samples. Surprisingly, however, I observed both loss and gain of Hg, when only significant loss was expected based on prior weathering studies. These findings improve the understanding of Hg mobility in nature and indicate that mass-independent fractionation can be modified after deposition in surprising ways.
ContributorsSullivan, Daniel Louis (Author) / Anbar, Ariel D (Thesis advisor) / Gordon, Gwyneth W (Committee member) / Hartnett, Hilairy E (Committee member) / Hervig, Richard L (Committee member) / Zheng, Wang (Committee member) / Arizona State University (Publisher)
Created2023
Description
Most stars in our galaxy are M–dwarfs, much cooler and smaller than the sun. The ubiquitous nature of these stars is also paired with the formation of terrestrial exoplanets orbiting them. The strategic placement of M-dwarfs between main-sequence stars and brown dwarfs, their uniqueness as exoplanet analogs, and their dominating presence in the galactic stellar population make them priority targets for study. This work investigates outstanding questions, including the need to acquire constraints on their chemical compositions to decode formation processes, evolution, and interaction with companion objects. Chapter 1 lays out a broad background emphasizing the importance of studying the most populous star in the galaxy, their far-reaching implications, and primarily the numerous challenges in characterizing the atmospheres and environments of these stars. Chapter 2 investigates the influence of M-dwarf star spots propagating into spectra of transiting terrestrial planets, showing that inaccurate modeling of M-dwarf photospheres leads to significant bias when inferring atmospheric properties of companion exoplanets. These biases persist despite correcting M-dwarf spot signatures imprinted onto the exoplanetary spectra, even with high-fidelity JWST observations. This result emphasizes the need for improved stellar atmosphere models as the first step to improving our understanding of the companion planets. To address this, chapter 3 introduces SPHINX—a new stellar atmosphere model grid for M-dwarfs. SPHINX provides improved constraints on fundamental properties of benchmark M-dwarf systems (e.g., temperature, surface gravity, radius, and chemistry). The improvement is significant relative to the state-of-the-art stellar model grid available today. Chapter 4 expands this model, applying it to mid-to-late type M-dwarfs, and investigating chemical trends in their atmospheric properties. Using low-resolution observations, both archival data (from SpeX Prism Library Database) and from previous empirical studies; this chapter presents constraints on fundamental atmospheric properties of 71 low-mass, late-type M-dwarfs to understand spectroscopic degeneracies arising due to stellar activity, cloud/dust condensation and convection. With SPHINX models, the chemical properties of these stars are compared against main-sequence stars to acquire a more holistic understanding of M-dwarfs as a class—in the quest to ultimately characterize their companions.
ContributorsIyer, Aishwarya (Author) / Line, Michael (Thesis advisor) / Patience, Jennifer (Committee member) / Young, Patrick (Committee member) / Shkolnik, Evgenya (Committee member) / O'Rourke, Joseph (Committee member) / Arizona State University (Publisher)
Created2023
Description
Banded iron formations (BIFs) are among the earliest possible indicators for oxidation of the Archean biosphere. However, the origin of BIFs remains debated. Proposed formation mechanisms include oxidation of Fe(II) by O2 (Cloud, 1973), photoferrotrophy (Konhauser et al., 2002), and abiotic UV photooxidation (Braterman et al., 1983; Konhauser et al., 2007). Resolving this debate could help determine whether BIFs are really indicators of O2, biological activity, or neither.
To examine the viability of abiotic UV photooxidation of Fe, laboratory experiments were conducted in which Fe-bearing solutions were irradiated with different regions of the ultraviolet (UV) spectrum and Fe oxidation and precipitation were measured. The goal was to revisit previous experiments that obtained conflicting results, and extend these experiments by using a realistic bicarbonate buffered solution and a xenon (Xe) lamp to better match the solar spectrum and light intensity.
In experiments reexamining previous work, Fe photooxidation and precipitation was observed. Using a series of wavelength cut-off filters, the reaction was determined not to be caused by light > 345 nm. Experiments using a bicarbonate buffered solution, simulating natural waters, and using unbuffered solutions, as in prior work showed the same wavelength sensitivity. In an experiment with a Xe lamp and realistic concentrations of Archean [Fe(II)], Fe precipitation was observed in hours, demonstrating the ability for photooxidation to occur significantly in a simulated natural setting.
These results lead to modeled Fe photooxidation rates of 25 mg Fe cm-2 yr-1—near the low end of published BIF deposition rates, which range from 9 mg Fe cm-2 yr-1 to as high as 254 mg Fe cm-2 yr-1 (Konhauser et al., 2002; Trendall and Blockley, 1970). Because the rates are on the edge and the model has unquantified, favorable assumptions, these results suggest that photooxidation could contribute to, but might not be completely responsible for, large rapidly deposited BIFs such those in the Hamersley Basin. Further work is needed to improve the model and test photooxidation with other solution components. Though possibly unable to fully explain BIFs, UV light has significant oxidizing power, so the importance of photooxidation in the Archean as an environmental process and its impact on paleoredox proxies need to be determined.
To examine the viability of abiotic UV photooxidation of Fe, laboratory experiments were conducted in which Fe-bearing solutions were irradiated with different regions of the ultraviolet (UV) spectrum and Fe oxidation and precipitation were measured. The goal was to revisit previous experiments that obtained conflicting results, and extend these experiments by using a realistic bicarbonate buffered solution and a xenon (Xe) lamp to better match the solar spectrum and light intensity.
In experiments reexamining previous work, Fe photooxidation and precipitation was observed. Using a series of wavelength cut-off filters, the reaction was determined not to be caused by light > 345 nm. Experiments using a bicarbonate buffered solution, simulating natural waters, and using unbuffered solutions, as in prior work showed the same wavelength sensitivity. In an experiment with a Xe lamp and realistic concentrations of Archean [Fe(II)], Fe precipitation was observed in hours, demonstrating the ability for photooxidation to occur significantly in a simulated natural setting.
These results lead to modeled Fe photooxidation rates of 25 mg Fe cm-2 yr-1—near the low end of published BIF deposition rates, which range from 9 mg Fe cm-2 yr-1 to as high as 254 mg Fe cm-2 yr-1 (Konhauser et al., 2002; Trendall and Blockley, 1970). Because the rates are on the edge and the model has unquantified, favorable assumptions, these results suggest that photooxidation could contribute to, but might not be completely responsible for, large rapidly deposited BIFs such those in the Hamersley Basin. Further work is needed to improve the model and test photooxidation with other solution components. Though possibly unable to fully explain BIFs, UV light has significant oxidizing power, so the importance of photooxidation in the Archean as an environmental process and its impact on paleoredox proxies need to be determined.
ContributorsCastleberry, Parker (Author) / Anbar, Ariel D (Thesis advisor) / Herckes, Pierre (Committee member) / Lyons, James (Committee member) / Arizona State University (Publisher)
Created2017
Description
The interactions that take place in the ionized halo of gas surrounding galaxies, known as the circumgalactic medium (CGM), dictates the host galaxy's evolution throughout cosmic time. These interactions are powered by inflows and outflows that enable the transfer of matter and energy, and are driven by feedback processes such as accretion, galactic winds, star formation and active galactic nuclei. Such feedback and the interactions that ensue leads to the formation of non-equilibrium chemistry in the CGM. This non-equilibrium chemistry is implied by observations that reveal the highly non-uniform distribution of lower ionization state species, such as Mg II and Si II, along with widespread higher ionization state material, such as O VI, that is difficult to match with equilibrium models. Given these observations, the CGM must be viewed as a dynamic, multiphase medium, such as occurs in the presence of turbulence. To better understand this ionized halo, I used the non-equilibrium chemistry package, MAIHEM, to perform hydrodynamic (HD) simulations. I carried out a suite of HD simulations with varying levels of artificially driven, homogeneous turbulence to learn how this influences the non-equilibrium chemistry that develops under certain conditions present in the CGM. I found that a level of turbulence consistent with velocities implied by observations replicated many observed features within the CGM, such as low and high ionization state material existing simultaneously. At higher levels of turbulence, however, simulations lead to a thermal runaway effect. To address this issue, and conduct more realistic simulations of this environment, I modeled a stratified medium in a Milky Way mass Navarro-Frenk-White (NFW) gravitational potential with turbulence that decreased radially. In this setup and with similar levels of turbulence, I alleviated the amount of thermal runaway that occurs, while also matching observed ionization states. I then performed magneto-hydrodynamic (MHD) simulations with the same model setup that additionally included rotation in the inner halo. Magnetic fields facilitate the development of an overall hotter CGM that forms dense structures within where magnetic pressure dominates. Ion ratios in these regions resemble detections and limits gathered from recent observations. Furthermore, magnetic fields allow for the diffusion of angular momentum throughout the extended disk and gas cooling onto the disk, allowing for the maintenance of the disk at late times.
ContributorsBuie II, Edward (Author) / Scannapieco, Evan (Thesis advisor) / Borthakur, Sanchyeeta (Committee member) / Groppi, Christopher (Committee member) / Jacobs, Danny (Committee member) / Young, Patrick (Committee member) / Arizona State University (Publisher)
Created2022
A Catalog of Stellar Evolution Profiles and the Effects of Variable Composition on Habitable Systems
Description
We present stellar evolution models for 0.5 - 1.2 M⊙ at scaled metallicities of 0.1 - 1.5 Z⊙ and O/Fe values of 0.44 - 2.28 O/Fe⊙. The time dependent evolution of habitable zone boundaries are calculated for each stellar evolution track based on stellar mass, effective temperature, and luminosity parameterizations. The rate of change of stellar surface quantities and the surrounding habitable zone position are strong functions of all three quantities explored. The range of orbits that remain continuously habitable, or habitable for at least 2 Gyr, are provided. The results show that the detailed chemical characterization of exoplanet host stars and a consideration of their evolutionary history are necessary to assess the likelihood that a planet found in the instantaneous habitable zone has had sufficient time to develop a biosphere capable of producing detectable biosignatures. This model grid is designed for use by the astrobiology and exoplanet communities to efficiently characterize the time evolution of host stars and their habitable zones for planetary candidates of interest.
ContributorsTruitt, Amanda (Author) / Young, Patrick (Author) / Spacek, Alexander (Author) / Probst, Luke (Author) / Dietrich, Jeremy (Author) / College of Liberal Arts and Sciences (Contributor)
Created2015-05-10