Ph.D. - Astronomy

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https://hdl.handle.net/10125/772

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    Vehicles to the Past: Characterizing the Activity of Long Period Comets
    (2024) Molnar-Bufanda, Erica; Meech, Karen; Astronomy
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    Hawai‘i Supernova Flows: A Peculiar Velocity Survey Using Over a Thousand Supernovae in the Near-Infrared
    (2024) Do, Aaron Joshua Matsuo; Shappee, Benjamin J.; Astronomy
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    Discovering and Exploring Extreme Black Holes and their Environments in the Transient Universe
    (University of Hawaii at Manoa, 2023) Payne, Anna Victoria; Shappee, Benjamin J.; Astronomy
    Supermassive black holes (SMBHs) at the centers of galaxies are some of the most luminous objects in the universe when feeding on surrounding material. This occurs when the SMBH is continuously accreting, called an active galactic nuclei (AGN). Or, a star can wander too close to the SMBH and becomes destroyed by the tidal forces, called a tidal disruption event (TDE), and the SMBH feeds on the stellar debris. The current landscape of astrophysics is rich with missions and telescopes ideal for exploring the transient universe, including AGNs and TDEs. Surveys, which image the entire night sky on various timescales ranging from minutes to days, provide crucial data to discover and characterize these nuclear transients. After developing a large sample of highly variable AGN observed by the ASAS-SN survey, I discovered a new, unique nuclear transient that undergoes periodic flares called ASASSN-14ko. One flare in 2014 was mistakenly classified as a supernova and the periodic behavior went undetected until my discovery. ASASSN-14ko is located within AGN ESO 253-G003 with optical flares present in the ASAS-SN light curve since 2014, and after modeling the peak times I find their timings are well-modeled with a period of 115.2 +1.3 -1.2 days and period derivative of -0.0026 +/- 0.0006. I organized, led, carried out, and analyzed data in an intensive multi-wavelength observing campaign to understand this unique object, and obtained X-ray, UV, and optical photometry and spectroscopy with numerous space- and ground-based facilities since 2020. All flares are characterized by a concurrent brightening in the UV and optical. The X-ray luminosities consistently decrease and the spectra become harder during the UV/optical rise but apparently without changes in absorption. The flares' photometric and spectroscopic signatures evolve rapidly. With the current suite of data, ASASSN-14ko is observationally consistent with a repeating partial tidal disruption event. This is the only such object ever discovered that has strongly periodic flares across X-ray, UV, and optical wavelengths, and provides a new laboratory to model TDEs and refine their theoretical frameworks.
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    Constraining Yarkovsky Acceleration With Thermophysical Modeling
    (University of Hawaii at Manoa, 2023) Hung, Denise; Tholen, David J.; Astronomy
    The Yarkovsky effect is a phenomenon that arises from an anisotropy in an asteroid's surface temperature distribution and resulting thermal emission that induces a secular change on the asteroid's semimajor axis, causing the asteroid to drift outwards or inwards in its orbit. It is the main mechanism by which the near-Earth asteroid population is maintained, where main belt asteroids (MBAs) are continuously moved into powerful resonances that eject them into near-Earth orbits. Despite hundreds of successful detection among near-Earth asteroids (NEAs), the Yarkovsky effect has never been detected in MBAs before. The Yarkovsky effect is difficult to predict as it relies on several physical parameters that are often completely unconstrained, but these parameters can be derived by way of thermophysical modeling. Using thermal flux measurements obtained by the Wide-field Infrared Survey Explorer (WISE) combined with shape models and spin information from the Database of Asteroid Models from Inversion Techniques (DAMIT), we obtained newly derived diameters, albedos, and thermal inertia for nearly 2000, primarily main belt, asteroids. With the parameters in hand, we were able to use a rudimentary Yarkovsky model to estimate the acceleration imparted to each of our asteroids. We attempted to confirm our predictions with the University of Hawai'i 88-inch telescope, where we observed a total of 135 MBAs and two NEAs in order to supplement the astrometric data drawn from the Minor Planet Center. Using the OrbFit software package to compute our asteroids' orbits, we found Yarkovsky signal strengths consistent with previous studies for our two NEAs. However, we were unable to find any reliable detections of Yarkovsky in our sample of MBAs due to the limitations of the historical astrometric data. Through tests with synthetic observations, we were able to identify the minimum observational arc length with modern astrometric accuracy needed to detect Yarkovsky for not only all of our sample MBAs, but also any theoretical MBA of a given size and semimajor axis, estimating that Yarkovsky should be detectable within a couple of decades for a 100-m MBA, a size range which should be detectable by the near-future Vera Rubin Observatory Legacy Survey of Space and Time. While current technology is not yet at a sufficient level to detect Yarkovsky in the main belt, the future is promising, especially with the gamut of upcoming infrared and wide-field surveys that will enable even more thermophysical modeling of higher precision in the future.
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    Spinning Red Clocks in Crowded Fields
    (University of Hawaii at Manoa, 2022) Dungee, Ryan; Chun, Mark; Van Saders, Jennifer L.; Astronomy
    To build a clear picture of the history of our Milky Way galaxy we need to be able to reliably measure the ages of the stars within it. Unfortunately the most common stars, M dwarfs, are resistant to most of our current age measuring techniques.This leaves gyrochronology, the use of an empirical relation between rotation and age, as our method for measuring M dwarf ages. Successful gyrochronology requires the study of open clusters spanning as wide a range of ages and metallicities as possible. In this dissertation I present the work I have done on advancing our understanding of spin-down in late K and early M dwarfs through observations of the 4 Gyr old open cluster M67. Future studies that include a wider sample of open clusters, and that push the observations to lower mass stars, will require high spatial resolution wide-field imaging to remain feasible. To this end, this dissertation also presents the work I have done in developing an advanced technique for the control systems of ground layer adaptive optics. This technique, which we have dubbed “temporal tomography” is expected to facilitate the widespread adoption of ground layer adaptive optics by reducing the number of guide stars needed to obtain accurate estimates of the ground layer turbulence. These systems will be capable of imaging crowded fields, such as the open clusters needed for calibrating gyrochronology.
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    The Space Photometry Revolution: Precise Star and Planet Properties in the Kepler, K2 & TESS Era
    (University of Hawaii at Manoa, 2022) Chontos, Ashley; Huber, Daniel; Astronomy
    The field of exoplanetary science recently surpassed a milestone by confirming more than five thousand planets beyond the solar system. However, more than 99% of known exoplanets are indirectly detected and therefore, the derived planet properties critically depend on fundamental stellar properties. The main theme of this thesis is to use the tools of stellar astrophysics to precisely characterize planetary systems. One powerful tool to measure precise stellar properties is asteroseismology, the study of stellar oscillations which provides masses, radii and ages of host stars with exquisite precision and accuracy. An open-source asteroseismology pipeline was developed from well-tested closed-source tools, benchmarked to the asteroseismic legacy sample to ensure reproducible Kepler mission results, and then publicly released for the broader astronomy community to analyze forthcoming TESS data. The pipeline successfully identified solar-like oscillations in benchmark systems like the solar analog alpha Men A as well as KOI 4, the first new planet host identified by the Kepler mission. KOI 4 remained unconfirmed for nearly a decade, the planet discovery which hinged on the recharacterization of the host star using asteroseismology. The asteroseismic constraints placed the star in a rare parameter space as a subgiant host, where planets are expected to undergo quick inward migration and engulfment by their host star. The lack of orbital period decay via star-planet tidal interactions in the KOI 4 system enabled the first observational constraint on the tidal quality factor in an evolved system. Even for non-oscillating subgiants though, the rapid phase in stellar evolution uniquely constrains system masses and ages, thus providing an ideal sample to test evolution theories for post-main-sequence systems. A homogeneous sample of subgiant planetary systems is presented as part of the TESS-Keck Survey (TKS), a complementary ground-based spectroscopic survey to follow up, confirm and characterize transiting TESS planets. Precise planet densities, system ages and long-term orbital properties are presented for 22 TESS subgiant systems, providing a first look into the differences between main-sequence and evolved planet distributions. Preliminary results suggest an increased efficiency of tidal interactions in evolved systems through the dearth of larger, more massive planets, which is consistent with predictions from tidal theories. In addition, more than half of the systems show strong evidence for planet multiplicity, which was previously thought to be uncommon around such stars. Further analysis of the planet sample can be used to place quantitative constraints on processes related to tidal dissipation like orbital period decay and circularization.