Master of Science, The Ohio State University, 1967, Graduate School

Committee: Not Provided (Other) Subjects:

Doctor of Philosophy, The Ohio State University, 2023, Astronomy

Stellar evolution—the study of how the observable properties of stars evolve over their life cycles—is linked to frameworks of how stars interact with their companion planets, how their structures evolve over time, and how they form the building blocks of galaxies like our own. Stellar models have historically been successful in describing the physical processes that occur within the interiors of stars as they evolve. With recent advances in modern ground- and space-based surveys, however, unprecedented tests of frameworks of stellar evolution have brought a number of discrepancies into focus. These startling set of tensions today exist between observations and standard stellar models, which suggest that a full physical explanation is missing. Such tensions include discrepancies in the ages of young stars, radii discrepancies in the most active stars, and rotational anomalies observed during the angular momentum evolution of stars. A leading hypothesis for the origin of these discrepancies is that stellar magnetism can systematically change the internal structure of stars. In strongly magnetic stellar surfaces, the emergence of magnetic starspots can make regions less efficient at transporting radiation by inhibiting convection. The star compensates by expanding—in doing so, active stars can appear cooler and inflated relative to inactive ones. A theory which explains observed rotational anomalies is called core-envelope decoupling, where the star's core and envelope spin down at different rates and exchange angular momentum as they return to solid body rotation. If the process of decoupling really occurs, strong velocity shears between the core and the envelope might drive dynamo action and enhanced magnetism over long timescales. In this dissertation, I discuss my work with magnetic stellar evolution models and provide a novel dataset of direct magnetic measurements. I demonstrate that tests with magnetic stellar evolution models show better agreement with the (open full item for complete abstract)

Committee: Marc Pinsonneault (Advisor); Jennifer Johnson (Committee Member); Krzysztof Stanek (Committee Member); Christopher Kochanek (Committee Member) Subjects: Astronomy; Astrophysics; Physics

Doctor of Philosophy, The Ohio State University, 2022, Astronomy

While the Universe might at first appear static and unchanging to a casual observer, it is teeming with variable sources and cataclysmic events that mark the births, lives, and deaths of the many and varied objects filling our Universe. In recent years, modern time-domain surveys have revolutionized the study of stellar variability by providing access to time series data for millions of stars in the Milky Way. The All-Sky Automated Survey for SuperNovae (ASAS-SN) monitors the entire visible sky daily using 20 robotic telescopes in Hawaii, Texas, Chile, and South Africa. In addition to the real-time detection of bright supernovae and other transients, archival ASAS-SN data allows for the time series characterization of over 100 million stars. By analyzing the ASAS-SN time series data for ~61 million stars, I made the first homogeneous all-sky catalog of bright variable stars and then uniformly classified them using machine learning techniques. This catalog includes the discovery of ~220,000 new variable stars and ~660,000 variables in total. I present studies that use this catalog combined with information from large scale spectroscopic surveys to study various populations of variable stars. Finally, I present examples of the discovery of rare and unusual variable stars using ASAS-SN, including the most extreme 'heartbeat'' star ever discovered.

Committee: Krzysztof Stanek (Advisor); Todd Thompson (Committee Member); Christopher Kochanek (Advisor) Subjects: Astronomy; Astrophysics; Physics

Doctor of Philosophy, The Ohio State University, 1981, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1980, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Master of Science, The Ohio State University, 1961, Graduate School

Committee: Not Provided (Other) Subjects:

PHD, Kent State University, 2024, College of Arts and Sciences / Department of Physics

This dissertation investigates the phase transitions from hadron to quark matter under non-equilibrium chemical conditions using the Chiral Mean Field (CMF) model, with a specific focus on the role of net strangeness and variations in charge and isospin fractions. By constructing detailed three-dimensional quantum chromodynamics (QCD) phase diagrams, this study elucidates how these factors influence chemical potentials, temperature, and the deconfinement process in environments similar to those found in protoneutron stars, binary neutron-star mergers, and heavy-ion collisions. The analysis reveals how these conditions impact the location of the critical endpoint, where the phase transition shifts from a first-order change to a crossover. Further exploration within this research addresses the significant impact of net strangeness on high-energy astrophysical phenomena, such as supernova explosions, neutron stars, and compact-star mergers. This work integrates recent theoretical developments in QCD critical points to provide a comprehensive overview of how non-equilibrium chemical conditions, particularly with respect to leptons, affect the deconfinement transition, narrowing the region of phase coexistence as observed in the interiors of cold catalyzed neutron stars. The findings emphasize the importance of including hyperons and strange quarks in theoretical models to better understand the complex nature of dense matter under extreme conditions. This dissertation contributes valuable insights into the dynamics of phase transitions in dense astrophysical objects, significantly advancing the field of high-energy astrophysics.

Committee: Veronica Dexheimer (Advisor); Antal Jakli (Committee Member); Hao Shen (Committee Member); Gokarna Sharma (Committee Member); Benjamin Fregoso (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2024, Astronomy

Brown dwarfs are substellar objects approximately an order of magnitude more massive than Jupiter but below the threshold necessary to burn hydrogen and become main-sequence stars. Although we have learned much about the atmospheres of brown dwarfs, and lower mass planetary-mass objects, over the past 30 years since the first observation of such an object, open questions remain about the driver of these objects' variability. Brown dwarfs, particularly those at the L/T spectral transition (Teff ~ 1300K), exhibit high spectroscopic and photometric variability which has been attributed to dynamic atmospheric processes. Theories invoking storms and vortices (spotted-models) and planetary-scale waves contained within Jupiter-like banded-structures (wave models) have been proposed to explain this variability. To address these mysteries, I first developed a unified analytical spectroscopic and photometric technique to infer surface inhomogeneities on stellar and substellar objects. The method was validated against archival data of Luhman 16B, a nearby, L/T transition brown dwarf. I then used the analytical technique to predict Doppler imaging performance for planned extremely large telescope spectrographs. I found these instruments capable of mapping the surfaces of brown dwarfs and planetary-mass objects such as VHS 1256 b, SIMP0136, and Beta Pic b. To further test my analytical framework, I applied it to real-world spectra collected at the Large Binocular Telescope, detecting a persistent polar dark spot on a pre-main sequence, T Tauri star. The extent of the spot suggested non-solar like dynamo processes. To conclude, I applied both spotted and wave models to multi-color, near-infrared (NIR) photometry of SIMP0136. Based on goodness-of-fit tests, I find wave models to be preferred. Moreover, the spotted models' inferred radii are unphysical based on the Rossby deformation radius and Rhines' scale. Through the correlation between light curves produced by the waves and as (open full item for complete abstract)

Committee: Ji Wang (Advisor); Marc Pinsonneault (Committee Member); Jennifer Johnson (Committee Member); B. Scott Gaudi (Committee Member) Subjects: Astronomy; Astrophysics

PhD, University of Cincinnati, 2023, Arts and Sciences: Physics

Galactic rotation curves (RCs), i.e. galactic rotational speeds vs. galactic radii, seem to infer the existence of a yet to be discovered type of matter commonly called dark matter (DM). Data for galactic RCs, like that in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database, can be used to test different models of galactic DM. Two popular DM candidates are QCD axions and axion-like particles (ALPs), which can make up some or all of the DM energy density of the universe. Being bosonic in nature, these particles can form Bose-Einstein condensates commonly called boson stars or axion stars. A class of light ALPs termed ultralight DM (ULDM) with masses of m ≲ 10-20 eV can form boson stars with sizes on the order of galactic cores, and have been a popular candidate for galactic DM. However, single flavor ULDM models have become heavily constrained, and multi-flavor ULDM models which can evade these constraints have gained traction in their stead. Here, we discuss QCD axions, ALPs, and the structures they form. We discuss the separate formalisms governing boson stars under fully relativistic conditions, and for those under conditions of weak gravity and weak binding energy. We then show various solutions and boson star properties for both single and multi-flavored ALP models. Changing gears, we then give the formalism behind galactic RCs, and show how different DM models can be tested using the RCs for observed galaxies. Finally, we discuss how ULDM models compare to other popular DM models, namely cold DM (CDM), and show the results of a statistical comparison of ULDM and CDM models to galactic RCs in the SPARC database.

Committee: L. C. R. Wijewardhana Ph.D. (Committee Chair); Joachim Brod (Committee Member); Jure Zupan Ph.D. (Committee Member); Rostislav Serota Ph.D. (Committee Member); Kay Kinoshita Ph.D. (Committee Member) Subjects: Physics

Master of Science (MS), Bowling Green State University, 2021, Physics

Studying stellar populations in the Milky Way's halo is one of several ways that astronomers can gain insight into how the halo formed. By studying the abundances, kinematics, and spatial distributions of the RR Lyrae population, a very old (>10 Gyr) group of variable stars with a particularly short period of less than a day, researchers can understand the conditions present during the Galaxy's formation. These stars are ideal for observation due to their narrowly defined absolute magnitude and distinctive light curves. However, much of the data we have on known RR Lyrae variables is outdated, and with the construction of new telescopes, astronomers now have the opportunity to bring the information up to date. Using data from the All-Sky Automated Survey for SuperNovae (ASAS-SN), we have obtained updated periods for the 106 RR Lyrae stars studied in Layden (1998). With these periods, new light curves were created, providing us with accurate mean magnitudes, epochs of maximum light, and amplitudes. These properties were used along with spectroscopic data to find the distances, radial velocities, and metal abundances of 84 of these stars, from which we then obtained net rotational velocities. The net rotational velocity for all stars in our sample was found to be 36 ± 19 km/s. When these stars were divided into metal-rich halo stars (-1.8 ≤ [Fe/H] < -1.0) and metal-poor halo stars ([Fe/H] < -1.8), we obtained rotational velocities of 41 ± 21 km/s and -17 ± 44 km/s, respectively. This difference in values is consistent with the existence of two distinct components of the Milky Way halo: a more metal-rich, prograde in-situ halo made up of stars native to the Milky Way, and a more metal-poor, kinematically hot accreted halo made up of stars added to the Galaxy by merger events.

Committee: Andrew Layden PhD (Advisor); John Laird PhD (Committee Member); Dale Smith PhD (Committee Member) Subjects: Astronomy; Physics

Master of Science (MS), Bowling Green State University, 2021, Physics

Globular clusters (GCs) are home to some of the oldest stellar populations. Many of the stars in GCs are near the end of their life and pulsate due to internal mechanisms that are not fully understood. These long-period variable stars (LPVs) vary in magnitude and radius and have periods of months up to hundreds of days. NGC 6553 is a metal-rich globular cluster located in the crowded bulge of the Milky Way. It has been studied extensively due to its spatial location and metal abundance, but the search for variable stars has been limited to short-period surveys. While 11 LPVs have been documented within NGC 6553, only one has a published period. The dataset consists of images taken in V and I passbands from the Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT) at Cerro Tololo Inter-American Observatory in Chile. Over four years, images were collected on NGC 6553 a few nights a month by previous BGSU students. Photometry was performed on each image using the program DAOPHOT to track changes in stars' magnitudes. Periods were established for 43 of the 46 stars that were investigated. Most of these variable stars show indications of multiple periods due to pulsations in various modes, and over half feature long-secondary periods (LSPs) which are not believed to be caused by pulsation. Light curves for the stars are shown and the process of identifying periods is detailed. Color-magnitude diagrams (CMD) were created to isolate the cluster sequence of NGC 6553. The locations of the LPVs on the CMD were used to determine whether the star was a member of cluster or the field. Of the stars examined, 23 appear to be cluster members.

Committee: Andrew Layden Ph.D. (Advisor); John Laird Ph.D. (Committee Member); Dale Smith Ph.D. (Committee Member) Subjects: Astronomy

Doctor of Philosophy, The Ohio State University, 2021, Astronomy

Rotation plays an important role in the life of stars, and offers a potential diagnostic to infer their ages and that of their planets. The idea of using stellar rotation as a chronometer is known as gyrochronology. While potentially fruitful over a wide range of ages and masses, gyrochronology has not been vetted across all the relevant regimes, and from a theoretical perspective the evolution of rotation is not accurately predicted by stellar models from first principles. For these reasons, empirical studies of rotation play a key role in stellar astrophysics. In recent years, much of the classical knowledge on the rotational evolution of low mass stars has been challenged by new data sets (e.g., Kepler and Gaia). In particular, contemporary results have raised concerns regarding the applicability of gyrochronology as a universal age diagnostic. In this context, this dissertation carries out three efforts to examine gyrochronology in different evolutionary regimes and stellar configurations. Regarding young stars (< 1 Gyr), I illustrate the impact that removing the non-member contamination by using the precise Gaia astrometry has on the rotational sequences of open clusters. The clean and updated sequences show that the overall fraction of rotational outliers decreases considerably but not completely; demonstrate that ground-based rotation periods can be as constraining as space-based periods; illustrate that 1.0 to 0.6 Msun stars populate a global maximum of rotation periods (with potential implications for exoplanet habitability); and suggest evidence that rapid rotators experience a lower torque than intermediate rotators in the saturated domain. Importantly for future calibrations and tests of stellar models, the percentiles of the revised rotational sequences are made publicly available. Regarding old stars (few Gyr), I carry out a search for common proper motion wide binaries in the Kepler field. These systems that can be thought of as the smallest ve (open full item for complete abstract)

Committee: Marc Pinsonneault (Advisor); Jennifer Johnson (Committee Member); Donald Terndrup (Committee Member); Matthew Stenzel (Committee Member) Subjects: Astronomy; Astrophysics

Doctor of Philosophy, The Ohio State University, 1985, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1981, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1981, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1981, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1981, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1980, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1980, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1971, Graduate School

Committee: Not Provided (Other) Subjects: Physics