Ph.D. - Astronomy

Permanent URI for this collectionhttps://hdl.handle.net/10125/772

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On the interaction between solar photospheric flow and magnetic field
(University of Hawai'i at Manoa, 2025) Liu, Jiayi; Sun, Xudong; Astronomy
The interaction between plasma flow and magnetic field transports energy and magnetic helicity through the solar photosphere. It plays a fundamental role in heating the corona and powering eruptions. In active regions, the rising, twisted magnetic flux tubes expand and unwind; the resulting net Lorentz force drives systematic photospheric flows that injects significant magnetic energy and helicity into the corona. In the quiet-Sun region, these interactions will introduce undulations in the magnetic field lines, resulting in the concentration, cancellation, and reconnection of the magnetic fields. Studying the photospheric flows and their interaction with the magnetic fields are of fundamental importance to understanding the dynamics of the lower atmosphere, and its role in transporting energy and helicity that power solar eruptions.An overarching goal of this dissertation is to accurately infer the flow fields from a time sequence of photospheric spectropolarimetric data, which is essential to quantifying the energy and helicity transport in both active regions and quiet Sun. The first part focuses on studying the flow field in active regions. Specifically, I investigate the apparently supersonic Doppler flows in the core of active region 12673 with a new flow tracking method and SDO/HMI and Hinode/SP observations. I show that the flows are parallel to the magnetic fields, and the large Doppler velocity is just a projection effect. In the second part, in preparation for quiet-Sun studies with the 4-m Daniel K. Inouye Solar Telescope (DKIST), I explore the capability of its high-resolution observation in quantifying the energy transport with the realistic magnetohydrodynamic simulation and forward modeling. I find that existing method can capture the majority of unsigned energy flux, but the net flux remains poorly estimated. In the third part, using active region 12673 as an example, I investigate the reasons for significant discrepancies between several widely used methods that estimate energy and helicity flux. I demonstrate that the curl-free part of the electric field, which is not well-constrained by observation and is treated in ad hoc fashion, is the main cause for the differences. The interaction between the flow field and the magnetic field in the solar photosphere is key to understanding the dynamics in the solar atmosphere. My work demonstrates the capability of new methods and observations in characterizing complex flow fields in both active regions and quiet Sun, though many challenges persist. The future multi-line, high spatial-, spectral-, and temporal-resolution observation as well as the observations from different vantage points will provide more constraints and expand our capabilities.
High-contrast imaging and polarimetry at solar-system scales
(University of Hawai'i at Manoa, 2025) Lucas, Miles Dylan; Bottom, Michael; Astronomy
Direct imaging is a uniquely powerful technique for studying exoplanets and circumstellar disks, offering spatially resolved observations of planetary systems and the environments in which they form. This dissertation presents a comprehensive investigation of high-contrast imaging and polarimetry at solar-system scales, with applications ranging from the detection of planetary companions to the detailed analysis of planet-forming disks.I begin with a theoretical and technical overview of direct imaging, outlining the key challenges of contrast, angular resolution, and wavefront aberrations. I then present a direct imaging search for second-generation planets around the white dwarf Sirius B, placing new limits on planetary companions within the dynamically stable orbital region. Next, I describe and characterize upgrades to the visible-light high-contrast imaging polarimeter VAMPIRES at the Subaru Telescope. These upgrades included photon-counting CMOS detectors, a novel observing mode for multiband imaging, coronagraphic optics, and new polarimetric optics. I then discuss the development of a dedicated data processing pipeline for VAMPIRES that handles calibration, image analysis, image registration, flux calibration, and polarimetry. This pipeline is designed to accommodate the high data volumes and diverse observing modes of VAMPIRES. Finally, I present a multi-epoch polarimetric imaging study of the HD 169142 planet-forming disk. This analysis highlights long-lived scattered-light substructures that persist over a twelve year observational baseline. By tracking the evolution of azimuthal features, I place dynamical constraints on potential planet-disk interactions. This study demonstrates the scientific merit of multi-epoch high-contrast polarimetric imaging for probing disk evolution and planet formation over multi-year timescales.
Stellar evolution and giant exoplanets
(University of Hawai'i at Manoa, 2025) Saunders, Nicholas; Huber, Daniel; van Saders, Jennifer; Astronomy
The fate of a planet is deeply influenced by the evolution of the star it orbits. With the sample of confirmed exoplanets nearing 6,000, we are now positioned to examine how stellar evolution affects the structure and dynamics of planetary systems across their full lifetimes. While many studies focus on main-sequence systems, planets orbiting evolved stars provide a unique lens into planetary evolution. In this dissertation, I conduct a focused investigation of exoplanets orbiting subgiant and red giant stars, coupling observations of their planetary architectures to changes in their host stars' internal structure. Using data from NASA's Transiting Exoplanet Survey Satellite, I developed and applied a specialized open-source pipeline to identify and confirm transiting planets orbiting evolved stars. This effort has so far led to the discovery and confirmation of fifteen new planets, including five detailed in this dissertation. The orbital architectures of these systems provide the first empirical evidence for post-main-sequence realignment between the orbital planes of short-period giant planets and the stellar spin axes of their hosts. These stars, originally hot and likely misaligned on the main sequence, have since cooled and developed convective envelopes, allowing tidal dissipation to realign their spins, marking a previously unobserved mechanism in planetary system evolution. Beyond orbital dynamics, these evolved systems also test key predictions for planet interior physics. Some of the planets discovered in this work appear inflated, as expected from models of post-main-sequence irradiation and energy deposition. However, others are not inflated despite receiving comparable levels of incident flux. A comparative analysis of their orbital eccentricities and radii supports the hypothesis that tidal heating during orbital circularization may play a critical role in radius inflation. To contextualize these planets within their host stars' evolutionary histories, I also investigate stellar spin-down as a probe of age. While gyrochronology offers precise age estimates for cool main-sequence stars, older stars are known to deviate from standard rotational evolution due to weakened magnetic braking. To model this effect, I trained a neural network emulator for stellar rotational evolution and used it to perform a population-level analysis of Kepler stars with asteroseismically measured rotation periods and ages. I find that deviations from standard spin-down begin slightly before solar age, and that while current weakened braking models improve fits to data, they remain incomplete beyond the age of the sun. Together, these projects demonstrate that evolved stars are powerful laboratories for understanding the long-term evolution of planetary systems. By combining new exoplanet discoveries, orbital dynamics, asteroseismic stellar characterization, and rotational modeling, this dissertation places new observational and theoretical constraints on key processes shaping planetary lifetimes.
Organosulfur chemistry in the birthplaces of planets: A comparative study of sulfur versus oxygen in interstellar ices
(University of Hawai'i at Manoa, 2025) Narayanan, Suchitra; Williams, Jonathan P.; Öberg, Karin I.; Astronomy
Of all the elements critical for life, sulfur (S) is the most poorly understood due to its 1–2 orders of magnitude depletion in the gas phase of star-forming regions relative to its cosmic abundance, called the "missing sulfur problem." To reconcile this, sulfur is believed to be locked up in icy grains, yet the sum of the solid sulfur inventory in ices accounts for only ≤ 4% of the cosmic sulfur abundance. The latest astrochemical gas-grain reaction network predicts that this observed sulfur depletion could be explained if the majority of the sulfur exists in the form of solid organosulfur species. However, due to the limited number of solid-state sulfur experiments, this model, like many others, heavily depends on the theoretical assumption that sulfur and oxygen (O) chemistry proceed comparably. By studying these organosulfurs, we not only test these predictions but also probe an important and understudied class of organic molecules that are significant in their own right, both for their prebiotic relevance and their potential to impact the chemistry of other icy species. In this dissertation, I characterize the simplest S-bearing complex organic molecule, methyl mercaptan (CH₃SH), with respect to its well-studied and relatively abundant O-bearing counterpart, methanol (CH₃OH). I present new laboratory experiments on CH₃SH's thermal desorption kinetics, entrapment behavior, and destruction pathways, while contextualizing all of the results with analogous CH₃OH experiments. This allows us to examine for the first time in the laboratory how, when, and why does S versus O chemistry proceed (dis)similarly. In most cases, I find that under identical experimental conditions, CH₃SH behaves strikingly differently from CH₃OH. These discrepancies cannot be fully explained with current computational chemistry capabilities. In particular, I find that the physical and chemical properties of a molecule (e.g., size, ability to form allotropes, bonding potential) significantly affects its behavior and stability in astrophysically relevant conditions. While CH₃OH is conducive to driving chemical complexity in ices, the majority of CH₃SH is destroyed efficiently and quickly into infrared-invisible refractory sulfur sinks, likely sulfur chains, potentially explaining why major sulfur carriers are elusive. Notably, I discover molecular size to play a crucial role in enhancing the volatile abundance in regions where planet formation is thought to be efficient: the water snow line. Such a size effect in ices has never been seen before and has reshaped our understanding of how simple organics in ice matrices can fundamentally change the distribution of volatiles across a protoplanetary disk. This research demonstrates how microphysics at the angstrom level can influence processes on planetary scales. By studying how two theoretically similar elements are empirically different, this work serves as a guide for further investigations into more complex molecules, helping us better predict their behavior based on whether they exhibit characteristics similar to sulfur or oxygen. This dissertation emphasizes the necessity of dedicated sulfur experiments and highlights the value of comparative chemistry for rationalizing observations and refining the theoretical understanding of (astro)chemistry.
Messy eaters: A diversity of supermassive black hole accretion behaviors revealed by nuclear transients
(University of Hawai'i at Manoa, 2025) Hinkle, Jason; Shappee, Benjamin J.; Astronomy
Optical transient surveys have ushered in the golden era of time-domain astronomy and exposed an unexpected breadth of transient phenomena, including nuclear transients: flares powered by accretion onto supermassive black holes (SMBHs). These transients provide a means to identify otherwise-quiescent SMBHs. More importantly, if their physics is well understood, they can be used to study the properties of SMBHs across a range of masses and redshifts. Here I detail my work on the major classes of nuclear transients through both case studies of well-observed objects—all discovered by UH-affiliated surveys—and population studies to investigate trends. Chapters 2-5 discuss various aspects of tidal disruption events (TDEs), Chapter 6 explains coronal line emitters (CLEs) as TDEs in gas-rich environments, Chapters 7-9 establish the growing group of ambiguous nuclear transients (ANTs) with observables similar to both TDEs and active galactic nuclei, and Chapter 10 introduces the class of extreme nuclear transients (ENTs) which are the most energetic transients known. Each class is powered by accretion, but it is clear that the nuclear environment shapes their observational signatures. I finally conclude by summarizing our current understanding of nuclear transients and promising future directions to leverage their utility as probes of SMBHs.
Vehicles to the Past: Characterizing the Activity of Long Period Comets
(University of Hawai'i at Manoa, 2024) Molnar-Bufanda, Erica; Meech, Karen; Astronomy
Long-period comets (LPCs) contain some of the solar system's most pristine volatiles, organics, and refractory materials, preserving chemical markers from the early proto-planetary disk. Despite their importance, LPCs are among the least studied in terms of activity. Observations reveal fewer LPCs with lower orbital energies (less eccentric orbits) than expected. Jan Oort suggested this scarcity results from LPCs losing volatile material after their initial passage through the inner solar system, causing their brightness to diminish over time. However, the precise physical processes behind cometary fading are still debated. To investigate these processes, we systematically modeled LPC activity and quantified volatile production rates as a function of solar distance, focusing on nearly 100 newly discovered LPCs with perihelia between 0.9 and 10 au. Our program combined multi-epoch photometric observations from CFHT and Gemini, supported by amateur and all-sky survey data. We use our new tool – the Volatile Activity Monitoring and Prediction (VAMP) python code - which is an augmented and improved version of an existing sublimation model - to measure nucleus sizes, identify sublimating ices, and determine active surface areas. For comets with delayed CO2 activity, we used a 1-D thermal model to estimate ice depths. Our findings indicate that CO2 sublimation plays a key role in outbursts and delayed activity onset, and we propose that periodic thermal processing of the nucleus, such as dust-mantle accumulation, and stochastic processes, such as activity caused by recondensing volatiles or avalanches that eventually lead to nucleus fracture may explain the scarcity of low-energy LPCs.
Dynamical Architecture of Transiting Planets with Outer Companions and Fourier Transform Spectroscopy for Directly Imaged Exoplanets
(University of Hawai'i at Manoa, 2024) Zhang, Jingwen; Huber, Daniel; Astronomy
Stellar and planetary companions are thought to play a crucial role in shaping the formation and migration of inner planets. However, the three-dimensional orbital architectures of these systems, including the mutual inclination between inner planets and outer companions, remain largely unexplored. In the first part of this dissertation, I compiled a volume-limited catalog of 66 stars within 300 pc, hosting planets and planet candidates from Kepler, K2, and TESS, and exhibiting significant Hipparcos-Gaia proper motion anomalies, which suggest the presence of outer planetary or stellar companions. To further investigate these systems, I monitored their radial velocities using Keck/HIRES and searched for stellar companions using high-resolution AO imaging from Keck/NIRC2. For planetary companions, I presented the discovery of long-period giant planets outside transiting planets in three systems: Kepler-129, HD 73344, and HD 118203. Our analysis revealed that small transiting planets could be misaligned relative to the stellar spin axis (Kepler-129) or the outer giant planets (HD 73344). The formation of such systems suggests the occurrence of dynamical events in the past, such as planet-planet scattering, or an alternative formation pathway compared to the solar system, such as planets forming in protoplanetary disks with misaligned inner and outer components. On the other hand, my results for the HD 118203 system reveal system-wide alignment, with the outer giant planet displaying low mutual inclination relative to both the inner hot Jupiter and the host star’s spin axis. This alignment helps to decipher the origin of the hot Jupiter: the system may have undergone coplanar high-eccentricity tidal migration. Furthermore, I presented the detection of 22 stellar companions to 20 TESS Object of Interests (TOIs) in my sample. Among them, I characterized the three-dimensional architecture of systems including LTT-1445, TOI-402 and TOI-369 using radial velocities, absolute astrometry from Gaia and Hipparcos, and relative astrometry from imaging. I identified a tentative correlation between the alignment of planetary and stellar companions and the periastron distances of the stellar companions. Specifically, stellar companions tend to exhibit low mutual inclination relative to transiting planets when their periastron distances are less than 20 AU. In contrast, at greater periastron distances, the mutual inclination distribution becomes more scattered, with significant misalignment in some cases (TOI-402). This discovery challenges the previous understanding that transiting planets are typically aligned with stellar companions, and suggests that we need to reconsider models of planet-disk-companion evolution. Second, I explored the potential of an imaging Fourier transform spectrograph (iFTS) to obtain atmospheric spectra from Earth-like planets. The Decadal Survey on Astronomy and Astrophysics 2020 has recommended a decade-long effort to mature technologies for a space-based direct imaging telescope, currently referred to as the Habitable Worlds Observatories (HWO), with the goal of observing around 25 terrestrial planets. A key challenge is the faintness of Earth-like exoplanets, with plant-star flux ratios on the order of 10^{-10}. Integral field spectrographs (IFS) have been a popular choice for the mission concepts. However, their performance is severely degraded by detector noise when observing extremely faint targets like terrestrial exoplanets. Therefore, I studied an alternative option of imaging Fourier transform spectrographs (iFTS), which are less sensitive to detector noise. I developed radiometric models to simulate the spectral observations of an Earth twin at 10 pc for the HWO using both an IFS and an iFTS. I compared their efficiency across a range of detector and optical system parameters. My simulations show that while an iFTS is not an efficient option in the visible wavelengths due to photon noise limitations, it holds potential in the near-infrared channels. Additionally, my results underscore the need for advancements in reducing detector noise to achieve the goal of spectrally characterizing habitable planets.
THE FRONTIERS OF GALAXY EVOLUTION IN THE NORTH ECLIPTIC POLE: A 10 SQUARE-DEGREE SURVEY TO MAP THE GROWTH OF STELLAR MASS IN GALAXIES ACROSS 10 BILLION YEARS
(University of Hawai'i at Manoa, 2024) Zalesky, Lukas Morse; Sanders, David; Astronomy
This dissertation completed the deepest survey spanning UV/optical to mid-infrared wavelengths over an area of ten square-degrees or more to date. Specifically, this survey targeted the widest and deepest field ever observed by the Spitzer Space Telescope in the northern sky, Euclid Deep Field North (EDF-N), located in the North Ecliptic Pole. Deep ultraviolet imaging (u < 26.2 mag) from the Canada France Hawaii Telescope (CFHT) and deep optical imaging from the Subaru telescope (griz < 27 mag) was acquired over the entire area. These data were combined with similarly deep imaging from a two square-degree region in the southern sky, Euclid Deep Field Fornax. Over six million galaxies were detected from these fields, and their photometry was measured using new tools developed as part of this work designed to self-consistently measure the flux across wide ranges of resolution and wavelength. Their redshifts and stellar masses were measured using two independent modeling codes and validated using an external reference catalog. Finally, these galaxies were used to measure the evolution of the galaxy stellar mass function, the number of galaxies per unit co-moving volume as a function of stellar mass, over a ten billion year time period (0.2 < z < 6.5). The unique volume of this dataset samples over cosmologically representative structure, providing new insight into the growth of stellar mass in galaxies. In addition to identifying the single largest sample of massive galaxies (M >10^10.5 Msun) above z > 3, three main results are obtained:(1) The observed abundance of massive galaxies at 3.5 < z < 6.5, 1-2 billion years after the Big Bang, requires that feedback mechanisms capable of regulating star-formation at late times are inefficient in the early universe; (2) The majority of massive quiescent galaxies are already formed by z ~ 3, two billion years after the Big Bang, and the increase in number density of such galaxies across the next 9 billion years is insubstantial; (3) For the first time, the environment (i.e., local density) is shown to influence the galaxy stellar mass function as early as z ~3.5, with massive galaxies being more frequently embedded within high-density environments.
Planets Made of Star Stuff: The Diversity of Earth-Sized Exoplanet Compositions In Relation To Their Host Stars
(University of Hawai'i at Manoa, 2024) Brinkman-Traverse, Casey Lynn; Huber, Daniel; Astronomy
Planets and their host stars are born from the same primordial cloud of gas and dust. We observe similar ratios of iron to rock-building elements (such as magnesium and silicon) on Earth as we do in Venus, Mars, and the Solar atmosphere—suggesting that rocky planets tend to have similar compositions to their host stars. We observe very different ratios of iron to rock on Mercury, however, demonstrating that planets occasionally undergo processing that alters its primordial composition and differentiates it from its host star. We have discovered roughly 1000 exoplanets similar in size to the Earth, indicating thatterrestrial planets are common in our galaxy. My dissertation addresses two main questions in exoplanetary science: (1) what is the diversity compositions for Earth-sized exoplanets, and (2) what is the relationship between the compositions of these planets and that of their host star? The mass and radius measurements of rocky exoplanets show such a diversity in density that suggest some are composed primarily of iron, while others hardly any. To test the diversity of Earth-sized exoplanet compositions, we collected high-precision Radial Velocity measurements on 10 super-Earth sized planets using Keck/HIRES, Keck/KPF, and Gemini/MAROON-X. We report updated mass measurements for seven of these planets, and confirm the existence of a new super-Earth TOI-1011 b. We then estimated the Core Mass Fraction (CMF) for each planet using updated radius measurements from TESS photometry for two planets (TOI-561 b and TOI-1011 b) and literature values for the rest. We findthat the CMF for all planets (except for TOI-561 b) became more similar to that of the Earth (CMF=0.33) after re characterization. We find that many planets classified as “super-Mercuries" have a lower CMF than previously thought after updating their masses using high-precision RVs (Kepler-100 b and HD 93963 A b), Gaussian Processes regression to account for stellar activity (Kepler-102 d), or homogeneously updating stellar parameters (Kepler-406 b, K2-106 b, K2-229 b, and K2-38 b). We also find that some planets (TOI-561 b and WASP-47 e) are more consistent with having a gaseous envelope of high mean molecular weight species than having a pure rocky composition, demonstrating that not all planets with radii R<1.5R⊕ can be assumed to be rocky. We then place these planet compositions in the context of their host stars. We find that 75% of planets have a CMF consistent with that of their host star to within 1σ, and that the best-fit slope of a linear fit to this relationship is consistent with planets and their host stars having a 1-1 correlation (indicating planets have the same composition of their host star). This is in contrast to previously determined slopes that suggest a much steeper relationship between host star and planet composition. At the present, however, the small sample of rocky planets and the relatively large uncertainties on CMF also place this slope within 2σ of 0 (indicating no strong correlation between star and planet) and future planet discoveries and characterization are needed to demonstrate a statistically significant correlation. Our analysis of rocky planet compositions, in the context of their host star abundances, has the power inform the planet formation and evolution pathways that are needed to produce all of the planets we observe—from iron-rich super-Mercuries to super-Earths with volatile rich envelopes. We have only begun to scratch the surface of detecting and characterizing planets similar in size to the Earth, however. Precise mass measurementsfrom instruments such as KPF, and atmospheric characterization from instruments such as JWST provide a means for us to continue answering these fundamental questions about the nature and origin of Earth-like planets throughout our galaxy.
A Multiwavelength Analysis of the Intrinsic Properties of X-ray Luminous AGN and their Host Galaxies
(University of Hawai'i at Manoa, 2024) Auge, Connor William; Sanders, David; Astronomy
Active galactic nuclei (AGN) represent a key stage in the evolution and growth of super massive black holes (SMBHs). To better understand how galaxies, form, grow, and evolve across cosmic time, it is vital to understand how their SMBHs grow and evolve as AGN. This work presents in depth analysis of the multiwavelngth properties of a large sample of X-ray luminous AGN. Detailed spectral energy distributions (SEDs) from the X-ray to FIR are constructed for 1246 X-ray selected AGN ($L_{0.5-10\rm{keV}}>10^{43}\ts$erg s$^{-1}$), with $z_{\rm{spec}}<1.2$. Data from three fields of differing depth and sky coverage (Stripe 82X, COSMOS, and GOODS-N/S) are utilized in order to identify sources spanning more than three orders of magnitude in intrinsic X-ray luminosities ($L_{0.5-10\rm{keV}} = 10^{43} - 10^{46}$erg s$^{-1}$) in addition to capturing the full range of SED emission features, from heavily obscured to unobscured AGN. Characteristic SED shapes are identified that fully capture the emission features of the entire sample. Each AGN is modeled with the SED fitting software CIGALE to determine intrinsic properties of the AGN and host galaxies such as $L_{\rm{AGN}}$ and the star formation rate. Imaging data of the AGN host galaxies are utilized to determine the galaxy morphology and to identify signs of past galaxy mergers, allowing for a more accurate identification of the AGN evolutionary stage. Finally, the multiwavelength properties for a sample of local luminous infrared galaxies (LIRGs, $L_{\rm{IR}}$ $> 10^{11} L_\odot$) and ultra-luminous infrared galaxies (ULIRGs, $L_{\rm{IR}}$ $> 10^{12}\ts L_\odot$), which have been identified as hosting heavily obscured AGN, are analyzed and directly compared to the high redshift sample. Each piece of this analysis works towards characterizing a potential AGN evolutionary scenario, described by key features in the SEDs.
Hawai‘i Supernova Flows: A Peculiar Velocity Survey Using Over a Thousand Supernovae in the Near-Infrared
(University of Hawai'i at Manoa, 2024) Do, Aaron Joshua Matsuo; Shappee, Benjamin J.; Astronomy
Observational cosmology directly benefits from the advancement of time-domain astronomy, primarily through the massively increased rate of Type Ia supernovae discovered by high-cadence all-sky surveys. I have developed essential infrastructure for coupling numerous streams of data to derive additional scientific utility through synergy. My work consolidates observational, theoretical, and computational elements in a project called Hawai`i Supernova Flows. The project's science goal is to map the structure of matter in the local (z<0.1) Universe. The objectives required to achieve this goal include measuring precise distances to as many Type Ia supernovae as possible, using those values to calculate peculiar velocities, and inferring the distribution of mass needed to produce those velocities. In this work I review the physical mechanisms connecting Type Ia supernovae to the study of the Universe, describe the methodology I employ in Hawai`i Supernova Flows, and present early scientific results.
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.
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.
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.
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.
Rotational Characterization of Tess Stars with Deep Learning
(University of Hawaii at Manoa, 2022) Claytor, Zachary R.; Van Saders, Jennifer L.; Astronomy
Rotation is a fundamental property of stars. The Kepler mission revolutionized the field of stellar rotation, delivering periods of over 50,000 stars near the plane of the Milky Way. The distribution of periods revealed unexpected gaps, dips, and edges that cannot be described by current rotational evolution models, demanding new physical explanations. To sharpen the features in the distribution and to disentangle the effects of star formation history, more measurements of rotation are needed across the entire sky. The TESS mission has the potential to probe stellar rotation in millions of stars across the entire sky, but mission systematics—instrumental noise, observing gaps, and changes in detector sensitivity—have prevented recovery of rotation periods longer than 13.7 days. We used deep learning to see through TESS systematics and recover periods from year-long light curves. Our approach uses a training set of synthesized light curves from realistic star spot evolution simulations, with real light curve systematics from quiet TESS stars. Evaluating the network on real TESS data, we estimated reliable periods for 9,837 cool dwarfs. We recovered key features of the Kepler and K2 distributions, including periods up to 60 days. We reproduced the intermediate rotation period gap for the first time using TESS, as well as a dip in photometric activity surrounding it. Combining our TESS rotation periods with spectroscopic temperatures and abundances from APOGEE, we examined the detectability of rotation across fundamental stellar parameters, finding a strong dependence on temperature and age. Using gyrochronology, we inferred masses, ages, and other fundamental properties for the 6,632 TESS stars with APOGEE spectroscopy and corroborated evolution trends of Galactic chemistry and magnetic activity seen with Kepler. Finally with new measurements of spot filling factor from APOGEE, we investigated the spottedness of stars across the period distribution. We found that stars exhibit elevated spot fractions in the same regime where magnetic braking temporarily stalls in young open cluster stars, suggesting a common cause. Now with the ability to estimate rotation periods, including long periods, across the entire sky, we can characterize stars along many more lines of sight than before, enabling detailed study of the Galaxy's stellar populations.
The Golden Era of Transient Astronomy
(University of Hawaii at Manoa, 2022) Tucker, Michael Andrew; Shappee, Benjamin J.; Astronomy
Astronomy has seen a massive increase in the discovery and classification of astrophysical transients, which are fleeting celestial events such as supernovae, flaring black holes, and tidal disruption events. The proliferation of sky surveys has produced an unprecedented data deluge, transforming how the astronomical community approaches research in the digital era. I have worked on various types of transients and how they shape their environment, or vice versa. The first 4 chapters discuss unique transients which provided special insight into the physical mechanisms governing their formation and evolution. Next, I take an in-depth look at how and why white dwarf stars explode as Type Ia supernovae. Finally, I present an overview of the Spectroscopic Classification of Transients (SCAT) survey. SCAT is a spectrophotometric transient survey designed to study the myriad of transient phenomena discovered by photometric sky surveys. I conclude by summarizing the current status of the survey, describing the data reduction pipeline, and highlighting preliminary results.
The Evolution of Magnetic Field Strengths in Low-mass Young Stars
(University of Hawaii at Manoa, 2022) Flores Gonzalez, Christian Alejandro; Connelley, Michael S.; Astronomy
Magnetic fields have a significant impact on the internal structure and atmospheric properties of low-mass stars. In particular, during the T Tauri phase, magnetic fields play a fundamental role in the star-disk interaction, in the stellar mass accretion process, and in regulating the angular momentum evolution of the system. Although magnetic fields have been measured for some T Tauri stars, little is known about the magnetic fields in protostars and older pre-main sequence sources. In this thesis, I present the hitherto largest and most comprehensive study of surface magnetic field strength of low-mass young stellar sources. I used iSHELL, a high-spectral resolution R~50,000 near-infrared spectrograph at IRTF to observe over 100 young stars in the K-band from different star-forming regions and young associations. Combining high-quality observations with a detailed magnetic radiative transfer code, I derive magnetic field strengths, temperatures, gravities, infrared veiling, and projected rotational velocities for 107 young sources with ages of <0.5 Myr to over 100 Myr and with masses between ~0.3 M_sun and ~1.3 M_sun. In this work, I performed the first survey of magnetic field strength in Class I and Flat Spectrum sources. I found that the magnetic field strength of Class I sources ranges from 0.5 kG to 4.1 kG with a median strength of 1.7 kG. The distribution of magnetic fields for the Class I sources is statistically indistinguishable from the magnetic fields of the Class II sources (or Classical T Tauri stars). Thus, no evolution in magnetic field strength is detected between the two classes. I also found that the gravities of Class I and II sources are statistically different, although a significant overlap exists. When combined with stellar evolutionary models, these results mean that about half of the Class I sources have ages of < 0.6 Myr and are likely in the protostellar phase, while the other half of Class I sources have gravities and ages consistent with Class II sources (or T Tauri stars). In a separate study, I discovered that T Tauri North is not in the same evolutionary stage as most T Tauri stars. Instead, its lower gravity, and thus earlier age <0.6 Myr, suggest that the iconic T Tau N source is a protostar ejected from the embedded southern binary companion shortly after its formation. In a series of studies, I discovered that infrared temperatures of Class II sources are almost always lower than their optical temperatures. Moreover, the observed temperature differences correlate with the magnetic field strengths of the stars and increase for hotter sources. I attribute this phenomenon to magnetically induced spots on the surface of the highly magnetic young stars. Since low-mass young stars contract isothermally as they descend the Hayashi track, an almost one-to-one correlation between temperature and stellar mass can be established. The discovery of an optical-infrared temperature difference necessarily implies that masses derived from optical temperatures are almost always higher than masses derived from infrared observations. By using independent mass measurements for a sub-sample of source, I found that K-band infrared temperatures produce more precise and accurate stellar masses than optical temperatures when combined with magnetic stellar evolutionary models. Analyzing the full sample of 107 young sources, I found that the magnetic field strength of sources less massive than M_star ~ 0.9 M_sun remains strong at a ~2 kG level during the first ~100 Myr. However, stars more massive than M_star ~ 0.9 M_sun often have magnetic field strengths below our detection limit of ~0.3 kG by the age of ~100 Myr. This suggests a change in the magnetic dynamo operating in these stars. Furthermore, by placing the stars in the theoretical HR diagram and overplotting stellar interior models, I found that the development of a radiative core has no effect on the measured magnetic field strength of the stars. The only substantial change in the magnetic field strength of the stars occurs when the convective layers in the stars thin to less than ~35 % in radial depth or below ~10% in stellar mass.
Large Adaptive Optics Survey for Substellar Objects (LASSO) at Wide Separations Around Young, Nearby, Low-Mass Stars
(University of Hawaii at Manoa, 2021) Salama, Maissa; Baranec, Christoph; Liu, Michael C.; Astronomy
The occurrence rates and population trends of massive substellar companions (giant planets and brown dwarfs, $2-70$~M$_{Jup}$) at wide separations ($>$50~AU) from their host star are critical to furthering our understanding of their origins and evolution. However, these objects are very rare and building up a large sample for population-level studies has been challenging. Therefore, large surveys are necessary to search for these objects, which requires technology that can efficiently observe large numbers of stars. One of the two main components of my dissertation was instrumentation development on the laser guide star robotic adaptive optics instrument Robo-AO. I commissioned a new infrared camera that uses a high-speed low-noise SAPHIRA avalanche photodiode detector, allowing us to simultaneously image in the visible and near-IR. I also report on performance optimizations of the instrument. This made possible the second component of my dissertation work, conducting the Large Adaptive optics Survey for Substellar Objects (LASSO) at wide separations around young ($\lesssim$300 Myrs), nearby ($<$100 pc), low-mass ($\approx$0.1--0.8 M$_{\odot}$) stars. Low-mass stars are the most common type of stars and offer the most favorable contrasts for detecting substellar companions. I directly imaged nearly 900 stars, making this survey one of the largest adaptive optics surveys searching for substellar companions to date. I observed 427 of the stars with Robo-AO successively mounted on the Kitt Peak 2.1-m and the Maunakea UH 2.2-m telescopes. With the Subaru/IRCS and Keck/NIRC2 adaptive optics systems, I directly imaged an additional 470 young low-mass stars. The majority of confirmed and pending candidates are stellar companions, with $\sim$5 potentially new substellar companions that will require follow-up observations to confirm. I also report on systems with measured proper motion accelerations from the Hipparcos-Gaia catalog of accelerations. The significance of the acceleration increases with decreasing companion separation and these young accelerating low-mass stars with companions will eventually yield dynamical masses and orbital information. I also report the detection of higher-order stellar multiple systems, which provide important constraints on stellar evolution models. In order to assess the multiplicity statistics of stars across spectral types and populations in a volume-limited sample, I analyzed 1157 stars of all spectral types within 25~pc observed with Robo-AO. I combined Robo-AO detections of close-in companions with a search for wide co-moving companions using Gaia and corrected for unresolved companions to calculate multiplicity fractions across SpTs. These large survey results allow us to better understand the different formation pathways by studying the full spectrum of substellar to stellar companions on a population level.
Tip of the Red Giant Branch Distances to Nearby Galaxies
(University of Hawaii at Manoa, 2021) Anand, Gagandeep Singh; Tully, Richard B.; Astronomy
The tip of the red giant branch (TRGB) is becoming the preferred technique to determine accurate distances to galaxies within the local Universe. The method involves using the brightest red giant branch stars as standard candles, and has now been applied to nearly 500 galaxies within 20 Mpc. Accurate distances are essential for transforming observed properties of galaxies into their physical counterparts (luminosity, physical size, etc.). In addition to their inherent benefits, distances allow us to disentangle the two components of a galaxy’s observed velocity, the component due to the Hubble Flow, and its peculiar velocity. With the latter, we map the patterns of large-scale galaxy flows, and provide insight into the formation and evolution of large-scale structure. In this dissertation, I provide science highlights from the ongoing TRGB component of the Cosmicflows program, and showcase several recent results from this work.

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