Dynamical Architecture of Transiting Planets with Outer Companions and Fourier Transform Spectroscopy for Directly Imaged Exoplanets
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2024
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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.
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Astronomy, exoplanets
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367 pages
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