Date of Award

8-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Committee Chair/Advisor

Jeffrey Fung

Committee Member

Sean Brittain

Committee Member

Jonathon Zrake

Committee Member

Xian Lu

Abstract

Observing and studying planets around young stars during their initial formation stages is essential for understanding the physics of their formation. Protoplanetary disks are the host sites of planets, and they exhibit substructures that hint towards the planets' presence, but the planet itself has rarely been detected. Analysis of the substructures in protoplanetary disks, especially towards the inner regions where most planets seem to lie, will provide information that can be used to refine theories about planet-disk interactions and, ultimately, planet formation. To probe the high-velocity inner regions, this study utilizes high-resolution spectroscopy, which has increased sensitivity with increasing velocities. This study focuses on the protoplanetary disks surrounding two young stars, CI Tau and AB Aurigae, that display substructures and host independently verified companions. As such, connecting the disk substructures to the planets inhabiting the disks of these systems is vital for the ongoing search for planets.

The disk around CI Tau has an undepleted inner region, determined from the infrared excess in its spectral energy distribution and it also hosts an eccentric massive companion, deduced from a radial-velocity survey. As such, observations of this disk can provide first-of-its-kind results of how planets can influence their natal disks. Theories exist describing such a planet-disk relationship, but it is unclear how accurate they are as very few planets have been detected while still deeply embedded in a disk. For example, due to the planet's influence, the region of the disk near the planet's immediate vicinity is expected to develop an eccentricity comparable to that of the planet's orbit. My analysis of CI Tau revealed that its inner disk, where the planet resides, has an eccentricity of $\approx$0.05, which was much smaller than the eccentricity attributed to the proposed planet at the time. As such, we concluded that the planet's eccentricity must be smaller. Another feature captured in CI Tau's inner disk was that the companion bisects it. Without forming a deep or detectable gap, the component of the disk that is interior to the planet's orbit is oppositely oriented to the exterior component. The location where the bisection occurs was $\approx$0.14~au, which is smaller than the planet's orbit size --- hinting that it may also be in a different location. The multi-component disk I observed from CI Tau was the first time such a feature was captured, and it can serve as the foundation of planet-disk interaction theory moving forward.

AB Aurigae's disk has been imaged numerous times and a multitude of substructures have been identified, such as rings/gaps, vortices, spirals, etc. The most famous substructure is the protoplanet candidate AB Aur b, a bright, concentrated emission source on a wide orbit. By targeting this star, we can understand the relationship between the luminosity of a forming planet with respect to that of the background disk. Observations were done for multiple position angles to capture varying emissions from the protoplanet. Doing this can allow us to place limits on the forming planet's luminosity --- an unknown quantity. Spectroscopic emission of CO directly from AB Aur b was not captured even though it has been a consistently imaged feature in multiple wavelengths. This non-detection of AB Aur b in the CO infrared emissions either indicates it is a rather cold feature or lacks a concentration of CO; both conclusions are essential to incorporate in planet formation models. Also, the non-detection can directly inform the theorized formation mechanisms based on their implied 'hot-start' or 'cold-start' scenarios.

Since this study targeted systems that have been independently verified to host protoplanets, our observations were expected to reinforce those discoveries. Instead, for both systems, our analysis hints at the presence of different planets. For example, instead of modifying the orbital parameters of CI Tau's previously known companion, our results suggest a new $\approx$1~M$_{\rm J}$ planet on a low $\sim$0.05 eccentricity orbit around 0.14~au. For AB Aurigae, instead of detecting AB Aur b, our results point to a different planet located about 65~au southwest of the star, which are distinctly different coordinates from AB Aur b's. Even though our detections are not definitive, our analysis techniques can be used in the future as an additional tool for verifying protoplanet discoveries.

Our high-resolution spectroscopic analysis of CI Tau and AB Auriage serves as foundational calibrations for theories relating to planet-disk interactions and planet formation. Said theories have been developing for decades but have had minimal observational backing as very few planets have been detected while forming in a protoplanetary disk. Our results here provide some of the first data points that can be incorporated into theoretical models to help refine them. When refined, these models can better capture the physics around young planetary systems and, ultimately, help answer the question of 'How do planets form?'

Author ORCID Identifier

0009-0002-7515-2925

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