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Dissertation/Thesis Abstract

Where Do We Find Exoplanets?
by Wise, Alexander, Ph.D., University of Delaware, 2019, 125; 22583650
Abstract (Summary)

With over 4000 planets discovered outside of the Solar System, exoplanet scientists are working both to better understand the exoplanets we have found and to discover exoplanets unlike any we have found. The mass-semimajor axis distribution of exoplanets summarizes our progress in detecting exoplanets and characterizing two of their most essential properties. In this thesis, I introduce two projects: a study of the physics behind the semimajor axis distribution of high-mass exoplanets including theory and numerical simulation, and a project devoted to improving our ability to detect earth-mass planets in 1 AU orbits around sun-like stars, filling in the space in the mass-semimajor axis distribution where Earth would go.

In the first project we investigate whether photoevaporation, which opens a gap in the inner few AU of gaseous disks before dissipating them, can significantly affect the final distribution of giant planets by modifying gas surface density and hence Type II migration rates near the photoevaporation gap. We first use an analytic disk model to demonstrate that newly-formed giant planets have a long migration epoch before photoevaporation can significantly alter their migration rates. Next we present new 2-D hydrodynamic simulations of planets migrating in photoevaporating disks, each paired with a control simulation of migration in an otherwise identical disk without photoevaporation. We show that in disks with surface densities near the minimum threshold for forming giant planets, photoevaporation alters the final semimajor axis of a migrating gas giant by at most 5% over the course of 0.1 Myr of migration. Once the disk mass is low enough for photoevaporation to carve a sharp gap, migration has almost completely stalled due to the low surface density of gas at the Lindblad resonances. We find that photoevaporation modifies migration rates so little that it is unlikely to leave a significant signature on the distribution of giant exoplanets. Finally, we argue that there is not sufficient observational evidence to say that a pileup exists in the distribution of confirmed giant exoplanets.

In the second project we show how new stellar activity indicators can be identified in high resolution spectra by correlating spectral line depths with a well-known activity index. We apply our correlation methods to archival HARPS spectra of ε Eri and ⍺ Cen B and use the results from both stars to generate a master list of activity-sensitive lines whose core fluxes are periodic at the star's rotation period. Our newly discovered activity indicators can in turn be used as benchmarks to extend the list of known activity-sensitive lines toward the infrared or UV. With recent improvements in spectrograph illumination stabilization, wavelength calibration, and telluric correction, stellar activity is now the biggest noise source in planet searches. Our suite of > 40 activity-sensitive lines is a first step toward allowing planet hunters to access all the information about spots, plages, and activity cycles contained in each spectrum.

Indexing (document details)
Advisor: Dodson-Robinson, Sarah E
Commitee: DeCamp, Matthew F, Fischer, Debra A, Petit, Veronique
School: University of Delaware
Department: Physics and Astronomy
School Location: United States -- Delaware
Source: DAI-B 81/4(E), Dissertation Abstracts International
Subjects: Astrophysics
Keywords: hydrodynamics, planet-disk interactions, protoplanetary disks, radial velocity, starspots, stellar activity
Publication Number: 22583650
ISBN: 9781687973726
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