The Population of Exoplanets as Sculpted by their Host Stars

Author: ORCID icon
Wilson, Robert, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Majewski, Steven, AS-Astronomy, University of Virginia

Over the last decade, exoplanet studies have experienced a revolution. This boom is in part due to large-scale photometric surveys such as the Kepler mission, which to date has discovered nearly 5000 credible exoplanet candidates. This vast dataset has stimulated exoplanet science to advance beyond the stage of discovery and characterization of single systems, to population studies of exoplanets within our Solar neighborhood. Evolving in a similar timeframe over the last decade are large-scale spectroscopic surveys, which provide means for efficient characterization of stars found by Kepler to host planets. With the combination of planet-finding missions and host star spectroscopic surveys, we are able to characterize more carefully and precisely the properties of planet-hosting stars, which in turn gives us a more detailed understanding of the planets themselves.

In this dissertation, we combine information about planet hosting stars as gleaned from the SDSS/APOGEE surveys and Kepler surveys to uncover the detailed and intertwined links that planets exhibit with the stars that they orbit. We first report on a relationship that planets show between their host star's metallicity and the planets orbital period/semi-major axis known as the period-metallicity relation. We provide a detailed characterization of this relationship, showing that there is a lack of hot planets in stars with low metallicities, and provide evidence that the population of planets found at distances less than 0.07 au from their host stars are distinctly different from those beyond that limit. We interpret this transition region as evidence for an inner protoplanetary disk boundary that is controlled, in part, by stellar metallicity.

We then extend the impact of stellar chemistry beyond that of just planet period and stellar metallicity, by examining the relationships between the planet occurrence rate (the average number of planets per star) and the enhancement of ten individual chemical elements (C, Mg, Al, Si, S, K, Ca, Mn, Fe, and Ni). We measure the planet occurrence rate density with orbital period and planet radius, and for the first time ever incorporate a detailed chemical understanding of planet host stars to track changes in the planet population due to chemistry. We find that the enhancement of all elements contributes equally to the enhancement of the planet occurrence rate, with stronger correlations given for planets with shorter orbital distances. This strong correlation presents a challenge in some sense, as it is the source of a strong confounding variable when considering changes in the planetary distribution function with stellar properties such as age and mass. After demonstrating how such a degeneracy can arise, we motivate the need for systematic, homogeneous, high-resolution spectroscopic surveys to properly characterize planet host stars.

In addition to large-scale spectroscopic surveys, the exoplanet field has also seen a large boom with the addition of Gaia, an astrometric survey which precisely measures distances to all the stars within the Solar neighborhood, allowing us to determine fundamental stellar properties for all of the stars observed by Kepler. To this end, we derive fundamental stellar properties for about 163,000 stars in the Kepler field using a homogeneous spectroscopic and photometric metallicity scale, enabling us to break degeneracies in mass and age. Motivated by the higher than expected fraction of evolved stars in the Kepler field and the difficulty in estimating occurrence rates for evolved stars due to an inadequate grid of transit templates searched, we develop a new transit detection algorithm, TraSH-DUMP (TRAnsiting planets with Subgiant Hosts -- Detection with an Unbiased Matched filter Pipeline). TraSH-DUMP utilizes precisely known stellar parameters to optimize a transit search for evolved stars, a host star target class not well searched by the original Kepler software pipeline. We describe the methodology behind TraSH-DUMP in detail and compare it to the current state of the art detection pipelines, demonstrating its competitive sensitivity. The combination of TraSH-DUMP and the catalog of precise stellar properties derived set the foundations for a detailed study of planet demographics and the evolution of planetary systems with age.

PHD (Doctor of Philosophy)
Astronomy, Exoplanets, Exoplanet Demographics, Stellar Characterization, Exoplanet Detection
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