A Census of Distant, Giant Exoplanets

Abstract

Hundreds of planets have recently been discovered around other stars,

revealing a startling diversity of planetary systems. However, these

exoplanets lie almost exclusively within a few astronomical units (AU)

of their host stars. A full picture of planet formation also requires

a census and statistical analysis of planets at wide separations.

This thesis uses the SEEDS direct imaging survey, together with

archival data, to search for massive, >~5 M_J

companions tens of AU from their host stars. These objects are not

sufficiently massive to fuse hydrogen, and simply cool and fade as

they radiate away their heat of formation. As a result, SEEDS targets

young, nearby stars using HiCIAO, a high-contrast infrared camera on

the Subaru telescope. I first present an analysis of HiCIAO, deriving

the distortion correction needed to reduce high-contrast data, and

optimizing HiCIAO's entrance pupil to vastly improve its performance.

I then describe ACORNS-ADI, software I have written to reduce HiCIAO

data. This software includes several new algorithms that both improve

its performance and efficiently compute each observation's

sensitivity. I use ACORNS-ADI to uniformly reduce data from the SEEDS

survey, including images of members of young moving groups and of debris

disk hosts. The ages of these stars, together with substellar cooling

models, are needed to convert our sensitivities from luminosities to

masses. I therefore present a uniform Bayesian analysis of all

targets, deriving a posterior age distribution for each using both

proposed moving group membership and observed stellar activity.

Finally, I combine the published SEEDS results with additional

archival imaging to assemble a diverse sample of nearly 200 stars. A

statistical analysis of five brown dwarfs and massive exoplanets

discovered by HiCIAO provides a limit of ~50--200 AU, depending

on the models used, beyond which the distribution of exoplanets at

small separations cannot extend. By treating massive planets and

brown dwarfs together, I further suggest that currently known

long-period exoplanets are not an extension of the short-period

planetary distribution. Instead, they may represent a low-mass

extension of the brown dwarf distribution function, formed by

gravitational fragmentation in a cloud or circumstellar disk.