Abstract
Over the past decade, a growing population of planetary-mass companions (<20 M_Jup; PMCs) orbiting young stars have been discovered. They are located at wide separations (>100 au) from their young host stars, challenging models of star and planet formation. It is unclear whether these systems represent the low-mass extreme of stellar binary formation, or the high-mass and wide-orbit extreme of planet formation theories, as various formation pathways inadequately explain their physical and orbital aspects. Determining which scenario best reproduces the observed characteristics of PMCs will come once a statistically robust sample of directly imaged planets are found and studied. PMC systems also provide an opportunity to witness planet assembly, thus characterizing the spectral energy distributions of PMCs will help with future interpretation of exoplanet observations.
The extensive Spitzer/IRAC data set of nearby young populations has great potential to be mined for wide companions to stars. For my thesis, I developed an automated pipeline to find faint PMCs via point spread function (PSF) subtraction in existing Spitzer/IRAC images. I identified candidates for further study and pursued follow-up observations of candidate companion systems as I endeavored to leverage the wealth of Spitzer images to find undiscovered companions.
I discovered two wide-orbit substellar companion systems and described characterization efforts of them. I measured the mid-infrared photometry of 16 wide-orbit companions, compared them to brown dwarfs in star-forming regions and the field, and determined the global disk frequency of young (<15 Myr) wide companions with low masses to be high (56%±12%). I determined that my PSF- subtraction infrastructure is sensitive to ∼2 M_Jup companions at ρ>300 au. I also expanded my search for companions to the Taurus star-forming region, constraining the frequency of 0.5–30 MJup companions on semi-major axes 50–5000 au to <3.7% at a 95% confidence level.
My thesis has set the stage to reveal the demographics of wide-orbit PMCs from which better constraints on the models of extreme binary star and planet formation will emerge, ultimately enhancing our understanding of where these systems come from, how they evolve, and where they fit into the paradigm of star and planet formation.
