Direct Imaging of Stellar and Substellar Companions to Accelerating Stars

Exoplanet astronomy has recently shifted from studies of individual planets using individual detection techniques to a statistical paradigm that combines multiple complementary methods, of which some now overlap in sensitivity. This holistic approach can begin to reveal a more comprehensive view of entire exoplanet populations. For instance, studying where planets tend to cluster in a parameter space of masses and orbital periods may begin to address important questions regarding formation and evolution. In this dissertation, we discuss three projects that attempt to bridge the gap between detection methods, focusing on the interface between Doppler radial velocity (RV) studies, astrometry, and high-contrast imaging. Chapter 1 provides a brief summary of the advantages and sensitivities of these various planet detection techniques; it also describes the current state of exoplanet demographics, and previews the Roman Space Telescope and Habitable World Observatory missions. Chapter 2 discusses the first step in the process of estimating planet yields for upcoming space observatories: generating synthetic planetary systems consistent with observed occurrence rates from multiple detection methods (Dulz et al. 2020}. In an attempt to self-consistently populate stars with orbiting planets, we found that naive extrapolation of occurrence rates (mass, semi-major axis) results in an unrealistically large number-density of Neptune-mass planets beyond the ice-line ($a \gtrsim 5$au), causing dynamic interactions that would destabilize orbits. We impose a stability criterion for multi-planet systems based on mutual Hill radii separation. Considering the influence of compact configurations containing Jovian-mass and Neptune-mass planets results in a marked suppression in the number of terrestrial planets that can exist at large radii. This result has a pronounced impact on planet yield calculations particularly in regions accessible to high-contrast imaging and micro-lensing. The dynamically compact configurations and occurrence rates that we developed were incorporated into imaging yield calculations to place meaningful limits on the number of detectable planets with future missions. Chapter 3 presents initial results of an ongoing observing program to combine astrometric, radial velocity, and high contrast imaging measurements to study the orbits of low-mass companions to accelerating stars. Starting from targets that show astrometric accelerations in the Brandt 2018 HGCA catalog, we have conducted high-contrast imaging of 25 stars. We report the detection and mass estimates of 5 likely binaries and one probable triple star system. While the program found no evidence of planetary mass companions in the systems observed, these systems may serve as benchmark objects for low-stellar-mass companion studies by providing insights into the physics of stellar atmospheres. Chapter 4 builds on the work of Montet et al. 2014, which combined long term radial velocity trends with direct imaging follow-up to search for massive companions of M-dwarf stars, by extending the method to K-dwarf stars. This project makes extensive use of legacy and new Keck HIRES RV data, as well as new imaging data from Keck NIRC2, in an attempt to make initial constraints on the occurrence rate of giant companion to K-stars. While the observation and statistical analysis for this project is ongoing, this chapter explores how archival and future ground-based imaging could place robust limits on planet occurrence rates as a function of stellar mass. In the next few decades, upcoming space observatories like the Roman Space Telescope and Habitable Worlds Observatory have a chance to revolutionize our understanding of life in the universe and will almost certainly change how we view the development of solar systems and planets. This dissertation concludes with a wide-angle look at how careful theoretical modeling, precursor observations and combining all the detection methods available to us will give these missions the best chance to discover new worlds and new interpretations of our own.