Discovery and Characterization of Giant Planets and Brown Dwarfs on Wide Orbits

Abstract

Giant planets and brown dwarfs that are wide-orbit (>500 au) companions to stars are excellent laboratories to study self-luminous ultracool atmospheres, yet such objects are rare and the current companion census is incomplete and heterogeneous. To establish a more complete sample, I have conducted a volume-limited survey for wide-orbit companions (COol Companions ON Ultrawide orbiTS; COCONUTS), targeting 300,000 primary stars within 100 pc from Gaia DR2. Mining astrometry and photometry from Pan-STARRS1 and AllWISE, I have identified candidates that are co-moving with their primary stars and have photometry expected for ultracool dwarfs. I have then confirmed new discoveries using ground-based astrometric, photometric, and spectroscopic follow-up, resulting in a high success rate (85%) in identifying brown dwarf companions. Thus far, COCONUTS has more than doubled the current census of wide-orbit substellar companions (13-70 M_Jup). Characterizing the physical properties of wide-orbit companions is commonly performed by comparing their spectra with model atmospheres, but these models rely on assumptions that are unavoidably simplified. To test state-of-the-art models, I have constructed a Bayesian forward-modeling framework and applied it to a large ensemble of near-infrared spectra for free-floating planets and brown dwarfs with cloudless atmospheres (55 T7-T9 dwarfs). Our data-driven analysis has found systematic offsets between data and models and studied such data-model inconsistencies as a function of wavelengths and physical parameters to identify specific shortcomings of model predictions. This work represents the first systematic examination of cloudless model atmospheres and has broad implications for the more complex models needed to interpret the majority of imaged exoplanets and brown dwarfs known to date. Combining our validated model atmospheres with the observed spectra of COCONUTS discoveries, we are producing the largest sample of wide-orbit ultracool companions with uniform and well-characterized physical properties. These objects will enhance our knowledge of the atmospheres on imaged exoplanets and brown dwarfs and also help us understand these objects' formation and evolution based on their demographics.