Program: GN-2019B-Q-139

Giant planets and brown dwarfs on wide orbits ($\gtrsim 500$~au) are valuable benchmarks to understand the composition and physical process of ultracool atmospheres, yet such objects are rare and their formation remains unknown. Constructing a comprehensive catalog of ultracool companions will significantly benefit the characterization of planetary and substellar atmospheres. Also, statistically studying the companion distributions is essential to understand how they formed, given that different formation scenarios predict vastly different companion distributions in separation and mass. However, such companion distributions are currently unavailable due to the incomplete census of wide-orbit ultracool companions. To establish a more complete sample, we are performing the largest and deepest search for wide-orbit ultracool companions, targeting 300,000 primary stars within $100$~pc selected from Gaia DR2. Based on multi-wavelength photometry and multi-epoch astrometry, we have used the Pan-STARRS1 (PS1), AllWISE, and UKIDSS surveys to identify candidate ultracool companions. (1)~We propose to use UKIRT/WFCAM to refine the proper motions of candidates that currently have too low signal-to-noise proper motions for robust companionship assessment. (2)~For confirmed co-moving candidates that are too faint for spectroscopy or likely L-type brown dwarfs, we propose to use CFHT/WIRCAM to obtain narrow-band photometry to measure the water and methane absorption of their atmospheres, to verify their ultracool nature. (3)~We propose to use Gemini/GNIRS to obtain near-infrared spectra for the co-moving candidates that are likely giant planets or T-type brown dwarfs to derive their spectral types and perform atmospheric modeling. Our proposed observations will discover new companions, derive their physical properties, investigate the companion distribution in separation and mass, and gain insight into the formation mechanism of wide companions. Our work is the first volume-limited survey of brown dwarfs and giant planets with orbital separations beyond $500$~au and will establish a robust, well-defined sample of wide ultracool companions. In addition, studying such an ensemble of wide companions with a uniform spectroscopic analysis will help provide unbiased observational constraints on planetary and brown dwarf atmospheres and evolution.