Program: GN-2019A-Q-318

Giant planets and brown dwarfs at wide orbits (>500 au) serve as valuable benchmarks to understand the physical processes of planetary and brown dwarf atmospheres, yet their formation remains unknown. Constructing a comprehensive catalog of ultracool companions and studying their distribution are perhaps the most promising means to understand the formation of such objects. Based on the Pan-STARRS1 (PS1) 3pi survey, we are performing the largest and deepest search for wide-orbit ultracool companions, targeting 200,000 primary stars within 100 pc selected from Gaia DR2. We have identified an initial high-quality sample of 69 L-dwarf and 9 T-dwarf companion candidates, based on multi-catalog photometry and consistent proper motion with their primary stars. Our L-type candidates are too faint for IRTF/SpeX and too numerous for Gemini/GNIRS, so we propose to confirm their ultracool nature by measuring their water-band absorption with W-band (1.45um) photometry from CFHT/WIRCAM. We also propose to use Gemini/GNIRS for spectroscopic confirmation of our 9 T-dwarf candidates. Should our proposed targets are all confirmed, we will triple the current census of wide-orbit ultracool companions, leading to the first companion distribution function at such wide separations and yielding insight into the formation of these objects. In addition, these companions will provide valuable benchmarks to test atmospheric models, leading to a better understanding of the thermal structure and composition of ultracool atmospheres.