# # Program information file # PROGRAM_ID 2021A075 PROGRAM_TITLE COCONUTS: COol Companions ON Ultrawide orbiTS PROGRAM_INV1 Zhoujian Zhang PROGRAM_INV2 Michael Liu PROGRAM_INV3 Eugene Magnier PROGRAM_INV4 William Best PROGRAM_INV5 PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG Giant planets and brown dwarfs on wide orbits [>500 au] are valuable benchmarks to understand the physical processes of ultracool atmospheres, yet such objects are rare and their formation remains uncertain. Constructing a comprehensive catalog of such substellar companions is essential to understand how they formed, since different formation scenarios predict different companion distributions in separation and mass. However, such companion distributions are currently unavailable due to the incomplete census of wide-orbit companions. We are continuing our COol Companions ON Ultrawide orbiTS [COCONUTS] program, to establish a more complete sample of wide-orbit ultracool companions around a volume-limited sample of 300,000 primary stars. We have identified candidate companions using multi-wavelength photometry and multi-epoch astrometry from Gaia DR2, Pan-STARRS1 [PS1], UKIDSS, and WISE. Thus far, our 2018B--2020B observations have discovered 52 new wide-orbit substellar companions, which have more than doubled the current census and represent an order-of-magnitude larger yield than any previous search. We propose to obtain SpeX spectra to measure metallicities and help estimate ages for primary stars of substellar companions identified from COCONUTS, as well as previously known companion host stars that lack dedicated stellar characterization. We will then use these stellar metallicities and ages to systematically examine model atmospheres over a large parameter space and robustly characterize the physical properties of companions. We will also obtain prism spectra for candidates whose companionship were previously unknown but now confirmed by new UKIRT astrometry. Our proposed spectroscopic follow-up will investigate the companion distribution in separation and mass and provide insight into the formation mechanism of wide companions. In addition, studying all these benchmarks with a uniform spectroscopic analysis will improve our understanding of planetary and brown dwarf atmospheres and evolution. PROGRAM_ABSTRACT_END