Program: GN-2019A-Q-317

The shape of the substellar mass function and its minimum mass can provide fundamental tests of theories of star formation. The Upper Sco OB association contains the largest stellar population at ages of <10 Myr and distances of <150 pc (~2000 stars). As a result, it offers the best available statistical constraints on the IMF of brown dwarfs. The recent data releases from Gaia, Pan-STARRS1, and the VISTA Hemisphere Survey combined with previous data from 2MASS, UKIDSS and WISE have now made it possible to perform a thorough survey for brown dwarfs in Upper Sco down to 0.005-0.01 solar masses (5-10 Jupiter masses). We have used parallaxes, proper motions, and color-magnitude diagrams measured with those databases to identify candidate brown dwarfs in Upper Sco. In 2018A, we were awarded time with GNIRS for spectroscopy of 70 candidates at K>14 (<0.04 solar masses) to confirm their membership. 29 of those targets were observed, 23 of which were confirmed as members. We propose to obtain spectra of the remaining candidates, as well as 4 additional candidates identified since the 2018A proposal. When combined with spectroscopy on other telescopes that we are proposing for brighter candidates at higher masses, the proposed GNIRS observations will provide a census of substellar members of Upper Sco that has completeness/detection limits of ~0.01/0.005 solar masses. Many of these targets are bright enough to be observed during poor conditions (IQ=85/CC=70).