Program: GN-2021A-Q-203

It took astronomers 21 years to discover 20 ultracool white dwarfs with temperatures below 4000 K. Some of these stars are the coolest and oldest white dwarfs currently known. We have recently presented a detailed model atmosphere analysis of the 100 pc white dwarf sample in the SDSS footprint, and demonstrated for the first time the existence of a well defined ultracool white dwarf sequence in color and magnitude. This enabled us to expand our selection to the entire Pan-STARRS 3pi survey footprint and identify 36 new ultracool white dwarf candidates based on colors and Gaia parallaxes. Here we propose to obtain low-resolution follow-up spectroscopy of 21 of these candidates visible in the spring to confirm their nature. Once confirmed, our model atmosphere analysis will provide temperatures, masses, and ages for these objects. We are in a unique position to make significant progress in this field through the proposed observations.