Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project

The Backyard Worlds: Planet 9 citizen science project aims to exhaustively search the vast WISE/NEOWISE mid-infrared data set for moving objects in the solar neighborhood, especially cold brown dwarfs that are not detectable with Gaia. Since the project’s inception in 2017, Backyard Worlds volunteers have discovered more than 1,700 motion-confirmed L, T and Y type brown dwarf candidates. Among this sample are a subset of ~100 prime Y dwarf candidates: moving objects detected exclusively in the WISE W2 (4.6 micron) band. We present the results of our Spitzer/IRAC follow-up campaigns to obtain photometry of these ~100 Backyard Worlds Y dwarf candidates, and derive phototypes based on Spitzer [3.6]-[4.5] color. Five of our Backyard Worlds discoveries have Y dwarf Spitzer phototypes. Four of these five have exceptionally red Spitzer colors indicating types of Y1 or later, adding appreciably to the small sample of known objects in this especially valuable low-temperature (T < 400 K) regime. Our Backyard Worlds Y dwarfs begin bridging a gap that previously separated the coldest known brown dwarf from the broader Y dwarf population, and therefore represent strong candidates for atmospheric characterization with JWST.