The Enigmatic Brown Dwarf WISEA J153429.75−104303.3 (aka "The Accident")

Continued follow-up of WISEA J153429.75−104303.3, announced in Meisner et al. (2020), has proven it to have an unusual set of properties. It is one of the few faint proper motion sources known with J−ch2 > 8 mag, indicating a very cold temperature consistent with the latest known Y dwarfs. Despite this, it has W1−W2 and ch1−ch2 colors ∼1.6 mag bluer than a typical Y dwarf. A new trigonometric parallax measurement from a combination of WISE, Spitzer, and HST astrometry confirms a nearby distance of 16.3+/-1.4 pc and a large transverse velocity of 207.4+/-15.9 km s known Y dwarfs, despite the highly discrepant W1−W2 and ch1−ch2 colors. We explore possible reasons for the unique traits of this object and conclude that it is most likely an old, metal-poor brown dwarf and possibly the first Y subdwarf. Given that the object has an HST F110W magnitude of 24.7 mag, broad-band spectroscopy and photometry from JWST are the best options for testing this hypothesis.