  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. New imaging data
from Keck/MOSFIRE and HST/WFC3 show that this object 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 $16.3^{+1.4}_{-1.2}$ pc and a large
transverse velocity of $207.4{\pm}15.9$ km/s. The absolute J, W2, and ch2
magnitudes are in line with the coldest 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.
