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

TL;DR

This paper reports on the discovery and detailed characterization of an unusually cold and fast-moving brown dwarf, WISEA J153429.75-104303.3, which may be the first Y subdwarf, challenging existing classifications.

Contribution

It provides new imaging, parallax, and astrometric data that reveal the object's unique properties and suggests it is an old, metal-poor Y subdwarf, expanding understanding of substellar populations.

Findings

Object is one of the faintest proper motion sources with J-ch2 > 8 mag.

Distance measured at approximately 16.3 parsecs.

Object has high transverse velocity of about 207 km/s.

Abstract

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.