Resolved Spectroscopy of a Brown Dwarf Binary at the T Dwarf/Y Dwarf Transition

TL;DR

This study presents high-resolution imaging and spectroscopy of a brown dwarf binary near the T/Y dwarf transition, revealing orbital motion, spectral classifications, and ammonia absorption features that inform substellar classification boundaries.

Contribution

First resolved near-infrared imaging and spectroscopic analysis of a T8.5 binary near the T/Y dwarf boundary, including detection of ammonia absorption and orbital motion evidence.

Findings

Confirmed common proper and radial motion of the binary

Detected ammonia absorption in the T9.5 component

Classified components as T8.5 and T9.5, near the T/Y boundary

Abstract

We report resolved near-infrared imaging and spectroscopic observations of the T8.5 binary WISEP J045853.90+643452.6AB obtained with Keck/NIRC2, Keck/OSIRIS and the Keck Laser Guide Star Adaptive Optics system. These data confirm common proper and radial motion for the two components, and we see the first indications of orbital motion (mostly radial) for this system. H-band spectroscopy identifies both components as very late-type brown dwarfs with strong H2O and CH4 absorption. The spectrum of WISE J0458+6434B also exhibits a compelling signature of NH3 absorption over 1.52-1.54 micron when compared to the T9 dwarf UGPS J072227.51-054031.2. Comparison to T8-Y0 spectral standards and H-band spectral indices indicate classifications of T8.5 and T9.5 for these two components, approaching the boundary between the T dwarf and Y dwarf spectral classes.