The discovery of a very cool binary system
  We report the discovery of a very cool d/sdL7+T7.5p common proper motion
binary system, SDSS J1416+13AB, found by cross-matching the UKIDSS Large Area
Survey Data Release 5 against the Sloan Digital Sky Survey Data Release 7. The
d/sdL7 is blue in J-H and H-K and has other features suggestive of
low-metallicity and/or high gravity. The T7.5p displays spectral peculiarity
seen before in earlier type dwarfs discovered in UKIDSS LAS DR4, and referred
to as CH4-J-early peculiarity. We suggest that CH4-J-early peculiarity arises
from low-metallicity and/or high-gravity, and speculate as to its use for
classifying T dwarfs. UKIDSS and follow-up UKIRT/WFCAM photometry shows the T
dwarf to have the bluest near infrared colours yet seen for such an object with
H-K = -1.31+/-0.17. Warm Spitzer IRAC photometry shows the T dwarf to have
extremely red H-[4.5] = 4.86+/-0.04, which is the reddest yet seen for a
substellar object. The lack of parallax measurement for the pair limits our
ability to estimate parameters for the system. However, applying a conservative
distance estimate of 5-15 pc suggests a projected separation in range 45-135
AU. By comparing H-K:H-[4.5] colours of the T dwarf to spectral models we
estimate that Teff = 500 K and [M/H]~-0.30, with log g ~ 5.0. This suggests a
mass of ~30 MJupiter for the T dwarf and an age of ~10 Gyr for the system. The
primary would then be a 75MJupiter object with log g ~ 5.5 and a relatively
dust-free Teff ~ 1500K atmosphere. Given the unusual properties of the system
we caution that these estimates are uncertain. We eagerly await parallax
measurements and high-resolution imaging which will constrain the parameters
further.
