|dc.description.abstract||I have used blue near-infrared colours to select a group of UKIDSS T dwarfs with spectral
types later than T4. From amongst these I identify two kinematic halo T-dwarf candi-
dates. Blue near-infrared colours have been attributed to collisionally-induced hydrogen
absorption, which is enhanced by either high surface gravity or low metallicity. Proper
motions are measured and distances estimated, allowing the determination of tangential
velocities. U and V components are estimated for our objects by assuming Vrad = 0.
From this, ULAS J0926+0835 is found to have U = 62 kms−1 and V = −140 kms−1 and
ULAS J1319+1209 is found to have U = 192 kms−1 and V = −92 kms−1. These values are
consistent with potential halo membership. However, surprisingly, these are not the bluest
objects in the selection. The bluest is ULAS J1233+1219, with J −K = −1.16±0.07, and
surprisingly this object is found to have thin disc-like U and V . Our sample also contains
Hip 73786B, which I find to be a companion to the star Hip 73786. Hip 73786 is a metal-
poor star, with [Fe/H]= −0.3 ± 0.1 and is located at a distance of 19±0.7 pc. U, V,W
space velocity components are calculated for Hip 73786A and B, finding that U = −48±7
kms−1, V = −75 ± 4 kms−1 and W = −44 ± 8 kms−1. From the properties of the pri-
mary, Hip 73786B is found to be at least 1.6Gyr old. As a metal poor object, Hip 73786B
represents an important addition to the sample of known T dwarf benchmarks.
Using mid-infrared data from WISE, I also identify T dwarfs with abnormally-red
H − W2 and consider possible causes for their extreme colours. In particular I exam-
ine three prominent examples of this phenomenon, ULAS J1416+1348B, 2MASS J0939-
2448 and BD+01o 2920B. A plot of spectral type against MW2-magnitude suggests that
ULAS J1416+1348B is potentially an unresolved binary, similar to 2MASS J0939-2448.
However, the plot also indicates that BD+01o 2920B is not an unresolved binary. I also
present new FIRE spectroscopy for ULAS J1416+1348B and 2MASS J0939-2448. These
data show that ULAS J1416+1348B has a similar shape to the Y -band spectrum to that of BD+01o 2920B, thus suggesting that the two objects have a similar metallicity, whereas
2MASS J0939-2448 appears to be a more metal-rich object. Using a new parallactic dis-
tance, I derive a luminosity of (6.9±0.7)×1020W for ULAS J1416+1348B. I also find a
radial velocity of −39 ± 1 kms−1 for this object. The agreement between this and that of
the L dwarf SDSS J1416+1348A confirms that these two objects are physically-associated.
I also present a set of simulated unresolved binaries; the colours of these systems do not
appear to redden significantly with the addition of cooler companions. From this, I suggest
that the colours of ULAS J1416+1348B and BD+01o 2920B cannot be solely attributed
to any possible unresolved companions; for these two objects, composition and/or surface
gravity must be playing a substantial role. Consideration of model predictions provides
extra evidence for this argument, showing as it does that high log g and low metallicity
can redden H − W2 colours by as much as »0.5mag as compared to a high-metallicity
and low log g object of the same effective temperature.
I also present kinematics and photometry for several more new candidate low-metallicity
T dwarfs. Spectra are also presented, where available. In addition I provide new follow-up
JHK spectroscopy for ULAS J0926+0835, ULAS J1233+1219 and ULAS J1319+1209.
These new spectra allow full JHK-based spectral typing for these objects.||en_US