Benchmark brown dwarfs are those objects for which fiducial constraints are
available, including effective temperature, parallax, age, metallicity. We
searched for new cool brown dwarfs in 186 sq.deg. of the new area covered by
the data release DR5+ of the UKIDSS Large Area Survey. Follow-up optical and
near-infrared broad-band photometry, and methane imaging of four promising
candidates, revealed three objects with distinct methane absorption, typical of
mid- to late-T dwarfs, and one possibly T4 dwarf. The latest-type object,
classified as T8-9, shares its large proper motion with Ross 458 (BD+13o2618),
an active M0.5 binary which is 102" away, forming a hierarchical low-mass
star+brown dwarf system. Ross 458C has an absolute J-band magnitude of 16.4,
and seems overluminous, particularly in the K band, compared to similar field
brown dwarfs. We estimate the age of the system to be less than 1 Gyr, and its
mass to be as low as 14 Jupiter masses for the age of 1 Gyr. At 11.4 pc, this
new late T benchmark dwarf is a promising target to constrain the evolutionary
and atmospheric models of very low-mass brown dwarfs. We present proper motion
measurements for our targets and for 13 known brown dwarfs. Two brown dwarfs
have velocities typical of the thick disk and may be old brown dwarfs.Comment: 15 pages, 10 figures and 6 tables. Accepted by MNRAS. Uses mn2e.cls
and aas_macr