A new benchmark T8-9 brown dwarf and a couple of new mid-T dwarfs from the UKIDSS DR5+ LAS

TL;DR

This paper reports the discovery of a new T8-9 brown dwarf, Ross 458C, in a hierarchical system, providing valuable data for understanding the properties and evolution of very low-mass brown dwarfs.

Contribution

The study introduces a new benchmark T8-9 brown dwarf, Ross 458C, and details its properties, including its association with a known star, aiding future atmospheric and evolutionary modeling.

Findings

Discovery of a new T8-9 brown dwarf, Ross 458C.

Ross 458C is part of a hierarchical system with a star.

Ross 458C is a promising target for brown dwarf studies.

Abstract

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.