Identifying ultra‐cool dwarfs at low Galactic latitudes: a southern candidate catalogue

ABSTRACT We present an ultra‐cool dwarf (UCD) catalogue compiled from low southern Galactic latitudes and mid‐plane, from a cross‐correlation of the Two Micron All‐Sky Survey (2MASS) and the SuperCOSMOS survey. The catalogue contains 246 members identified from 5042 deg2 within 220° ≤ ℓ ≤ 360° and 0° < ℓ ≤ 30°, for |b| ≤ 15°. Sixteen candidates are spectroscopically confirmed in the near‐infrared as UCDs with spectral types from M7.5V to L9, the latest being the unusual blue L dwarf 2MASS J11263991−5003550. Our catalogue selection method is presented enabling UCDs from ∼M8V to the L–T transition to be selected down to a 2MASS limiting magnitude of Ks≃14.5 mag (for S/N ≥ 10). This method does not require candidates to have optical detections for catalogue inclusion. An optimal set of optical/near‐IR and reduced proper‐motion selection criteria have been defined that includes: an RF and IN photometric surface gravity test; a dual RF‐band variability check and an additional photometric classification scheme to selectively limit numbers of potential contaminants in regions of severe overcrowding. We identify four candidates as possible companions to nearby Hipparcos stars – observations are needed to identify these as potential benchmark UCD companions. We also identify 12 UCDs within a possible distance 20 pc, three are previously unknown of which two are estimated within 10 pc, complementing the nearby volume‐limited census of UCDs. An analysis of the catalogue spatial completeness provides estimates for distance completeness over three UCD MJ ranges, while Monte Carlo simulations provide an estimate of catalogue areal completeness at the 75 per cent level. We estimate a UCD space density of ρ(total) = (6.41 ± 3.01) × 10−3 pc−3 over the range of 10.5≤MJ≲14.9, similar to values measured at higher Galactic latitudes (|b|≳10∘) in the field population and obtained from more robust spectroscopically confirmed UCD samples.