Discovery of a nearby L-T transition object in the Southern Galactic plane

We present the discovery of 2MASS J11263991-5003550identified as part of an ongoing survey to discover ultra-cool dwarfs in the Southern Galactic Plane, using data from the 2MASS and SuperCOSMOS Sky Surveys. Strong FeH and H2O features in the near-infrared JH-band spectrum reveal characteristics seen in both mid-L, and L-T transition type dwarfs. We suggest these may be attributable to holes in the condensate cloud layers in the atmosphere of a single sub-stellar object, but cannot at present completely rule out the role of binarity as the possible cause. We also identify this object as a blue L dwarf, and explore the similar observable characteristics of these objects with those of the L-T transition. From this comparison we suggest that the temperature (and thus spectral type) at which the condensate cloud later begins to break-up/rain out, may be highly sensitive to small variations in metallicity. However, the JH-band spectrum of this object does not resemble that of the known L subdwarfs, and therefore extreme metal deficiency may not in fact be responsible for the discordant features. We estimate a spectral type of L9±1, and measure a large proper-motion of µ(tot)=1. ′′ 65 ± 0. 03 yr −1 . Also, a spectrophotometric distance of 8.2pc is estimated, possibly making this object the nearest easily observable single L-T transition object in the southern hemisphere.