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

Abstract

We present the discovery of 2MASS J11263991-5003550 identified as part of an ongoing survey to discover ultracool 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 substellar 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 layer 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. Also, a spectrophotometric distance of 8.2 pc is estimated, possibly making this object the nearest easily observable single L-T transition object in the Southern hemisphere. © 2007 The Authors. Journal compilation © 2007 RAS.