An L-Type Substellar Object in Orion: Reaching the Mass Boundary between Brown Dwarfs and Giant Planets

We present J-band photometry and low-resolution optical spectroscopy(600-1000 nm) for one of the faintest substellar member candidates in the youngsigma Ori cluster, SOri 47 (I=20.53, Bejar et al. 1999). Its very red(I-J)=3.3+/-0.1 color and its optical spectrum allow us to classify SOri 47 asan L1.5-type object which fits the low-luminosity end of the clusterphotometric and spectroscopic sequences. It also displays atmospheric featuresindicative of low gravity such as weak alkaline lines and hydride and oxidebands, consistent with the expectation for a very young object still undergoinggravitational collapse. Our data lead us to conclude that SOri 47 is a truesubstellar member of the sigma Ori cluster. Additionally, we present thedetection of LiI in its atmosphere which provides an independent confirmationof youth and substellarity. Using current theoretical evolutionary tracks andadopting an age interval of 1-5 Myr for the sigma Ori cluster, we estimate themass of SOri 47 at 0.015+/-0.005 Msun, i.e. at the minimum mass for deuteriumburning, which has been proposed as a definition for the boundary between browndwarfs and giant planets. SOri 47 could well be the result of a naturalextension of the process of cloud fragmentation down to the deuterium burningmass limit; a less likely alternative is that it has originated from aprotoplanetary disc around a more massive cluster member and later ejected fromits orbit due to interacting effects within this rather sparse (~12objects/pc^3) young cluster.Comment: 9 pages, 3 figures, accepted for publication in ApJ Letter