An L-type substellar object in Orion: reaching the mass boundary between brown dwarfs and giant planets

Abstract

We present J-band photometry and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young sigma 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 as an L1.5-type object which fits the low-luminosity end of the cluster photometric and spectroscopic sequences. It also displays atmospheric features indicative of low gravity such as weak alkaline lines and hydride and oxide bands, consistent with the expectation for a very young object still undergoing gravitational collapse. Our data lead us to conclude that SOri 47 is a true substellar member of the sigma Ori cluster. Additionally, we present the detection of LiI in its atmosphere which provides an independent confirmation of youth and substellarity. Using current theoretical evolutionary tracks and adopting an age interval of 1-5 Myr for the sigma Ori cluster, we estimate the mass of SOri 47 at 0.015+/-0.005 Msun, i.e. at the minimum mass for deuterium burning, which has been proposed as a definition for the boundary between brown dwarfs and giant planets. SOri 47 could well be the result of a natural extension of the process of cloud fragmentation down to the deuterium burning mass limit; a less likely alternative is that it has originated from a protoplanetary disc around a more massive cluster member and later ejected from its orbit due to interacting effects within this rather sparse (~12 objects/pc^3) young cluster.

9 pages, 3 figures, accepted for publication in ApJ Letters

References in corpus (3)

• L Dwarfs and the Substellar Mass Function
• A Search for Very Low-mass Stars and Brown Dwarfs in the Young sigma Orionis Cluster
• A 2MASS Survey for Brown Dwarfs Towards the Hyades

Cited by in corpus (26)

• The Formation of a Star Cluster: Predicting the Properties of Stars and Brown Dwarfs
• Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages
• Theory of Low-Mass Stars and Substellar Objects
• New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence
• The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification
• The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters
• Infrared Spectroscopy of Substellar Objects in Orion
• Lithium and Halpha in stars and brown dwarfs of sigma Orionis
• Identifying Young Brown Dwarfs Using Gravity-Sensitive Spectral Features
• The Substellar Mass Function in sigma Orionis
• New isolated planetary mass objects and the stellar and substellar mass function of the sigma Orionis cluster
• Photometry and Proper Motions of M, L, and T Dwarfs from the Pan-STARRS1 3$\pi$ Survey
• A Methane Isolated Planetary Mass Object in Orion
• The substellar mass function in sigma Orionis. II. Optical, near-infrared and IRAC/Spitzer photometry of young cluster brown dwarfs and planetary-mass objects
• Initial Results from the Palomar Adaptive Optics Survey of Young Solar-Type Stars: a Brown Dwarf and Three Stellar Companions
• Optical spectroscopy of isolated planetary mass objects in the sigma Orionis cluster
• Rotational periods of very young brown dwarfs and very low-mass stars in ChaI
• Keck NIRC Observations of Planetary-mass Candidate Members in the sigma Orionis Open Cluster
• Photometric variability of young brown dwarfs in the sigma Orionis open cluster
• Contamination by field late-M, L and T dwarfs in deep surveys
• Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey
• Near-infrared low-resolution spectroscopy of Pleiades L-type brown dwarfs
• Activity at the Deuterium-Burning Mass Limit in Orion
• Effective Temperatures of late L dwarfs and the onset of Methane signatures
• Substellar Mass Function of Young Open Clusters as Determined through a Statistical Approach Using 2MASS and GSC Data
• Activity and Kinematics of M and L Dwarfs