The Substellar Population in σ Orionis

The a Orionis cluster (rv3 Myr, 350 pc) is an ideal site to investigate the early evolution of substellar (brown dwarf and planetary mass) objects. To date, the cluster photometric and spectroscopic sequence of free-floaters is known for a wide mass range from 1 M 0 down to roughly 3 MJup. The substellar domain covers spectral types that go from mid-M classes to the recently defined "methane" T-types, i.e., surface temperatures between rv3000K and 800 K. We derive a rising initial substellar mass function in the mass interval of 150-5 MJup (dN/ dM rv M':", with Q == 0.9 ± 0.4). We also find evidence for a extension of this mass function toward lower masses down to 2-3 MJup. This indicates that the population of isolated planetary mass objects with masses below the deuterium burning threshold is rather abundant in the cluster.

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