The σ Orionis Cluster (original) (raw)
2008
Theσ Orionis cluster is the group of stars surrounding the high ma ssσ Ori stellar system. It is kinematically distinct from the Orion OB1a and OB1b associations, against which it is projected. The cluster could contain up t o ∼700 stars and substellar mass objects, with a total mass of about 225 M ⊙ within a radius of about 30 arcmin. The age is 2–3 million years. The distance, from main sequenc e fitting corrected for the sub-solar metallicity, is 420 ±30 pc. The mass function is similar to the field star mass function, and has been traced well into the planetary mass re gime. The disk fraction is normal for its age. The cluster appears to be an older and less ma ive analog of the Orion Nebula Cluster.
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A near-infrared/optical/X-ray survey in the centre of sigma Orionis
Astron Nachr, 2007
Because of the intense brightness of the OB-type multiple star system σ Ori, the low-mass stellar and substellar populations close to the centre of the very young σ Orionis cluster is poorly know. I present an IJHK_s survey in the cluster centre, able to detect from the massive early-type stars down to cluster members below the deuterium burning mass limit. The near-infrared and optical data have been complemented with X-ray imaging. Ten objects have been found for the first time to display high-energy emission. Previously known stars with clear spectroscopic youth indicators and/or X-ray emission define a clear sequence in the I vs. I-K_s diagram. I have found six new candidate cluster members that follow this sequence. One of them, in the magnitude interval of the brown dwarfs in the cluster, displays X-ray emission and a very red J-K_s colour, indicative of a disc. Other three low-mass stars have excesses in the K_s band as well. The frequency of X-ray emitters in the area is 80±20 %. The spatial density of stars is very high, of up to 1.6±0.1 arcmin-2. There is no indication of lower abundance of substellar objects in the cluster centre. Finally, I also report two cluster stars with X-ray emission located at only 8000-11000 AU to σ Ori AB, two sources with peculiar colours and an object with X-ray emission and near-infrared magnitudes similar to those of previously-known substellar objects in the cluster.
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