Spatial distribution of stars and brown dwarfs in σ Orionis |
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| Authors: | José A Caballero |
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| Institution: | Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany;Departamento de Astrofísica y Ciencias de la Atmósfera, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain |
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| Abstract: | I have re-visited the spatial distribution of stars and high-mass brown dwarfs in the σ Orionis (σ Ori) cluster (～3 Ma, ～360 pc). The input was a catalogue of 340 cluster members and candidates at separations less than 30 arcmin to σ Ori AB. Of them, 70 per cent have features of extreme youth. I fitted the normalized cumulative number of objects counting from the cluster centre to several power-law, exponential and King radial distributions. The cluster seems to have two components: a dense core that extends from the centre to r ≈ 20 arcmin and a rarified halo at larger separations. The radial distribution in the core follows a power law proportional to r 1, which corresponds to a volume density proportional to r ?2 . This is consistent with the collapse of an isothermal spherical molecular cloud. The stars more massive than 3.7 M⊙ concentrate, however, towards the cluster centre, where there is also an apparent deficit of very low mass objects ( M < 0.16 M⊙) . Last, I demonstrated through Monte Carlo simulations that the cluster is azimuthally asymmetric, with a filamentary overdensity of objects that runs from the cluster centre to the Horsehead Nebula. |
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| Keywords: | stars: formation stars: low mass brown dwarfs open clusters and associations: individual: σ Orionis |
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