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Young stars and brown dwarfs surrounding Alnilam (ɛ Orionis) and Mintaka (δ Orionis)
Aims. We look for new regions to search for substellar objects. Methods: Two circular areas, 45 arcmin-radius each, centred on the youngmassive star systems Alnilam and Mintaka in the Orion Belt, wereexplored. The regions are very young (less than 10 Ma), have lowextinction, and are neighbours to σ Orionis (~3 Ma), a young opencluster very rich in brown dwarfs and planetary-mass objects. We usedVirtual Observatory tools, the astro-photometric Tycho-2, DENIS and2MASS catalogues, 10 control fields at similar galactic latitudes, aswell as X-ray, mid-infrared, and spectroscopic data from the literature. Results: We compiled exhaustive lists of known young stars and newcandidate members in the Ori OB1b association and of fore- andbackground sources. A total of 136 stars display features of extremeyouth, like early spectral types, lithium in absorption, or mid-infraredflux excess. Other two young brown dwarf and 289 star candidates havebeen identified from an optical/near-infrared colour-magnitude diagram.We list another 74 known objects that might belong to the association.This catalogue can serve as input for characterising the stellar andhigh-mass substellar populations in the Orion Belt. Finally, weinvestigated the surface densities and radial distributions of youngobjects surrounding Alnilam and Mintaka and compared them with those inthe σ Orionis cluster. We report on a new open cluster centred onMintaka. Conclusions: Both regions can be analogues to the σOrionis cluster, but more massive, more extended, slightly older, andless radially concentrated.Tables A.1 to A.18 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/485/931
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