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Young stars and brown dwarfs surrounding Alnilam (ϵ Orionis) and Mintaka (δ Orionis)^*

¹ Dpto. de Astrofísica y Ciencias de la Atmósfera, Facultad de Física, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Laboratorio de Astrofísica Espacial y Física Fundamental, INSA, Apdo. 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 17 February 2008
Accepted: 9 April 2008

Abstract

Aims.We look for new regions to search for substellar objects.

Methods.Two circular areas, 45 arcmin-radius each, centred on the young massive starsystems Alnilam and Mintaka in the Orion Belt, were explored.The regions are very young (less than 10 Ma), have low extinction, and areneighbours toσ Orionis (~3 Ma), a young open cluster very rich inbrown dwarfs and planetary-mass objects.We used Virtual Observatory tools, the astro-photometric Tycho-2, DENIS and2MASS catalogues, 10 control fields at similar galactic latitudes, as well asX-ray, mid-infrared, and spectroscopic data from the literature.

Results.We compiled exhaustive lists of known young stars and new candidatemembers in the Ori OB1b association and of fore- and background sources.A total of 136 stars display features of extreme youth, like earlyspectral types, lithium in absorption, or mid-infrared flux excess.Other two young brown dwarf and 289 star candidates have been identified froman 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, we investigated the surface densities and radial distributions ofyoung objects surrounding Alnilam and Mintaka and compared them with those intheσ Orionis cluster.We report on a new open cluster centred on Mintaka.

Conclusions.Both regions can be analogues to theσ Orionis cluster, but moremassive, more extended, slightly older, and less radially concentrated.