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

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 star systems Alnilam and Mintaka in the Orion Belt, were explored. The regions are very young (less than 10 Ma), have low extinction, and are neighbours to σ Orionis (∼3 Ma), a young open cluster very rich in brown dwarfs and planetary-mass objects. We used Virtual Observatory tools, the astro-photometric Tycho-2, DENIS and 2MASS catalogues, 10 control fields at similar galactic latitudes, as well as X-ray, mid-infrared, and spectroscopic data from the literature. Results. We compiled exhaustive lists of known young stars and new candidate members in the Ori OB1b association and of foreand background sources. A total of 136 stars display features of extreme youth, like early spectral types, lithium in absorption, or mid-infrared flux excess. Other two young brown dwarf and 289 star candidates have been 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 and high-mass substellar populations in the Orion Belt. Finally, we investigated the surface densities and radial distributions of young objects surrounding Alnilam and Mintaka and compared them with those in the σ Orionis cluster. We report on a new open cluster centred on Mintaka. Conclusions. Both regions can be analogues to the σ Orionis cluster, but more massive, more extended, slightly older, and less radially concentrated.

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

“…It has a very red colour of J − K s = 1.77±0.03 mag (Caballero & Solano 2008). From our light curve, PU Ori is an irregular variable with minimum and maximum magnitudes V 0 ∼ 12.5 and 14.5 mag.…”

“…It may be the optical/near infrared counterpart of IRAS Z05322-0039. V472 Ori is a probable member in the Mintaka cluster (Caballero & Solano 2008). Its ASAS light curve has a peak-to-magnitude larger than 1 mag.…”

“…According to Caballero & Solano (2008), the star is a member in the young Ori OB1 b stellar population surrounding Alnilam, and is proably associated to the [OS98] 29J, [OS98] 29H, and [OS98] 29K remnant molecular clouds (Ogura & Sugitani 1998). Based on its V 0 − K s colour and the empirical scale of effective temperatures of main-sequence stars of Alonso et al (1996), and extrapolating it to the pre-main sequence, V469 Ori is likely a late Ge or early Ke star.…”

“…It is a K4-type T Tauri star with Hα in emission and Li I in absorption (Stephenson 1986;Maheswar et al 2003;Caballero & Solano 2008). The star, which is located in the surroundings of Alnilam, has a wide amplitude of variability, with maximum and minimum brighteness at V 0 ≈ 11.9 and 13.3 mag, respectively, and time scales of variation of a few days.…”

“…Here lies the σ Orionis cluster, one of the most suitable sites for discovering and characterising substellar objects (Béjar et al 1999;Zapatero Osorio et al 2002;Sherry et al 2004;). The population of stars surrounding Alnilam (ǫ Ori) and Mintaka (δ Ori), to the west of the Belt, are older (5-10 Ma) and spread over a wider area than the much denser σ Orionis cluster (Caballero & Solano 2008). Surrounding the Belt, there is another dispersed population of 10-20 Ma-old stars, mostly associated to the Ori OB1 a subgroup association (Jeffries et al 2006; and its agglomerates, such as the 25 Orionis group or the η Orionis overdensity (Briceño et al 2007;Caballero & Dinis 2008).…”