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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
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| The Program to Link the HIPPARCOS Reference Frame to an Extragalactic Reference System Using the Fine Guidance Sensors of the Hubble Space Telescope
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.2796H&db_key=AST
| Speckle interferometry of HIPPARCOS link stars
Speckle interferometry has been carried out on 39 stars that had beenselected as provisionally suitable for linking Hipparcos positionalmeasurements to an extragalactic reference frame by Space Telescope. TheImperial College, London, speckle interferometer and the autocorrelatoron the Anglo-Australian 3.9-m telescope are used. The result is thatabout 30 percent of the stars selected turned out to be confirmed orsuspected multiple stars in the range 0.03-0.7 arcsec.
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