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Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| The Sirius Supercluster and Missing Mass near the Sun
The Hipparcos results confirm some 50 members of the Sirius superclusterin the Bright Star Catalogue. The resulting, well-definedcolor-luminosity array indicates an age of 4 x 10^8 yr from conventional(no overshoot) models. A comparison of the luminosity function obtainedfrom members in the Bright Star Catalogue, unbiased as to proper motion,and in the Catalogue of Nearby Stars (~27 pc), which are strongly biasedtoward large proper motions, reveals that either most of the smallproper-motion stars near the Sun remain to be identified or theluminosity function of the supercluster is drastically different fromthat of the field stars. A search for low-mass members in a 6 deg x 6deg field in the center of the Ursa Major cluster yielded 10 possiblemembers that lie on the white dwarf sequence, 5 mag below the mainsequence. Luyten (field LP 131) found 368 stars in this field withproper motion exceeding 0.08", and only 19 of these have a positionangle of their proper motion between 0 deg and 98 deg (i.e., thequadrant containing UMa cluster members). The assumption of clustermembership yields a parallax very close to that obtained for brightmembers. Aside from white dwarfs, the possibilities are discussed thatthese stars are (1) brown dwarfs, (2) members of a cluster in the lineof sight to UMa but 100 pc more distant, or (3) reflecting a chain ofvery unlikely coincidences.
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