The Sirius Supercluster and Missing Mass near the Sun

Abstract

The Hipparcos results confirm some 50 members of the Sirius supercluster in the Bright Star Catalogue. The resulting, well-defined color-luminosity array indicates an age of 4 x 10^8 yr from conventional (no overshoot) models. A comparison of the luminosity function obtained from members in the Bright Star Catalogue, unbiased as to proper motion, and in the Catalogue of Nearby Stars (~27 pc), which are strongly biased toward large proper motions, reveals that either most of the small proper-motion stars near the Sun remain to be identified or the luminosity function of the supercluster is drastically different from that of the field stars. A search for low-mass members in a 6 deg x 6 deg field in the center of the Ursa Major cluster yielded 10 possible members that lie on the white dwarf sequence, 5 mag below the main sequence. Luyten (field LP 131) found 368 stars in this field with proper motion exceeding 0.08", and only 19 of these have a position angle of their proper motion between 0 deg and 98 deg (i.e., the quadrant containing UMa cluster members). The assumption of cluster membership yields a parallax very close to that obtained for bright members. Aside from white dwarfs, the possibilities are discussed that these stars are (1) brown dwarfs, (2) members of a cluster in the line of sight to UMa but 100 pc more distant, or (3) reflecting a chain of very unlikely coincidences.