Olin J. Eggen

Kinematics and Metallicity of Stars in the Solar Region

Several samples of nearby stars with the most accurate astrometric and photometric parameters are searched for clues to their evolutionary history. The main samples are (1) the main-sequence stars with b - y between 0.29 and 0.59 mag (F3 to K1) in the Yale parallax catalog, (2) a group of high-velocity subgiants studied spectroscopically by Ryan & Lambert, and (3) high-velocity main-sequence stars in the extensive investigation by Norris, Bessel, & Pickles. The major conclusions are as follows: (1) The oldest stars (halo), t ≥ 10–12 Gyr, have V-velocities (in the direction of Galactic rotation and referred to the Sun) in the range from about -50 to -800 km s-1 and have a heavy-element abundance [Fe/H] of less than about -0.8 dex. The age range of these objects depends on our knowledge of globular cluster ages, but if age is correlated with V-velocity, the youngest may be M22 and M28 (V ~ -50 km s-1) and the oldest NGC 3201 (V ~ -500 km s-1) and assorted field stars. (2) The old disk population covers the large age range from about 2 Gyr (Hyades, NGC 752) to 10 or 12 Gyr (Arcturus group, 47 Tuc), but the lag (V) velocity is restricted to less than about 120 km s-1 and [Fe/H] ≥ -0.8 or -0.9 dex. The [Fe/H] ~ -0.8 dex division between halo and old disk, near t ~ 10–12 Gyr, is marked by a change in the character of the CN index (Cm) and of the blanketing parameter K of the DDO photometry. (3) The young disk population, t < 2 Gyr, is confined exclusively to a well-defined area of the (U, V) velocity plane. The age separating young and old disk stars is also that separating giant evolution of the Hyades (near main-sequence luminosity) and M67 (degenerate helium cores and a large luminosity rise) kinds. The two disk populations are also separated by such indexes as the g-index of Geveva photometry. There appears to be no obvious need to invoke exogeneous influences to understand the motion and heavy-element abundance distributions of the best-observed stars near the Sun. Individual stars of special interest include the parallax star HD 55575, which may be an equal-component binary, and the high-velocity star HD 220127, with a well-determined space velocity near 1000 km s-1.

The Age Range of Hyades Stars

On the basis of canonical models, the age of Hyades supercluster stars, whether in the Hyades and Praesepe clusters or the noncluster field, ranges from (5–6) × 108 to 109 yr. The difference between the parallax derived from the supercluster motion and that obtained from Hipparcos observations has a dispersion only twice that of the mean dispersion of the individual Hipparcos values. The supercluster appears not to contain red giants on the first ascent of the red giant branch, but only asymptotic giant branch (clump) stars. The masses obtained for individual components of binary stars in the supercluster show a dispersion of less than 10% when compared with model predictions.

The Scorpius OB2 Complex

The Sco OB2 complex is a member of the Local Association. The association contains pre–main-sequence stars in addition to objects some 3 × 107 yr old. If it is assumed that stars in wide binaries are coeval, then the He-weak variables in the supercluster are pre–main-sequence, elevated above that sequence by an amount similar to that of the 3 × 107 yr old, normal stars but contracting toward the sequence, not evolving from it. The apparent great depth of the association is probably a result of some foreground supercluster members, superposed on Sco OB2, and a rift in the absorption cloud between Scorpius and Ophiuchus. The adjoining Chamaeleon complex also appears to be a member of the supercluster.

The Sirius Supercluster and Missing Mass near the Sun

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 × 108 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° × 6° 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 008, and only 19 of these have a position angle of their proper motion between 0° and 98° (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.

The HR 1614 Group and Hipparcos Astrometry

The evidence for the existence of the important HR 1614 supercluster and group of rare rich CN stars is examined with the use of Hipparcos astrometry. Group membership is supported by Hipparcos parallaxes except for two stars for which available evidence suggests uncertainty in the Hipparcos values.