Kinematics and Metallicity of Stars in the Solar Region

Abstract

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.