Nearby stars of the Galactic disk and halo. III.

Abstract

AbstractHigh‐resolution spectroscopic observations of about 150 nearby stars or star systems are presented and discussed. The study of these and another 100 objects of the previous papers of this series implies that the Galaxy became reality 13 or 14 Gyr ago with the implementation of a massive, rotationally‐supported population of thick‐disk stars. The very high star formation rate in that phase gave rise to a rapid metal enrichment and an expulsion of gas in supernovae‐driven Galactic winds, but was followed by a star formation gap for no less than three billion years at the Sun's galactocentric distance. In a second phase, then, the thin disk – our “familiar Milky Way” – came on stage. Nowadays it traces the bright side of the Galaxy, but it is also embedded in a huge coffin of dead thick‐disk stars that account for a large amount of baryonic dark matter. As opposed to this, cold‐dark‐matter‐dominated cosmologies that suggest a more gradual hierarchical buildup through mergers of minor structures, though popular, are a poor description for the Milky Way Galaxy – and by inference many other spirals as well – if, as the sample implies, the fossil records of its long‐lived stars do not stick to this paradigm. Apart from this general picture that emerges with reference to the entire sample stars, a good deal of the present work is however also concerned with detailed discussions of many individual objects. Among the most interesting we mention the blue straggler or merger candidates HD 165401 and HD 137763/HD 137778, the likely accretion of a giant planet or brown dwarf on 59 Vir in its recent history, and HD 63433 that proves to be a young solar analog at τ ∼ 200 Myr. Likewise, the secondary to HR 4867, formerly suspected non‐single from the Hipparcos astrometry, is directly detectable in the highresolution spectroscopic tracings, whereas the visual binary χ Cet is instead at least triple, and presumably even quadruple. With respect to the nearby young stars a complete account of the UrsaMajor Association is presented, and we provide as well plain evidence for another, the “Hercules‐Lyra Association”, the likely existence of which was only realized in recent years. On account of its rotation, chemistry, and age we do confirm that the Sun is very typical among its G‐type neighbors; as to its kinematics, it appears however not unlikely that the Sun's known low peculiar space velocity could indeed be the cause for the weak paleontological record of mass extinctions and major impact events on our parent planet during the most recent Galactic plane passage of the solar system. Although the significance of this correlation certainly remains a matter of debate for years to come, we point in this context to the principal importance of the thick disk for a complete census with respect to the local surface and volume densities. Other important effects that can be ascribed to this dark stellar population comprise (i) the observed plateau in the shape of the luminosity function of the local FGK stars, (ii) a small though systematic effect on the basic solar motion, (iii) a reassessment of the term “asymmetrical drift velocity” for the remainder (i.e. the thin disk) of the stellar objects, (iv) its ability to account for the bulk of the recently discovered high‐velocity blue white dwarfs, (v) its major contribution to the Sun's ∼220 km s−1 rotational velocity around the Galactic center, and (vi) the significant flattening that it imposes on the Milky Way's rotation curve. Finally we note a high multiplicity fraction in the small but volume‐complete local sample of stars of this ancient population. This in turn is highly suggestive for a star formation scenario wherein the few existing single stellar objects might only arise from either late mergers or the dynamical ejection of former triple or higher level star systems. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)