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The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Proper motion and X-ray selected search for new members of the young TW Hya association
We have searched for new members of the TW Hya association (TWA) amongunidentified ROSAT X-ray sources by identifying them in proper motioncatalogues and selecting those that would be consistent with kinematicalmembership to the TWA. Spectroscopic follow-up observations of 19 membercandidates revealed the detection of moderate lithium absorption linesfor the following three stars: GSC 7206 845, TYC 7216-55, and TYC7247-12. The isochronal ages of the latter TYC stars are estimated to be 20 Myr while the other one has 100 Myr age based on akinematic distance estimate that assumes TWA membership. However, themoderately Li-rich stars are not likely to be new pre-main sequencemembers of TWA partly because of the discrepant radial velocities.Infrared follow-up imaging in the H-band for the 3 stars shows companioncandidates near two of them. While one system (TYC 7216-55) is probablya near-equal-magnitude stellar binary, our follow-up H-band spectrum ofthe faint companion candidate near GSC 7206 845 shows that it is insteada background K-type star rather than a companion.Based on observations obtained in ESO programs 66.C-0540, 66.C-0138B,and 68.C-0009 on La Silla, Chile, as well as with ROSAT and theUniversity of Hawaii Institute for Astronomy 2.2m telescope on MaunaKea.
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