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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
| The ages of the globular clusters M 71 and 47 Tuc from Strömgren uvby photometry. Evidence for high ages
New uvby CCD photometry for the fairly metal-rich globular clusters M 71(NGC 6838) and 47 Tuc (NGC 104) is presented. We derive the clusterdistances using a sample of field subdwarfs with metallicitiesdetermined from uvby photometry and accurate parallaxes from theHipparcos mission. The biases associated with the main-sequence fittingtechnique are discussed and only that due to metallicity is found to besignificant, corresponding to a -0.05 mag change in distance modulus.Our main results are that: 1) The distance moduli of 47 Tuc and M 71 aresomewhat shorter than that derived by Reid (\cite{Rei98}, AJ 115, 204).For M 71 and 47 Tuc we find (metallicity corrected) (m-M)V =13.71+/- 0.04+/- 0.1 and (m-M)V = 13.33+/- 0.04+/- 0.1, foradopted reddenings of E(B-V) = 0.28 and E(B-V) = 0.04 respectively(first errorbar denotes random errors and the second systematic errors).The main source of difference with Reid is the selection of subdwarfswith this study having more intrinsically faint field subdwarfs; 2)These values lead to ages of nearly 12 Gyr when using the isochrones ofVandenBerg et al. (\cite{Vane00}, ApJ, 532, 430); this estimate does notinclude the effects of He diffusion. 3) A differential comparison of thecluster colour-magnitude diagrams show that the age difference betweenthe two is very small - less than one billion years. 4) The observedscatter in the c1 index (due to star-to-star nitrogenvariations) among main-sequence stars does not allow us to use the[(v-y)0, c0] diagram for a distance-independentage determination. Based on observations made with the Nordic OpticalTelescope, operated on the island of La Palma jointly by Denmark,Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrofisica de Canarias.Based on observations obtained with the Danish 1.5 m telescope at theEuropean Southern Observatory, La Silla, Chile.
| A Survey of Ca II H and K Chromospheric Emission in Southern Solar-Type Stars
More than 800 southern stars within 50 pc have been observed forchromospheric emission in the cores of the Ca II H and K lines. Most ofthe sample targets were chosen to be G dwarfs on the basis of colors andspectral types. The bimodal distribution in stellar activity first notedin a sample of northern stars by Vaughan and Preston in 1980 isconfirmed, and the percentage of active stars, about 30%, is remarkablyconsistent between the northern and southern surveys. This is especiallycompelling given that we have used an entirely different instrumentalsetup and stellar sample than used in the previous study. Comparisons tothe Sun, a relatively inactive star, show that most nearby solar-typestars have a similar activity level, and presumably a similar age. Weidentify two additional subsamples of stars -- a very active group, anda very inactive group. The very active group may be made up of youngstars near the Sun, accounting for only a few percent of the sample, andappears to be less than ~0.1 Gyr old. Included in this high-activitytail of the distribution, however, is a subset of very close binaries ofthe RS CVn or W UMa types. The remaining members of this population maybe undetected close binaries or very young single stars. The veryinactive group of stars, contributting ~5%--10% to the total sample, maybe those caught in a Maunder Minimum type phase. If the observations ofthe survey stars are considered to be a sequence of snapshots of the Sunduring its life, we might expect that the Sun will spend about 10% ofthe remainder of its main sequence life in a Maunder Minimum phase.
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Datos observacionales y astrométricos
| Constelación: | Altar |
| Ascensión Recta: | 17h13m23.19s |
| Declinación: | -53°06'20.7" |
| Magnitud Aparente: | 8.875 |
| Distancia: | 39.92 parsecs |
| Movimiento Propio en Ascensión Recta: | -44 |
| Movimiento Propio en Declinación: | 44.9 |
| B-T magnitude: | 9.713 |
| V-T magnitude: | 8.945 |
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