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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 multiperiodicity of the gamma Doradus stars HD 224945 and HD 224638 as detected from a multisite campaign
We discuss new photometric data collected on the gamma Dor variables HD224945 and HD 224638. Multiperiodicity was detected in both stars,thanks to the clear spectral window of a multisite campaign thatinvolved five observatories. HD 224945 shows the shortest period amongthe gamma Dor stars, i.e., 0.3330 d. The pulsation behaviour is verydifferent: HD 224945 displays a set of frequencies spread over aninterval much larger than that of HD 224638. We clearly found evidencefor amplitude variations in the excited modes by comparing data fromdifferent years. HD 224945 and HD 224638 are among the best examples ofgamma Dor stars that show multimode pulsations, which make them veryinteresting from an asteroseismological point of view. Based onobservations partially collected at ESO-La Silla (Proposals 54.E-018 and56.E-0308).
| BH Piscium=HD 224639: a δ Scuti star with a rich pulsation spectrum and changing mode amplitudes.
New B and V observations of the δ Scuti star BH Psc (64 hoursduring 13 quasi-consecutive nights in October and November 1994),combined with the previous ones obtained in 1991, have allowed us to getan improved resolution of its light curve, confirming the presence of alarge number (at least 13) of excited modes. There is strong evidencethat some modes have large amplitude variations over timescales shorterthan a few years, while others seem to be rather stable. There is alsoevidence that the pulsation energy in 1994 was lower than in 1991. Theratios between the amplitudes of the light and colour curves and theirphase shifts, available for the first time, tend to suggest the presenceof excited modes with l<=2.
| The complex photometric behaviour of the δ Scuti star HD 224639.
HD 224639 was photometrically observed for 20h in 1989 and 120h in 1991(454 and 2567 datapoints, respectively). The star shows a very complexpulsational content with 3 components with semi-amplitudes of about15mmag and many other terms with a smaller semi-amplitude. The 1991dataset allowed us to single out 11 components in the frequency rangebetween 5 and 12 c/d, but many more are probably present since theresidual rms of the fit is 8.6mmag, while the noise level is expected tobe 3.7mmag. By subdividing the 1991 dataset into two parts and bycomparing the results of the frequency analysis with those obtained fromthe 1989 dataset we found independent evidence in support of thereliability of four frequencies: 6.21, 8.49, 9.54 and 11.42 c/d. Sincemost of the 11 components are nonradial modes, their identification ishampered by the high rotational velocity (vsin i=110km/s) which veryprobably produces the splitting of an l mode into a multiplet(-l<=m<=+l) and, in any case, causes an appreciable shift infrequency of the central value (m=0).
| Spurious effects in presence of a variable extinction coefficient in photoelectric photometry
Observational evidence was found that even in good photometric sites theextinction coefficient can display large variations during a night. Ifnot corrected, this variability increases the measurement errors andintroduces low-frequency spurious peaks in the power spectra. In orderto apply the procedures based on the instantaneous determination of theextinction coefficient, an analytical method is proposed to determine areliable value for the extra-atmosphere instrumental magnitude for areference star. The whole process was applied to different couples ofstars and the improvements in the frequency analysis with respect toclassical procedures are emphasized. It allowed us to better extract asmall amplitude signal in presence of a rapid variability of theatmospheric extinction coefficient and to obtain a clear picture of theextinction coefficient at La Silla during September and October 1991.
| UBVRI photometry of stars useful for checking equipment orientation stability
Several sets of stars have been measured, equally spaced around the skyin right ascension, over a range in declination from -75 deg to +30 deg.The goal was to obtain a number of well-observed stars with qualityinternally consistent magnitudes and color indices. The stars wereneeded to check orientation effects of photometric equipment.
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| Takýmyýldýz: | Baliklar |
| Sað Açýklýk: | 00h06m46.97s |
| Yükselim: | -04°21'00.1" |
| Görünürdeki Parlaklýk: | 8.219 |
| Uzaklýk: | 110.132 parsek |
| özdevim Sað Açýklýk: | 59.2 |
| özdevim Yükselim: | -88.1 |
| B-T magnitude: | 8.871 |
| V-T magnitude: | 8.273 |
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