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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
| A search for debris discs around stars with giant planets
Eight nearby stars with known giant planets have been searched forthermal emission in the submillimetre arising from dust debris. The nullresults imply quantities of dust typically less than 0.02 Earth massesper star. Conversely, literature data for 20 Sun-like stars with debrisdiscs show that <= 5 per cent have gas giants inside a fewastronomical units - but the dust distribution suggests that nearly allhave more distant planets. The lack of overlap in these systems - i.e.few stars possess both inner planets and a disc - indicates that thesephenomena either are not connected or are mutually exclusive. Comparisonwith an evolutionary model shows that debris masses are predicted to below by the stellar ages of 2-8 Gyr (unless the colliding parent bodiesare quite distant, located beyond 100-200 au), but it remains to beexplained why stars that do have debris should preferentially only havedistant planets. A simple idea is proposed that could produce theselargely different systems, invoking a difference in the primordial discmass. Large masses promote fast gas giant growth and inwards migration,whereas small masses imply slow evolution, low-mass gas giants andoutwards migration that increases the collision rate of Kuiper Belt-likeobjects. This explanation neglects other sources of diversity betweendiscs (such as density and planetesimal composition and orbits), but itdoes have the merit of matching the observational results.
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Stellar atmospheres of nearby young solar analogs
High-resolution (/R~90,000) spectra of 34 nearby, young Sun-like starswere analyzed using stellar atmosphere models to estimate effectivephotosphere temperatures, surface gravities, and the abundance ofcertain heavy elements (C, Na, Mg, Si, S, Ca, Ti, Fe, and Ni). Theeffective temperatures derived from spectroscopy were compared withtemperatures estimated using optical and near-infrared photometry. Inmany cases the spectroscopic temperatures are significantly higher thanthe photometric estimates, possibly as a result of spottedness orchromospheric activity on these active stars. Values of effectivetemperature, surface gravity, and luminosity were compared totheoretical stellar evolution tracks and the evolutionary status ofthese objects was evaluated. The correlation between heavy elementabundance patterns and kinematics (space motion) was also examined. Twonearby stars that were tentatively assigned to the Hyades cluster basedon kinematics have Fe abundances that are also consistent withmembership in that cluster. Members of the Ursa Major kinematic groupexhibit a range of [Fe/H] values but have monotonic [Si/Fe]. These twoobservations suggest that heterogeneous incorporation of the heavyelements into protostars is creating the variation in metallicity. LocalAssociation members have a distinctly different Si/Fe that probablyreflects their distinct origin and chemical inheritance.
| GasDust Shells around Some Early-Type Stars with an IR Excess (of Emission)
The results of an investigation of IR (IRAS) observations of 58OBAF stars of different luminosity classes, whichare mainly members of various associations, are presented. The colorindices of these stars are determined and two-color diagrams areconstructed. The emission excesses at 12 and 25 mm (E 12 and E 25) arealso compared with the absorption A1640 of UV radiation. It is concludedthat 24 stars (of the 58 investigated) are disk systems of the Vegatype, to which Vega = N 53 also belongs. Eight known stars of the Vegatype are also given in the figures for comparison. The remaining 34stars may have gasdust shells and/or shelldisks. The IRemission excesses of the 34 investigated stars and 11 comparison stars(eight of them are Be-Ae stars) are evidently due both to thermalemission from grains and to the emission from freefreetransitions of electrons in the gasdust shells of these stars.
| A revision of the solar neighbourhood metallicity distribution
We present a revised metallicity distribution of dwarfs in the solarneighbourhood. This distribution is centred on solar metallicity. Weshow that previous metallicity distributions, selected on the basis ofspectral type, are biased against stars with solar metallicity orhigher. A selection of G-dwarf stars is inherently biased againstmetal-rich stars and is not representative of the solar neighbourhoodmetallicity distribution. Using a sample selected on colour, we obtain adistribution where approximately half the stars in the solarneighbourhood have metallicities higher than [Fe/H]=0. The percentage ofmid-metal-poor stars ([Fe/H]<-0.5) is approximately 4 per cent, inagreement with present estimates of the thick disc. In order to have ametallicity distribution comparable to chemical evolution modelpredictions, we convert the star fraction to mass fraction, and showthat another bias against metal-rich stars affects dwarf metallicitydistributions, due to the colour (or spectral type) limits of thesamples. Reconsidering the corrections resulting from the increasingthickness of the stellar disc with age, we show that the simpleclosed-box model with no instantaneous recycling approximation gives areasonable fit to the observed distribution. Comparisons with theage-metallicity relation and abundance ratios suggest that the simpleclosed-box model may be a viable model of the chemical evolution of theGalaxy at solar radius.
| Spectroscopy and Photometry of Nearby Young Solar Analogs
We present new photometry and spectroscopy of 34 stars from a catalog of38 nearby (d<25 pc) G and K dwarfs selected as analogs to the earlySun. We report that the least active star in our sample is also slowlyrotating and probably of solar age. Two other stars appear to be evolvedobjects that have recently acquired angular momentum. A fourth star maybe a spectroscopic binary. Many of the other stars belong to previouslyidentified common proper-motion groups. Space motions, lithiumabundances, and Ca II emission of these stars suggest ages between 70and 800 Myr.
| Ages of Late Spectral Type Vega-like Stars
We have estimated the ages of eight late-type Vega-like stars by usingstandard age-dating methods for single late-type stars, e.g., locationon the color-magnitude diagram, Li λ6708 absorption, Ca II H andK emission, X-ray luminosity, and stellar kinematic population. With theexception of the very unusual pre-main-sequence star system HD 98800,all the late-type Vega-like stars are the same age as the Hyades cluster(600-800 Myr) or older.
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Observational Constraints on Late Heavy Bombardment Episodes around Young Solar Analogs
Thermal emission from dust is investigated around 36 nearby G and Kstars selected as analogs to the young Sun during the period of lateheavy bombardment on the Moon (prior to 3.8 Ga). Measurements or upperlimits at 12, 25, and 60 μm wavelengths were obtained fromobservations by the Infrared Astronomical Satellite (IRAS), andphotosphere contributions were subtracted using the models of Kurucz andground-based optical and near-infrared data. A single star, HD 128400,has circumstellar emission that is consistent with a 490 K blackbodywith an effective area ~7x10^3 times that of the present solar systemzodiacal cloud. Emission from 260 K dust at a mean level exceeding 900times the present zodiacal cloud can be ruled out for the remaining 35stars. A model of dust production by cometary impactors scaled to thelunar cratering record shows that dust from late heavy bombardmentepisodes around these stars would fall below IRAS detection levels after500 Myr (4.1 Ga). The emission from HD 128400 is broadly consistent withthe model at the star's 300 Myr age.
| A Multiplicity Survey of Chromospherically Active and Inactive Stars
Surveys of three samples of solar-type stars, segregated bychromospheric emission level, were made to determine their multiplicityfractions and to investigate the evolution of multiplicity with age. Intotal, 245 stars were searched for companions with DeltaV <= 3.0 andseparations of 0.035" to 1.08" using optical speckle interferometry. Byincorporating the visual micrometer survey for duplicity of theLamontHussey Observatory, the angular coverage was extended to 5.0" withno change in the DeltaV limit. This magnitude difference allows massratios of 0.63 and larger to be detected throughout a search region of2-127 AU for the stars observed. The 84 primaries observed in thechromospherically active sample are presumably part of a youngpopulation and are found to have a multiplicity fraction of 17.9% +/-4.6%. The sample of 118 inactive, presumably older, primaries wereselected and observed using identical methods and are found to have amultiplicity fraction of only 8.5% +/- 2.7%. Given the known linkbetween chromospheric activity and age, these results tentatively implya decreasing stellar multiplicity fraction from 1 to 4 Gyr, theapproximate ages of the two samples. Finally, only two of the 14 veryactive primaries observed were found to have a companion meeting thesurvey detection parameters. In this case, many of the systems areeither very young, or close, RS CVn type multiples that are unresolvableusing the techniques employed here.
| Nearby Young Solar Analogs. I. Catalog and Stellar Characteristics
We present a catalog of 38 young solar analogs within 25 pc, stars thatare uniquely well suited for observations of their circumstellarenvironments to improve our understanding of conditions within the solarsystem during the Hadean/early Archean eons (prior to 3.8 Gyr ago).These G and early K stars were selected from the Hipparcos astrometriccatalog based on lack of known stellar companions within 800 AU,bolometric luminosities close to that of the zero-age Sun and consistentwith the zero-age main sequence, and ROSAT X-ray luminositiescommensurate with the higher rotation rate and level of dynamo-drivenactivity in solar-mass stars less than 0.8 Gyr old. While many of theseobjects have been previously identified, this sample is novel in tworespects: The selection criteria specifically consider the planetaryenvironment, and the selection is uniform and all-sky. The X-rayemission from these young analogs is spectrally soft and consistent witha coronal origin. Calcium H and K emission, rotation periods, lithiumabundances, and kinematics support an age range of 0.2-0.8 Gyr for mostof these stars. Three stars have exceptionally high space motions withrespect to the local standard of rest and may be old disk or halo starsthat are anomalously X-ray luminous.
| Metallicity effects on the chromospheric activity-age relation for late-type dwarfs
We show that there is a relationship between the age excess, defined asthe difference between the stellar isochrone and chromospheric ages, andthe metallicity as measured by the index [Fe/H] for late-type dwarfs.The chromospheric age tends to be lower than the isochrone age formetal-poor stars, and the opposite occurs for metal-rich objects. Wesuggest that this could be an effect of neglecting the metallicitydependence of the calibrated chromospheric emission-age relation. Wepropose a correction to account for this dependence. We also investigatethe metallicity distributions of these stars, and show that there aredistinct trends according to the chromospheric activity level. Inactivestars have a metallicity distribution which resembles the metallicitydistribution of solar neighbourhood stars, while active stars appear tobe concentrated in an activity strip on the logR'_HKx[Fe/H] diagram. Weprovide some explanations for these trends, and show that thechromospheric emission-age relation probably has different slopes on thetwo sides of the Vaughan-Preston gap.
| Polarimetric observations of southern hemisphere young solar-type stars
{A report is given on some linear and circular polarimetric observationsof young solar-type stars. Despite a somewhat high level of instrumentalpolarization, the measurements were able to detect polarizations ~0.0001 or\ 0.01%. Temporal variability has been discovered for 59 Virwith an indication that circular polarization is also present. Apolarization at a level of p ~ 1% was found associated with the star SAO251015. Marginal polarizations are also suspected for the stars HD155555 and HD 1835. For SAO 154972 (TU Pyx), a sudden change inpolarization ~ 2% occurred over some tens of minutes and an associatedcircular polarization was also detected.}
| 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.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
| Preliminary Version of the Third Catalogue of Nearby Stars
Not Available
| Large and kinematically unbiased samples of G and K type stars. I - The dwarfs
Four-color, H-beta, and (R,I) photometry for the little-evolvedmain-sequence stars from the Bright Star Catalogue, South Galactic Pole,Griffin (1971), and Moore-Paddock-Wayman (Moore and Paddock 1950, andWayman 1960) samples are analyzed. The luminosity and heavy-elementabundances for these stars are calculated in terms of the Hyadessupercluster, the Wolf 630 group, the Sirius supercluster, and theKapteyn star group. The data reveal the presence of a metal-abundancedependent discontinuity near M(v) = +7 mag in the photometric parametersof dwarfs. The distributions of the abundances and the space motions ofthe sample stars are discussed.
| Preliminary UVBY calibrations for G and K type dwarf stars
Four-color uvby photometry for several hundred late-type dwarf stars oftypes G, K and M is discussed. Mean values of photometric indices aregiven for MK spectral types between F8/G0V and M2V. Preliminary standardrelations between the four-color indices are derived. Based onparallaxes and results from high-dispersion spectroscopic analyses,calibrations of the observed indices in terms of Mv, log Te and Fe/Hhave been derived. The means errors are 0.29 mag, 0.009 dex, and 0.17dex, respectively. The calibrations are valid for class-V stars of allpopulations between G0 and M2. If extreme population-II dwarfs areexcluded, the mean error of the abundance calibration decreases to 0.13dex. Calibrations in terms of log g have been attempted,, but the lowaccuracy of the spectroscopic g determinations does not inspireconfidence in the results. The possible contribution of a'fourth-parameter' variation to the mean errors of the calibrations isbriefly discussed. This fourth parameter could be the intrinsic heliumabundance of the stars.
| The chemical evolution of the solar neighborhood. I - A bias-free reduction technique and data sample
The possible ways of measuring the age-metallicity relation for thegalactic disk in the neighborhood of the sun are discussed. It is shownthat the use of a field star sample chosen on the basis of effectivetemperature introduces a bias which results in a monotonic increase inthe metal abundance of the disk with time. However, if theage-metallicity relation for the disk can be shown to satisfy certaincriteria, the bias introduced in such a sample can be neglected: thegalactic disk apparently satisfies the criteria. It is concluded that asample analyzed through the use of uvby and H(beta) photometry inconjunction with a self-consistent set of theoretical isochronesprovides the least biased, most accurate estimate of the age-metallicityrelation for the disk.
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