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Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| On Ca II Emission as an Indicator of the Age of Young Stars
Chromospheric emission in the Ca II H and K lines has often been used asan age diagnostic for solar mass stars. For 20 such stars with ages lessthan a few hundred megayears, we compare Ca II ages derived by Wright etal. with ages we derive based on a combination of lithium abundance,X-ray activity, and Galactic space motion. Typically, the Ca II ages arenoticeably older than the lithium/X-ray ages, suggesting that arecalibration of the Ca II ages may be necessary.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| Chemical enrichment and star formation in the Milky Way disk. III. Chemodynamical constraints
In this paper, we investigate some chemokinematical properties of theMilky Way disk, by using a sample composed by 424 late-type dwarfs. Weshow that the velocity dispersion of a stellar group correlates with theage of this group, according to a law proportional to t0.26,where t is the age of the stellar group. The temporal evolution of thevertex deviation is considered in detail. It is shown that the vertexdeviation does not seem to depend strongly on the age of the stellargroup. Previous studies in the literature seem to not have found it dueto the use of statistical ages for stellar groups, rather thanindividual ages. The possibility to use the orbital parameters of a starto derive information about its birthplace is investigated, and we showthat the mean galactocentric radius is likely to be the most reliablestellar birthplace indicator. However, this information cannot bepresently used to derive radial evolutionary constraints, due to anintrinsic bias present in all samples constructed from nearby stars. Anextensive discussion of the secular and stochastic heating mechanismscommonly invoked to explain the age-velocity dispersion relation ispresented. We suggest that the age-velocity dispersion relation couldreflect the gradual decrease in the turbulent velocity dispersion fromwhich disk stars form, a suggestion originally made by Tinsley &Larson (\cite{tinsley}, ApJ, 221, 554) and supported by several morerecent disk evolution calculations. A test to distinguish between thetwo types of models using high-redshift galaxies is proposed.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/517
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| 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
| New Members of the TW Hydrae Association, β Pictoris Moving Group, and Tucana/Horologium Association
We have identified five new members of the TW Hydrae association (TWA),11 new members of the β Pic moving group, and 11 newTucana/Horologium association members. These are the three youngest(<~30 Myr) known kinematic stellar groups near the Earth. Newlyidentified β Pic group members are located mostly in the northernhemisphere, and they have a slightly different U-component of Galacticvelocity compared to that of previously known members. Tracing themotion of β Pic members backward in time for 12 Myr indicates thatthey might have formed in a small region with an initial velocitydispersion of ~8 km s-1. A couple of mid-M spectral typeβ Pic members show emission features [He Iλ5876+λ6678) and Na D λ5890+λ5896)] seenamong earlier spectral type stars in the TWA and β Pic groups. Toderive the distances of the non-Hipparcos members of these groups, wehave constructed a V-K versus MK color-magnitude diagram thatis very useful in separating young K/M stars from older main-sequencecounterparts and constraining theoretical pre-main-sequence evolutionarytracks. All newly identified K- and M-type members of the three groupsshow saturated X-ray activity(LX/Lbol~10-3). One newly identifiedTWA member, SSS 101727-5354, is estimated to be only 22 pc away fromEarth. Its extreme youth, late spectral type (~M5), and proximity toEarth make SSS 101727-5354 perhaps the best target for direct imagingdetection of cooling planets.
| Detection of moving clusters by a method of cinematic pairs.
Not Available
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| First radial velocities for 146 bright F- and G-type stars
We have obtained an average of 5.7 radial velocities for 146 northernstars of types F and G (all luminosity classes) listed in 'A Supplementto the Bright Star Catalogue' as having no previously known radialvelocity measures. Those were obtained with charge coupled devices(CCDs) and a cross-correlation technique; the intrinsic velocityaccuracy, based on stars of apparently constant velocity, is probablyless than +/- 0.3 km/s per mean. Of those stars 14% are newly discoveredSB2 stars. The prevelance of rapid profile variations (in minutes orhours) in most of the broad-lined F-type stars makes it difficult toobtain accurate measures for them.
| 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
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Some revisions to the Bright Star Catalogue and its supplement
Not Available
| The chromospheric emission-age relation for stars of the lower main sequence and its implications for the star formation rate
An attempt is made to formulate the relationship between age andchromospheric emission (CE) in late-type dwarf stars. Evidence isreviewed that a deterministic relationship of this type actually exists,and that for stars of known age, either a power-law relation or a curvecorresponding to a constant star formation rate fits equally well.Further observations should be able to demonstrate either that there isa real excess of young stars near the sun or that the evolution of CEfor a low-mass star goes through a slow initial decline, a rapid declineat intermediate ages, and finally a slow decline for old stars like thesun.
| Absolute far-ultraviolet spectrophotometry of hot subluminous stars from Voyager
Observations, obtained with the Voyager ultraviolet spectrometers, arepresented of absolute fluxes for two well-known hot subluminous stars:BD + 28 deg 4211, an sdO, and G191 - B2B, a hot DA white dwarf. Completeabsolute energy distributions for these two stars, from the Lyman limitat 912 A to 1 micron, are given. For BD + 28 deg 4211, a single powerlaw closely represents the entire observed energy distribution. For G191- B2B, a pure hydrogen model atmosphere provides an excellent match tothe entire absolute energy distribution. Voyager absolute fluxes arediscussed in relation to those reported from various sounding rocketexperiments, including a recent rocket observation of BD + 28 deg 4211.
| Maximum separations among cataloged binaries
The paper classifies many of the widest common-motion binaries listed inthe Aitken catalog and list 72 physical pairs with known photoelectricphotometry, 31 physical pairs without good photometry, and 27 opticalpairs. As a function of primary types, the physical systems have upperlimits to their separations that are exceeded by some of the opticalpairs. The fact that optical pairs occur with larger separations impliesthat the limits are real ones and not just catalog limitations. Thoselimits (in AU) are expressed by 2500 M1 exp 1.54 for B5-KO main-sequenceprimaries. The same limits hold for the Trapezium and hierarchicalsystems studied previously.
| Photoelectric observations of CPM stars in the BVRI system
A photometric program to estimate the frequency of close unresolvedcompanions in visual binary systems has been developed. The sensitivityof the detection depends on the accuracy of the differential photometricmeasurements of binary components. Unresolved companions up to sevenmagnitudes fainter than the primary are detectable in the BVRIphotometric systems.
| Lithium in F stars of known age
High resolution spectra of 25 F stars in visual binary systems wereobtained with the Canada-France-Hawaii Telescope in a search for acorrelation between Li abundance and the age of F stars. Computerizedscans were made for equivalent widths of the Li 6707 A resonance, the CaI 6717 A line and five neighboring Fe I lines. Stars cooler than 6300 Kexhibited an order of magnitude spread in Li abundance, symptomatic ofage-dependent depletion of Li in late F and early G stars. Li wasdepleted by factors of 6-20 in stars with temperatures from 6300-6700 K,a depletion/temperature pattern also seen in Hyades and mid-F stars, andwhich may be affected by the presence of companion objects.
| Common proper motion stars in the AGK 3
A search was made of common-proper-motion (CPM) systems among AGK 3stars. The selection of physical systems was based upon the ratiobetween the angular separation (rho) and the proper motion (mu); the CPMstars found are presented in two tables. Table I lists systems withrho/mu less than 1000 years. It contains 326 entries, and the proportionof optical pairs is estimated to be 1 percent. Table II lists systemswith rho/mu in the range 1000 to 3500 years; it contains 113 systems,but only 60 percent of them are physical. Nevertheless, these systemsoften have separations larger than 10,000 AU and are the mostinteresting for the study of the tail of the distribution function ofthe semimajor axes.
| Photometry of the comet Halley : solar analogs selected along the path (November 1985 - Mat 1986)
Not Available
| Visual multiples. VIII - 1000 MK types
A total of 1000 new classifications are given for stars brighter than B= 8.0 mag in the Aitken double star catalog. The classificationssupplement 865 classifications obtained in 1981 and 1984. Among thenewly discovered stars are 12 new Ap stars, eight Lambda Bootis stars,one Ba II star, and 60 Am stars. A detailed list of the newclassifications is given.
| A sample of solar-type stars of known age
A sample of field F and G dwarfs of known ages is presented. All thedwarfs are secondaries of visual binaries in which the temperature andgravity of the hotter primary has been estimated from Stromgrenphotometry, and its age derived from reference to isochrones. This ageis taken to apply to the secondary. Even in the case of F-typesecondaries, which themselves have measured Stromgren indices, it ismuch better to estimate ages from the primaries as it is demonstratedthat use of isochrones to determine age of stars not far evolved fromthe main sequence can lead to significant systematic overestimates ofages when the photometric measurements have typical random errors. Anumber of systems in which photometry is available for both componentshave primaries which appear much younger than the secondaries,suggesting a need for further investigation.
| Photometric boxes in the four-color system
Photometric boxes derived from the latest four-color catalogue compiledby Hauck and Mermilliod (1980) containing uvby and H-beta observationsfor more than 15,000 stars are discussed. The construction andpopulation of stellar boxes, the distribution of boxes with spectraltype, the relations between a geneva box and a four-color box, and theuse of stellar boxes for photometric classification are considered. Itis concluded that the method of stellar boxes is a very useful tool inphotometric investigation.
| Errata in Magnitudes of Stars in AGK3
Not Available
| UVBY beta study of A-and F-type visual binaries. I.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976AJ.....81..107M&db_key=AST
| Lithium abundance in stellar atmospheres
Data on the lithium abundance in the atmospheres of 491 stars arecompiled and reduced to a single system. The lithium abundances of thesestars are shown to differ by more than a factor of 1 million, and thedependence of lithium abundance on spectral type is determined forvarious groups of stars. It is found that cooler stars have lowerlithium abundances than hotter stars and that young stars have higherabundances than old stars. Data on the Li-6/Li-7 isotope ratio in theatmospheres of 30 stars are presented which indicate that Li-6 is notobserved in most cases and that the exceptions comprise magneticvariable stars with Li-6/Li-7 ratios of 0.10 to 2.00. It is concludedthat the observational results as a whole are satisfactorily explainedby the hypothesis that the same high lithium abundance characterized allstars at birth but then decreased in the course of evolution due tointermixing of matter.
| Catalog of Indidual Radial Velocities, 0h-12h, Measured by Astronomers of the Mount Wilson Observatory
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970ApJS...19..387A&db_key=AST
| MK classifications for F and G-type stars. I.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1969AJ.....74..916H&db_key=AST
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| Görünürdeki Parlaklýk: | 7.04 |
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