תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
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
| HIPPARCOS subdwarf parallaxes - Metal-rich clusters and the thick disk
We have used main-sequence fitting to calibrate the distances to theglobular clusters NGC 6397, M5, NGC 288, M71, and 47 Tucanae, matchingthe cluster photometry against data for subdwarfs with precise Hipparcosparallax measurements and accurate abundance determinations. Both thecluster and subdwarf abundance scales are tied to high-resolutionspectroscopic analyses. The distance moduli that we derive for the fiveclusters are 12.24, 14.52, 15.00, 13.19, and 13.59 mag, withuncertainties of 0.15 mag. These distances are higher than those derivedin pre-Hipparcos investigations. The calibrated cluster color-magnitudediagrams provide fiducial sequences in the (MV, B - V)-plane, outliningthe distribution expected for stars of a particular abundance. We havecombined the photometric data for NGC 6397, M5, and 47 Tucanae with themean color-magnitude relation delineated by nearby FGK dwarfs to definea reference grid in the (MV, B - V)-plane, and we have matched this gridagainst data for stars drawn from the Lowell Proper Motion Survey.Limiting the comparison to nonbinaries, there are significantly fewersubluminous stars than expected given the spectroscopic metallicitydistribution. Inverting the analysis, this implies a reduction by afactor of three in the proportion of stars contributing to themetal-poor tail of the Galactic disk. We discuss the implications ofthese results.
| 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
| Spectroscopic analyses of metal-poor stars. III. Magnesium abundances.
We present magnesium abundances and Mg/Fe ratios for 56 metal-poordwarfs and subgiants based on differential LTE model atmosphere analysesof spectra that have a typical S/N of 50-100 and a resolution of about20000-30000. The distribution of Mg/Fe abundance ratios with Feabundances is characterized by a steep increase of [Mg/Fe] near[Fe/H]=-0.6. At that iron abundance we recognize an abrupt change from asolar value for stars with [Fe/H]>=-0.6 to an upper limit of +0.4dexfor the metal-poor stars. Our data suggest that this step in the Mg/Feabundance ratio at [Fe/H]=-0.6 is the result of the onset of Galacticdisk formation, whereas [Mg/Fe]=+0.4 apparently is an upper limitarising from the constraints of nucleosynthesis in massive SN II eventsduring the first stellar generations, the yields of which we observe inthe most metal-poor halo stars.
| Spectroscopic analyses of metal-poor stars. II. The evolutionary stage of subdwarfs.
Models of post-main sequence stellar evolution of VandenBerg & Bell(???) have been applied to determine spectroscopic masses and distancesfor metal-poor stars. Careful consideration of the most important errorsources published in more recent papers such as VandenBerg (???) for thefirst time allow us to draw firm statistical conclusions. It is shownthat the evolutionary calculations qualitatively fit to the observedstellar parameters whereas quantitatively they predict too high ages formetal-poor stars. As an important result we confirm that evolutionarysequences need to be calibrated with respect to their metal abundance inorder to use their absolute predictions of temperature and luminosity.It turns out that this can be achieved by a simple shift of theevolutionary tracks and isochrones in effective temperature with values{DELTA}log T_eff_<~0.03 which accounts for possible changes of themixing-length and the O/Fe ratio with metallicity. The stellarluminosities and surface gravities obtained from evolutionary models aremuch more reliable than their effective temperatures. Therefore weconclude that the accuracy of the corresponding spectroscopic stellargravities is systematically affected by deviations from LTE, inparticular along the subgiant sequence where systematic errors less than{DELTA}log g =~0.3 must be ascribed to the non-LTE ionizationequilibrium of Fe II/Fe I. In our spectroscopic analyses the strongdependence between surface gravity and abundances determined from Fe Ilines restricts the accuracy of Fe abundances in subgiants to 0.1 dex atbest. The most remarkable result of our evolutionary and kinematicinvestigations of halo stars refers to the large fraction of slightlyevolved subgiants among the so-called subdwarfs. Since conventionalphotometric approaches often assume that the great majority ofmetal-poor stars are dwarfs this results in distances that aresystematically too low for their samples. Consequently, significantdifferences are found when comparing evolutionary and kinematicparameters obtained from either photometric or spectroscopic data. Wedemonstrate this by comparing the space velocities of the stars. Itappears that stars with particularly high space velocities derived fromspectroscopic distances show very often much lower velocities based ontheir main sequence parallaxes. We find that results refering to mainsequence parallaxes are doubtful and can be used only with greatestcare. An advantageous side-effect of the application of spectroscopicdata to evolutionary calculations is the possibility to identify binarysystems that are either standing out from the Toomre diagram with theirunusually high space velocities, or from a log g - log T_eff_ diagramwith apparently contradictory luminosities.
| Spectroscopic analyses of metal-poor stars. 1: Basic data and stellar parameters
Accurate stellar parameters have been obtained from the analyses of morethan thousand spectra of 115 metal-poor dwarfs and subgiants with visualmagnitudes brighter than V approximately equal to 12. The stellar samplewas selected mainly by high proper motion, with additional restrictionsfrom B-V colors and U-B excesses. The effective temperatures cover arange from 5000 to 6500 K while abundances are found between (M/H) =-0.1 and -0.3 dex. Based on homogeneous ODF blanketed model atmospheresin local thermodynamic equilibrium (LTE) and working differentially withrespect to the Sun we derive a consistent set of stellar parameters,effective temperature, surface gravity, metal abundance andmicroturbulence velocity. Individual profile synthesis is applied to anumber of spectral lines for each star, which has led as a rule toaccuracies in Teff of better than 100 K, in log g of betterthan 0.15, and in (Fe/H) of better than 0.1 dex. Because of theconsistent treatment with only one type of model atmosphere, this sampleprovides an oppurtunity to examine the individual parametersstatistically and investigate in detail their relation to the formationand evolution of the Galaxy. One aspect of this analysis is a generalshift to higher iron abundances for the most metal-poor stars. Alongwith the recently preferred meteoritic solar iron abundance andeffective temperatures from conistent Balmer line profile fits that tendto be 100-200 K hotter than found from photometric calibrations,discrepancies of up to 0.5 dex in (Fe/H) can be explained in comparisonwith other abundance analyses. The most important results refer to theevolutionary status of the bona fide subdwarf sample. Irrespective ofthe different effective temperatures found here, there exists a severeproblem when comparing post main sequence evolutionary models of coolstars with our observed parameters. Even more interesting is the fact,demonstrated by the results of a consistent analysis of the ironionization equilibrium, that roughly half of the subdwarfs aresubgiants, some of them having nearly reached the bottom of the giantbranch.
| 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
| Balmer lines in cool dwarf stars II. Effective temperatures and calibration of colour indices
Effective temperatures obtained from synthesis of the extended profilewings of the first four Balmer lines are presented for more than 100dwarfs and subgiants of different metal abundances and surface gravitiesin the temperature range from 5000 to 6500 K. Line formation is based onhomogeneous ODF blanketed model atmospheres in LTE. The resultingtemperatures of the more metal-rich stars differ systematically fromthose determined by reference to synthetic broad- or intermediate-bandcolours such as B-V , b-y , R-I or V-K . While the Balmer linetemperatures give room to only very small individual errors and resultin a convincingly small mean error for all four lines, the scatteragainst temperatures determined from broad-band colours is by faroutside the internal errors claimed in recent applications. This may beattributed to either (a) observational errors, (b) dependence on therelative mixture of metal abundances, (c) unknown line blocking in mostof the visible and near-infrared spectrum or (d) the inhomogeneity foundin the granular patterns of stellar surfaces. Our results suggest thatbroad-band colours are insufficient individual temperature indicators,reliable only in a statistical sense.
| Astrometric and astrophysical discontinuities between the galactic old disk and halo stellar populations
Intermediate band, RI, and DDO photometry of the weak-lined stars in thefirst three volumes of the Michigan catalogs of spectral type arediscussed on the basis of luminosity and heavy element abundance. Theinterface between the old disk (Fe/H greater than -0.8 dex) and halo(Fe/H less than -1.2 dex) populations represents discontinuities in boththe stellar motions and the stellar physics. The CN strengths of bothevolved and unevolved halo stars decrease with decreasing temperature,in a mirror image of the increase with decreasing temperature for thedisk objects. The result for the halo giants has been attributed to deepmixing in the stellar atmospheres but the similar result for unevolveddwarfs indicates a difference in formation rather than in evolutionaryprocess of the two populations.
| Population studies. I - The Bidelman-MacConnell 'weak-metal' stars
BRVI and DDO photometry are presented for 309 Bidelman-MacConnell'weak-metal' stars. Radial velocities are calculated for most of thestars having Fe/H abundances of no more than -0.8. The photometricobservations were carried out using the 0.6-meter and 1.0-metertelescopes of the Siding Spring Observatory. Photometric taxonomy wasused to classify the stars as dwarfs, giants, red-horizontal branchstars, and ultraviolet-bright stars, respectively. It is found that 35percent of the stars are giants; 50 percent are dwarfs; and 5 percentbelong to the red-horizontal branch group. The role of selection effectsin investigations of the formation of the Galaxy is discussed on thebasis of the photometric observations and the observational constraintsproposed by Eggen et al. (1962).
| Weak-lined stars in the Michigan spectral survey. I - A-, F-, and G-type stars
Data from intermediate-band and H-beta photometric observations of 89early-type stars classified as weak-lined by Bidelman and MacConnell(1973), Houk and Cowley (1975), or Houk (1978 and 1982) are presented intables and graphs and analyzed. The group is found to comprise 55 diskstars and 34 halo stars (Fe/H less than -0.65), including 21 near themain-sequence turnoff, seven red horizontal-branch stars, one bluehorizontal-branch star, one suprahorizontal-branch object, and four bluestragglers; five of the newly identified halo stars are shown to haveradial velocities in excess of 100 km/s. Special consideration is givento the 17 early-type members of Kapetyn's star group (Eggen, 1977), andthe mean luminosity and Fe/H ratio of the four very-short-periodCepheids of this group are determined as 0.75 mag and -1.7.
| Southern metal-poor stars - UBVRI photometry
Considering the study of subdwarf kinematics and metallicities by Eggen,Lynden-Bell, and Sandage (1962), UBVRI photometry and normalizedultraviolet excesses are presented for 178 metal-poor stars, 144 ofwhich are contained in the kinematically unbiased list of Bidelman andMacConnell (1973). The Lowell 0.6 m telescope at Cerro Tololo was used,equipped with a single-channel photometer and a Ga-As photomultiplier.The final magnitudes and colors, number of observations, value ofdelta(U-B)0.6 (if B-V lies between 0.35 and 0.90), B and M class, andpublished spectral types for these stars are presented; severalextremely metal-poor stars are evident. In addition, sixteen nearbyvisual companions of the stars were measured, and their magnitudes andcolors are given.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
קטלוגים וכינוים:
|
