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Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars
Aims.Parameters for 55 nearby metal-poor stars are determined usinghigh-resolution spectroscopy. Together with similar data taken from arecent analysis, they are used to show trends of their Galacticevolution with stellar [Fe/H] or [Mg/H] abundances. The separation ofabundance ratios between disk and halo stars is used as a basiccriterion for population membership. Methods.After carefulselection of a clean subsample free of suspected or known binaries andpeculiar stars, abundances of Mg, Na and Al are based on NLTE kineticequilibrium calculations applied to spectrum synthesis methods. Results.The relation between [Na/Mg] and [Fe/H] is a continuousenrichment through all three Galactic populations spanning a range ofvalues between a metal-poor plateau at [ Na/Mg] = -0.7 and solar values.[Al/Mg] displays a step-like difference between stars of the Galactichalo with overline[Al/Mg] -0.45 and the two disk populations withoverline[Al/Mg] +0.10. [Al/Mg] ratios, together with the [Mg/Fe]ratios, asymmetric drift velocities V, and stellar evolutionary ages,make possible the individual discrimination between stars of the thickdisk and the halo. At present, this evidence is limited by the smallnumber of stars, and by the theoretical and empirical uncertainties ofstellar age determinations, but it achieves a high significance. Conclusions.While the stellar sample is not complete with respect tospace volume, the resulting abundances indicate the necessity to revisecurrent models of chemical evolution to allow for an adequate productionof Al in early stellar generations.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Tertiary companions to close spectroscopic binaries
We have surveyed a sample of 165 solar-type spectroscopic binaries (SB)with periods from 1 to 30 days for higher-order multiplicity. Asubsample of 62 targets were observed with the NACO adaptive opticssystem and 13 new physical tertiary companions were detected. Anadditional 12 new wide companions (5 still tentative) were found usingthe 2MASS all-sky survey. The binaries belong to 161 stellar systems; ofthese 64 are triple, 11 quadruple and 7 quintuple. After correction forincompleteness, the fraction of SBs with additional companions is foundto be 63% ± 5%. We find that this fraction is a strong functionof the SB period P, reaching 96% for P<3d and dropping to34% for P>12^d. Period distributions of SBs with and withouttertiaries are significantly different, but their mass ratiodistributions are identical. The statistical data on the multiplicity ofclose SBs presented in this paper indicates that the periods and massratios of SBs were established very early, but the periods of SB systemswith triples were further shortened by angular momentum exchange withcompanions.
| Elemental abundance survey of the Galactic thick disc
We have performed an abundance analysis for F- and G- dwarfs of theGalactic thick-disc component. A sample of 176 nearby (d<= 150pc)thick-disc candidate stars was chosen from the Hipparcos catalogue andsubjected to a high-resolution spectroscopic analysis. Using accurateradial velocities combined with the Hipparcos astrometry, kinematics (U,V and W) and Galactic orbital parameters were computed. We estimate theprobability for a star to belong to the thin disc, the thick disc or thehalo. With a probability P>= 70 per cent taken as certain membership,we assigned 95 stars to the thick disc, 13 to the thin disc, and 20 tothe halo. The remaining 48 stars in the sample cannot be assigned withreasonable certainty to one of the three components.Abundances of C, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni,Cu, Zn, Y, Ba, Ce, Nd and Eu have been obtained. The abundances for thethick-disc stars are compared with those for the thin-disc members fromReddy et al. The ratios of α-elements (O, Mg, Si, Ca and Ti) toiron for thick-disc stars show a clear enhancement compared to thin-discmembers in the range -0.3 < [Fe/H] < -1.2. There are also otherelements - Al, Sc, V, Co, and possibly Zn - which show enhanced ratiosto iron in the thick disc relative to the thin disc. The abundances ofNa, Cr, Mn, Ni and Cu (relative to Fe) are very similar for thin- andthick-disc stars. The dispersion in abundance ratios [X/Fe] at given[Fe/H] for thick-disc stars is consistent with the expected scatter dueto measurement errors, suggesting a lack of `cosmic' scatter.A few stars classified as members of the thick disc by our kinematiccriteria show thin-disc abundances. These stars, which appear older thanmost thin-disc stars, are also, on average, younger than the thick-discpopulation. They may have originated early in the thin-disc history, andbeen subsequently scattered to hotter orbits by collisions. The thickdisc may not include stars with [Fe/H] > -0.3. The observedcompositions of the thin and thick discs seem to be consistent with themodels of galaxy formation by hierarchical clustering in a Lambda colddark matter (ΛCDM) universe.
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| Abundances of A/F and Am/Fm stars in open clusters as constraints to self-consistent models including transport processes
We describe the current status of a programme we started a few years agoto observe a large number of A/F and Am/Fm stars in open clusters ofvarious ages. Spectra were obtained with the AURELIE and ELODIEspectrographs at a resolving power of about 40000 and S/N ratios from100 up to 500. Abundances of 11 chemical elements have been derived byusing Takeda's (1995) procedure. A short review on previous abundancedeterminations of A and F dwarfs in open clusters and a progress reporton the current status of this project are presented. New abundancedeterminations for 24 A and F dwarfs in the Coma Berenices cluster arepresented. These abundance determinations serve to set constraints toself-consistent evolutionary models of A and F stars including transportprocesses.
| Statistical properties of solar-type close binaries
Two Coravel radial velocity surveys dedicated to F7-K field dwarfs andto open clusters are merged in order to investigate the statisticalproperties of binaries with periods up to 10 years. Thanks to theaccurate trigonometric parallaxes provided by Hipparcos, an unbiasedsample of spectroscopic binaries (SB) is selected. After correction forthe uncertainties of the measurements, the following results areobtained: 1. The distribution of mass ratios exhibits a peak forequal-mass binaries (twins), which is higher for short-period binariesthan for long-period binaries. 2. Apart from the twins, the distributionof mass ratios exhibits a broad peak from 0.2 to 0.6. 3. The orbitaleccentricities of twins are slightly smaller than those of otherbinaries. 4. An excess of SB is observed with periods shorter than about50 days in comparison with the Duquennoy and Mayor log-normaldistribution of periods. These features suggest that close binary starsare generated by two different processes. A possible difference couldcome from the accretion onto the binary, for instance from a commonenvelope or from a circumbinary disk. Alternatively, twins could comefrom dynamic evolution of multiple systems. It is not clear whether theformation models are already sufficiently elaborated to reproduce ourstatistics.
| 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
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| On the Mass-Ratio Distribution of Spectroscopic Binaries
In this paper we derive the mass-ratio and secondary-mass distributionsof a large, well-defined, complete sample of 129 spectroscopic binarieswith periods between 1 and 2500 days. The binaries, whose orbits werepublished recently, were detected by a systematic radial-velocity surveyof a sample of more than 1400 large proper motion stars. Three featuresstand out in the mass-ratio distribution: a rise as the mass ratio goesdown to q~0.2, a sharp drop below q~0.2, and a smaller peak at q~0.8.Another way to characterize the results is to state that thedistribution includes two ``populations,'' one with a high asymmetricpeak at q~0.2 and another with a smaller peak at q~0.8, while theminimum between the two populations is centered at q~0.55. The size ofthe binary sample allows us to divide it into two subsamples and lookfor differences in the mass-ratio distributions of the two subsamples.We performed two different divisions: one into Galactic halo versus diskpopulations, and the other into high- and low-mass primary stars (aboveand below 0.67 Msolar). The former division yieldsdifferences with moderate statistical significance of 88%, while thelatter is more significant at a level of 97%. Our analysis suggests thatthe rise toward low mass ratios does not appear in the mass-ratiodistribution of the halo binaries. The other separation shows a broadpeak at mass ratio of q~0.8-1 for the subsample of binaries withlow-mass primaries but no corresponding peak in the subsample withhigh-mass primaries. We discuss our findings and their application totheories of binary formation.
| The CHARA Catalog of Orbital Elements of Spectroscopic Binary Stars
Optical interferometry is entering a new age, with several ground-basedlong-baseline observatories now making observations of unprecedentedresolution. Interferometers bring a new level of resolution to bear onspectroscopic binaries, enabling the full extraction of the physicalparameters for the component stars with high accuracy. In the case ofdouble-lined systems, a geometrically determined orbital parallaxbecomes available as well. The first step in preparing to observespectroscopic binaries is to list them, which has not been done sincethe 1989 publication of the Eighth Catalogue of the Orbital Elements ofSpectroscopic Binaries by Batten et al. We present a new catalog withroughly half again as many listings as the Eighth Catalogue. Angularseparation predictions are made for each catalog entry. The numbers ofspectroscopic binaries available for study as a function of severalimportant observational parameters are explored, and in particular, thenumber of spectroscopic binaries as a function of expected separation isdiscussed.
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/721
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries
We report 25,563 radial velocity measurements for 1359 single-linedstars in the Carney-Latham sample of 1464 stars selected for high propermotion. For 171 of these, we present spectroscopic orbital solutions. Wefind no obvious difference between the binary characteristics in thehalo and the disk populations. The observed frequency is the same, andthe period distributions are consistent with the hypothesis that the twosets of binaries were drawn from the same parent population. Thissuggests that metallicity in general, and radiative opacities inparticular, have little influence over the fragmentation process thatleads to short-period binaries. All the binaries with periods shorterthan 10 days have nearly circular orbits, while the binaries withperiods longer than 20 days exhibit a wide range of eccentricities and amedian value of 0.37. For the metal-poor high-velocity halo binaries inour sample, the transition from circular to eccentric orbits appears tooccur at about 20 days, supporting the conclusion that tidalcircularization on the main sequence is important for the oldestbinaries in the Galaxy. Some of the results presented here usedobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| The Spectra of T Dwarfs. I. Near-Infrared Data and Spectral Classification
We present near-infrared spectra for a sample of T dwarfs, including 11new discoveries made using the 2 Micron All Sky Survey. These objectsare distinguished from warmer (L-type) brown dwarfs by the presence ofmethane absorption bands in the 1-2.5 μm spectral region. A firstattempt at a near-infrared classification scheme for T dwarfs is made,based on the strengths of CH4 and H2O bands andthe shapes of the 1.25, 1.6, and 2.1 μm flux peaks. Subtypes T1 V-T8V are defined, and spectral indices useful for classification arepresented. The subclasses appear to follow a decreasing Teffscale, based on the evolution of CH4 and H2O bandsand the properties of L and T dwarfs with known distances. However, wespeculate that this scale is not linear with spectral type for cooldwarfs, due to the settling of dust layers below the photosphere andsubsequent rapid evolution of spectral morphology aroundTeff~1300-1500 K. Similarities in near-infrared colors andcontinuity of spectral features suggest that the gap between the latestL dwarfs and earliest T dwarfs has been nearly bridged. This argument isstrengthened by the possible role of CH4 as a minor absorber,shaping the K-band spectra of the latest L dwarfs. Finally, we discussone peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infraredcolors (J-Ks=-0.89+/-0.24) due to suppression of the 2.1μm peak. The feature is likely caused by enhanced collision-inducedH2 absorption in a high-pressure or low-metallicityphotosphere.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| On the Nature of Low-Mass Companions to Solar-like Stars
Low-mass companions (mass <70 Jupiter masses) to solar-like stars arecompared statistically to stellar-mass secondaries in binaries ofsimilar primary spectral types and orbital scales, based largely on thesurvey of Duquennoy & Mayor. To within the limits imposed byobservational constraints, the orbital properties of these low-masscompanions (LMCs) are statistically indistinguishable from those ofbinary systems. In both populations, orbital periods (P), semimajor axes(a), angular momenta (L), and binding energies (U) are all distributedapproximately as f(x)~x-1 for x=P,a,L,U. In both populations,eccentricities are broadly distributed approximately asf(e)~e-0.5, with no significant correlation with otherorbital elements, apart from a marked circularization of close orbits.The distribution of LMC masses is approximately a power law with indexbetween -1 and -2 there is ambiguous evidence in the data for a massspectrum bimodality about approximately 10 Jupiter masses. In bothpopulations the joint distributions of mass with all orbital propertiesare largely scattergrams, with no statistically significantcorrelations. The overall statistical properties of LMCs are suggestiveof a common formation mechanism with binary star systems. The similarform of the distributions of all orbital dynamic quantities in bothpopulations may result from postformation dissipative orbital decay.
| Homogeneous photometry and metal abundances for a large sample of Hipparcos metal-poor stars*
Homogeneous photometric data (Johnson V, B-V, V-K, Cousins V-I andStrömgren b-y), radial velocities, and abundances of Fe, O, Mg, Si,Ca, Ti, Cr and Ni are presented for 99 stars with high-precisionparallaxes measured by the Hipparcos satellite. These data have beenpreviously used to assist the derivation of accurate distances and agesof Galactic globular clusters. Magnitudes and colours for the programmestars were obtained by combining and standardizing carefully selectedliterature data available in the Simbad data base, and V and B-V valuesmeasured by the Hipparcos/Tycho mission. Comparison of colours for ourtargets suggests: (i) ground-based and Tycho B-V values agree well forcolours bluer than 0.75 mag, but have a lot of scatter for reddercolours; (ii) the Hipparcos V-I colours have a very large scatter and azero-point offset of +0.02 mag compared to the literature values.The programme stars have metal abundances in the range-2.5<[Fe/H]<0.2. The spectroscopic observational data set consistsof high-dispersion (15 000200) spectraobtained at the Asiago and McDonald Observatories for 66 stars. Theanalysis is carried out following the same precepts as those used inprevious papers of this series, and includes corrections for departuresfrom LTE in the formation of the O lines. The main results are: (i) theequilibrium of ionization of Fe is well satisfied in late F-early Kdwarfs; (ii) oxygen and α-elements are overabundant by ~0.3 dex.This large homogeneous abundance data set has been used to recalibratethe abundance scales of Schuster & Nissen, Ryan & Norris andCarney et al. (1994).
| Evolutionary Oddities in Old Disk Population Clusters
With a luminosity zero point fixed by the kinematics of old disksuperclusters (HR 1614, t = 6 Gyr, [Fe/H] = +0.1 dex) and groups(Arcturus, t = 14 Gyr, [Fe/H] = -0.65 dex), the luminosities and colorsof evolved old disk stars, especially red horizontal branch (RHB), earlyasymptotic branch [AGB(1)], thermally pulsing asymptotic giant branch[AGB(2)], and sdOB stars in old disk clusters (NGC 6791, 47 Tuc, M71,M67, Mel 66, NGC 2420, NGC 2204, and NGC 2443) are discussed. (1) TheRHB stars in the old disk all have M_V = +0.7 +/- 0.1 (M_K = -1.3 +/-0.1) mag. (2) Large-amplitude red variables (LARVs) with quasi-stableperiods and light curves are old disk stars on AGB(2). (3) AGB(1)objects include CH stars and semiregular (SRa) variables. (4) Thepopulous and overabundant cluster NGC 6791 may be the only disk clusterwith sdOB stars, populating the lower portion of the bifurcated extendedhorizontal branch that is usual in most ``blue tailed'' and high-densityhalo clusters. (5) Post-red giant branch (RGB) stars in old diskclusters show a B - V (b - y) defect when compared with RGB stars,possibly because of a change in the character of the atmospheres. (6) Ifthe bulk of the LARVs are pulsating in the fundamental mode, R Vir (P =145 days) is possibly a first-overtone pulsator. (7) The overabundantold disk clusters are within the solar circle, with Liller 1 being atthe Galactic center. (8) Several probable RHB stars at the southGalactic pole are identified. (9) The period-age relation, combined withthe known spatial distribution of Galactic LARVs, leads to a relationbetween age and scale height of distribution that monotonicallyincreases with age, leaving no obvious reason for a bifurcation of thepopulation.
| Kinematics and Metallicity of Stars in the Solar Region
Several samples of nearby stars with the most accurate astrometric andphotometric parameters are searched for clues to their evolutionaryhistory. The main samples are (1) the main-sequence stars with b - ybetween 0.29 and 0.59 mag (F3 to K1) in the Yale parallax catalog, (2) agroup of high-velocity subgiants studied spectroscopically by Ryan &Lambert, and (3) high-velocity main-sequence stars in the extensiveinvestigation by Norris, Bessel, & Pickles. The major conclusionsare as follows: (1) The oldest stars (halo), t >= 10-12 Gyr, haveV-velocities (in the direction of Galactic rotation and referred to theSun) in the range from about -50 to -800 km s^-1 and have aheavy-element abundance [Fe/H] of less than about -0.8 dex. The agerange of these objects depends on our knowledge of globular clusterages, but if age is correlated with V-velocity, the youngest may be M22and M28 (V ~ -50 km s^-1) and the oldest NGC 3201 (V ~ -500 km s^-1) andassorted field stars. (2) The old disk population covers the large agerange from about 2 Gyr (Hyades, NGC 752) to 10 or 12 Gyr (Arcturusgroup, 47 Tuc), but the lag (V) velocity is restricted to less thanabout 120 km s^-1 and [Fe/H] >= -0.8 or -0.9 dex. The [Fe/H] ~ -0.8dex division between halo and old disk, near t ~ 10-12 Gyr, is marked bya change in the character of the CN index (C_m) and of the blanketingparameter K of the DDO photometry. (3) The young disk population, t <2 Gyr, is confined exclusively to a well-defined area of the (U, V)velocity plane. The age separating young and old disk stars is also thatseparating giant evolution of the Hyades (near main-sequence luminosity)and M67 (degenerate helium cores and a large luminosity rise) kinds. Thetwo disk populations are also separated by such indexes as the g-indexof Geveva photometry. There appears to be no obvious need to invokeexogeneous influences to understand the motion and heavy-elementabundance distributions of the best-observed stars near the Sun.Individual stars of special interest include the parallax star HD 55575,which may be an equal-component binary, and the high-velocity star HD220127, with a well-determined space velocity near 1000 km s^-1.
| Library of high-resolution UES echelle spectra of F, G, K and M field dwarf stars
We present a library of Utrecht echelle spectrograph (UES) observationsof a sample of F, G, K and M field dwarf stars covering the spectralrange from 4800 \ Angstroms to 10600 \ Angstroms with a resolution of55000. These spectra include some of the spectral lines most widely usedas optical and near-infrared indicators of chromospheric activity suchas Hβ , Mg i b triplet, Na i D1, D2, He iD3, Hα , and Ca ii IRT lines, as well as a large numberof photospheric lines which can also be affected by chromosphericactivity. The spectra have been compiled with the aim of providing a setof standards observed at high-resolution to be used in the applicationof the spectral subtraction technique to obtain the active-chromospherecontribution to these lines in chromospherically active single andbinary stars. This library can also be used for spectral classificationpurposes. A digital version with all the spectra is available via ftpand the World Wide Web (WWW) in both ASCII and FITS formats. Based onobservations made with the William Herschel Telescope operated on theisland of La Palma by the Isaac Newton Group at the Spanish Observatoriodel Roque de Los Muchachos of the Instituto de Astrof\'{\i
| Statistical dynamics of solar-like binaries
A statistical study has been made of dynamic properties of nearbybinaries with solar-like primaries, based largely on the survey ofDuquennoy and Mayor (1991). Emphasis is placed on those quantitieslikely to serve as observational constraints on binary star formation:mass ratios, semimajor axes, orbital angular momenta, and bindingenergies. Nonparametric statistical models have been employed to deducestatistical distributions of these quantities for those classes ofbinaries for which individual values cannot be inferred from theobservations. There is little correlation observed between semimajoraxes and mass ratios or eccentricities, aside from a slight tendency forwide binaries to have low-mass companions and for circularization ofclose binary orbits. The mass ratio distribution for all binaries in theprogram is significantly weighted toward low values, with a possiblesecondary maximum near unity. The distributions of semimajor axes,orbital periods, orbital angular momenta, and binding energies all showapproximately the same nearly featureless, monotonically decreasing,scale-free form spanning several orders of magnitude; there is noevidence for the presence of more than one population of binaries or ofmultiple formation mechanisms. The specific angular momenta of binarysystems are approximately two orders of magnitude less than those of thesmallest dense cloud cores from which they presumably form.
| Ages of Globular Clusters from HIPPARCOS Parallaxes of Local Subdwarfs
We report here initial but strongly conclusive results for absolute agesof Galactic globular clusters (GGCs). This study is based onhigh-precision trigonometric parallaxes from the HIPPARCOS satellitecoupled with accurate metal abundances ([Fe/H], [O/Fe], and [ alpha/Fe]) from high-resolution spectroscopy for a sample of about thirtysubdwarfs. Systematic effects due to star selection (Lutz-Kelkercorrections to parallaxes) and the possible presence of undetectedbinaries in the sample of bona fide single stars are examined, andappropriate corrections are estimated. They are found to be small forour sample. The new data allow us to reliably define the absolutelocation of the main sequence (MS) as a function of metallicity. Theseresults are then used to derive distances and ages for a carefullyselected sample of nine globular clusters having metallicitiesdetermined from high-dispersion spectra of individual giants accordingto a procedure totally consistent with that used for the fieldsubdwarfs. Very precise and homogeneous reddening values have also beenindependently determined for these clusters. Random errors for ourdistance moduli are +/-0.08 mag, and systematic errors are likely of thesame order of magnitude. These very accurate distances allow us toderive ages with internal errors of ~12% (+/-1.5 Gyr). The main resultsare: 1. HIPPARCOS parallaxes are smaller than corresponding ground-basedmeasurements, leading, in turn, to longer distance moduli (~0.2 mag) andyounger ages (~2.8 Gyr). 2. The distance to NGC 6752 derived from our MSfitting is consistent with that determined using the white dwarf coolingsequence. 3. The relation between the zero-age HB (ZAHB) absolutemagnitude and metallicity for the nine program clusters isMV({ZAHB})=(0.22+/-0.09)([{Fe}/{H}]+1.5)+(0.49+/-0.04) . Thisrelation is fairly consistent with some of the most recent theoreticalmodels. Within quoted errors, the slope is in agreement with that givenby the Baade-Wesselink (BW) analysis of RR Lyrae stars by Fernley andClementini et al., while it is somewhat shallower than the relationgiven by Sandage. The zero-point is 0.2 to 0.3 mag brighter than thatobtained with BW, while it agrees fairly well with that given bySandage. A comparison with alternative relationships is brieflydiscussed. 4. The corresponding LMC distance modulus is (m - M)0 = 18.60+/- 0.07, in good agreement with the recent values of 18.70 +/- 0.10 and18.54 +/- 0.2 derived by Feast & Catchpole and van Leeuwen et al.,respectively, from HIPPARCOS parallaxes of Galactic Cepheid and Miravariables. 5. The age of the bona fide old globular clusters (OosterhoffII and BHB), based on the absolute magnitude of the turnoff (atheoretically robust indicator) is {Age}=11.8^{+2.1}_{-2.5} {Gyr} ,wherethe error bar is the 95% confidence range. The rms scatter of individualages around the mean value is ~10%, in agreement with expectations fromobservational errors alone (that is, we do not find it necessary tointroduce a real age scatter among these clusters). A reliable study ofthe relative ages requires the use of age indicators better suited tothis purpose and data for a larger sample of GGCs. 6. Allowing for aminimum delay of 0.5 Gyr from the birth of the universe until theformation of globular clusters, our age estimate is compatible with anEinstein-de Sitter model if H0 <= 64 km s-1 Mpc-1, or H0 <= 83 kms-1 Mpc-1 in a flat universe with Omega m = 0.2. Since these upperlimits are well within the confidence range of most determinations ofH0, we conclude that the present age of globular clusters does notconflict with standard inflationary models of the universe.
| ROSAT all-sky survey observations of PopII field binaries: X-ray activity of old, metal-poor stellar coronae.
This study represents the first X-ray observations of an extended sampleof Pop II field binaries, aimed at investigating the properties of old,metal-poor stellar coronae. Analysing X-ray observations from the ROSATall-sky survey, we detected only 13 (out of 86) Pop II systems (15%detection rate). The X-ray luminosity function, taking into account bothdetections and upper limits, has its median at logL_x_<=28.1erg/s,indicating a low average X-ray luminosity, with a high-luminosity tailat logL_x_~29-31erg/s. The only extreme metal-poor system detected is HD89499. Thus, the detection rate of extreme Pop II systems is lower thanof intermediate Pop II, possibly indicating extreme Pop II to betypically less luminous. The X-ray luminosity is not very wellcorrelated with orbital period; long-period Pop II binaries may havehigh X-ray luminosities and, surprisingly, short-period systems are notper se strong X-ray emitters. For a subsample of emission-line Pop IIbinaries, i.e. the halo component analogs to the RS CVn binaries, themedian X-ray luminosity is at least one order of magnitude lower thanfor the RS CVns. The lower activity levels of the Pop II systems may becaused in part by the presence of fewer evolved stars in the sample andlower metallicity. The extremely old age of Pop II binaries may alsogive rise to the unexpectedly low X-ray luminosities of some systems(e.g., CD-481741, BD+53080).
| Eccentricity versus Mass for Low-Mass Secondaries and Planets
Spectroscopic orbits have been reported for six unseen companionsorbiting solar-type stars with minimum possible masses in the range0.5--10 Jupiter masses. The four least massive companions, around 51Peg, 47 UMa, 55 Cnc, and tau Boo, have nearly circular orbits, while thetwo most massive companions, around HD 114762 and 70 Vir, haveeccentricities of 0.35 and 0.40. We compare the orbital eccentricitiesof these six planet candidates with the eccentricities of the planets inthe solar system, of the three planets found around the pulsar PSRB1957+12, and of the low-mass secondaries in a subsample of thespectroscopic binaries from the Carney-Latham proper-motion survey. Thedistribution of eccentricities for the combined samples displays astriking pattern: the companions with masses smaller than about 5Jupiter masses have circular orbits, while the more massive companionshave eccentric orbits. We outline four possible scenarios that mighthave produced this pattern of eccentricity versus mass.
| Detecting faint secondaries of spectroscopic binaries: HD 101177B and 149414A
We have observed HD 101177B and 149414A, two known single-linedbinaries, and directly measured the radial velocities of theirsecondaries, enabling us to derive the mass ratios of the two systems.Using high signal-to-noise ratio spectra obtained mostly with theWilliam Herschel Telescope, and a new algorithm to analyse compositespectra, TODCOR, we show that the mass ratios of HD 101177B and 149414Aare 0.48+/-0.03 and 0.57+/-0.02, respectively. These are two of thelowest mass ratios ever measured around normal G or K dwarf stars. Thesetwo systems demonstrate the potential of the present approach, which canturn many of the known single-lined binaries into double-lined systems.Such work has important implications for the study of the mass-ratiodistribution of many samples of binaries.
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| Sað Açýklýk: | 12h29m42.74s |
| Yükselim: | -03°19'58.7" |
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