Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Metallicity and absolute magnitude calibrations for UBV photometry
Calibrations are presented here for metallicity ([Fe/H]) in terms of theultraviolet excess, [δ(U - B) at B - V = 0.6, hereafterδ0.6], and also for the absolute visual magnitude(MV) and its difference with respect to the Hyades(ΔMHV) in terms of δ0.6 and(B - V), making use of high-resolution spectroscopic abundances from theliterature and Hipparcos parallaxes. The relation[Fe/H]-δ0.6 has been derived for dwarf plus turn-offstars, and also for dwarf, turn-off, plus subgiant stars classifiedusing the MV-(B - V)0 plane of Fig. 11, which iscalibrated with isochrones from Bergbusch & VandenBerg (and alsoVandenBerg & Clem). The [Fe/H]-δ0.6 relations inour equations (5) and (6) agree well with those of Carney, as can beseen from Fig. 5(a). Within the uncertainties, the zero-points,+0.13(+/-0.05) of equation (5) and +0.13(+/-0.04) of equation (6), arein good agreement with the photometric ones of Cameron and of Carney,and close to the spectroscopic ones of Cayrel et al. and of Boesgaard& Friel for the Hyades open cluster. Good quantitative agreementbetween our estimated [Fe/H] abundances with those from uvby-βphotometry and spectroscopic [Fe/H]spec values demonstratesthat our equation (6) can be used in deriving quality photometric metalabundances for field stars and clusters using UBV data from variousphotometric surveys.For dwarf and turn-off stars, a new hybrid MV calibration ispresented, based on Hipparcos parallaxes withσπ/π <= 0.1 and with a dispersion of +/-0.24in MV. This hybrid MV calibration containsδ0.6 and (B - V) terms, plus higher order cross-termsof these, and is valid for the ranges of +0.37 <= (B - V)0<= +0.88,- 0.10 <= δ0.6 <= +0.29 and 3.44<= MV <= 7.23. For dwarf and turn-off stars, therelation for ΔMHV is revised and updated interms of (B - V) and δ0.6, for the ranges of -0.10<= δ0.6 <= +0.29, and +0.49 <= (B -V)0 <= +0.89, again making use of Hipparcos parallaxeswith σπ/π <= 0.1. These parallaxes formetal-poor dwarf and turn-off stars in our sample reveal that thedifference of ΔMHV(B - V) relative to Hyadesat (B - V) = +0.70 should be 1.37mag, instead of the 1.58mag given byLaird et al. In general, Hipparcos parallaxes are larger thanground-based ones, causing a divergence of ourΔMHV(B - V,δ0.6) relation(the solid line in Fig. 15b), from the one of Laird et al. (the dashedline) for the range +0.10 <= δ0.6 <= +0.29 ourabsolute magnitudes are fainter, as has been confirmed for localsubdwarfs by Reid. Our final calibrations forΔMHV(B - V, δ0.6),equations (16) and (17), are third-order polynomials inδ0.6, pass through the origin, and provide photometricdistances in reasonable agreement with those obtained directly fromHipparcos parallaxes (Fig. 18).
| 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.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Statistical Cataloging of Archival Data for Luminosity Class IV-V Stars. III. The Epoch 2004 [Fe/H] and Temperature Catalogs
In this paper, an updated catalog containing averaged values of [Fe/H]is presented for FGK stars on and near the main sequence. The input datafor the catalog are values of [Fe/H] derived from weak and moderatelystrong lines and published before 2005 July 1. Those data are correctedto a uniform temperature scale, and a statistical analysis is thenapplied to a subset of the data that did not contribute to a previousversion of the catalog. In this way, it is found that an accurate zeropoint for the catalog can be established with an rms error of 0.005 dex.After corrections are applied to a number of the newly added data, it isshown that those corrections help to produce satisfactory zero-pointcoherence among the catalog entries. Standard errors that are derivedfor the catalog data are shown to be accurate. It is also shown thatthose standard errors are based on pervasive scatter in the input data,as is expected if those data are affected by genuine random effects.Samples of the metallicity catalog and an accompanying temperaturecatalog are displayed and discussed. To make possible an effectiveextension of the catalog to stars without catalog entries, a databasecontaining photometric metallicities derived by Nordström et al. isconsidered. Standard errors for those metallicities are derived, andzero-point corrections required to put those data on the catalog zeropoint are presented.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| 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
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are 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/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| 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.
| 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
| High proper motion stars with declinations between -30o and -40o, and right ascensions between 00 h and 10 h 40 m
Proper motions, positions, finding charts and magnitudes are given for147 newly discovered stars with proper motions larger than 0.15arcsec/year. They were found in a search for high proper motion starscarried out in thirteen 5o x 5o areas, locatedbetween -30o and -40o in declination, and 00 h and10 h 40 m in right ascension. Their blue photographic magnitudes rangefrom approximately 9.0 to 18.5. Nine objects from the above sample haveproper motions larger than 0.4 (0.404 to 0.550) arcsec/year. The sametype of data is also presented for 149 Luyten Catalogue (LTT, Luyten\cite{Luyten57}) stars re-discovered during the above search. Anestimated precision level between 6 and 22 mas/year was achieved for theproper motions. Tables 2 and 3 are only available in electronic form atthe CDS, via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/725 Figures 4 and 5are only available in electronic form at http://www.edpsciences.org
| Abundances of light elements in metal-poor stars. III. Data analysis and results
We present the results of the analysis of an extensive set of new andliterature high quality data concerning Fe, C, N, O, Na, and Mg. Thisanalysis exploited the T_eff scale determined in Gratton et al. (1996a),and the non-LTE abundance corrections computed in Gratton et al.(1999a). Results obtained with various abundance indices are discussedand compared. Detailed comparison with models of galactic chemicalevolution will be presented in future papers of this series. Our non-LTEanalysis yields the same O abundances from both permitted and forbiddenlines for stars with T_eff >4600 K, in agreement with King (1993),but not with other studies using a lower T_eff -scale for subdwarfs.However, we obtain slightly smaller O abundances for the most luminousmetal-poor field stars than for fainter stars of similar metallicities,an effect attributed to inadequacies of the adopted model atmospheres(Kurucz 1992, with overshooting) for cool stars. We find a nearlyconstant O overundance in metal-poor stars ([Fe/H]<-0.8), at a meanvalue of 0.46+/- 0.02 dex (sigma =0.12, 32 stars), with only a gentleslope with [Fe/H] ( ~ -0.1); this result is different from the steeperslope recently obtained using OH band in the near UV. If only bonafideunmixed stars are considered, C abundances scale with Fe ones (i.e.[C/Fe]~ 0) down to [Fe/H] ~ -2.5. Due to our adoption of a differentT_eff scale, we do not confirm the slight C excess in the most metalpoor disk dwarfs (-0.8<[Fe/H]<-0.4) found in previousinvestigations. Na abundances scale as Fe ones in the high metallicityregime, while metal-poor stars present a Na underabundance. None of thefield stars analyzed belong to the group of O-poor and Na-rich starsobserved in globular clusters. Na is deficient with respect to Mg inhalo and thick disk stars; within these populations, Na deficiency maybe a slow function of [Mg/H]. Solar [Na/Mg] ratios are obtained for thindisk stars. Tables~ 2 to 9 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strabg.fr/Abstract.html
| A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out followingthe classical recipe: LTE line formation and homogeneous,plane-parallel, flux-constant, and LTE model atmospheres. We reviewdifferent results in the literature that have suggested significantinconsistencies in the spectroscopic analyses, pointing out thedifficulties in deriving independent estimates of the stellarfundamental parameters and hence, detecting systematic errors. Thetrigonometric parallaxes measured by the Hipparcos mission provideaccurate appraisals of the stellar surface gravity for nearby stars,which are used here to check the gravities obtained from thephotospheric iron ionization balance. We find an approximate agreementfor stars in the metallicity range -1.0<=[Fe/H]<=0, but thecomparison shows that the differences between the spectroscopic andtrigonometric gravities decrease toward lower metallicities for moremetal-deficient dwarfs (-2.5<=[Fe/H]<=-1.0), which casts a shadowupon the abundance analyses for extreme metal-poor stars that make useof the ionization equilibrium to constrain the gravity. The comparisonwith the strong-line gravities derived by Edvardsson and Fuhrmannconfirms that this method provide systematically larger gravities thanthe ionization balance. The strong-line gravities get closer to thephysical ones for the stars analyzed by Fuhrmann, but they are evenfurther away than the iron ionization gravities for the stars of lowergravities in Edvardsson's sample. The confrontation of the deviations ofthe iron ionization gravities in metal-poor stars, reported here withdepartures from the excitation balance found in the literature, showthat they are likely to be induced by the same physical mechanism.
| Stellar Iron Abundances: Non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II inthe atmosphere of late-type stars. We used atomic models for Fe I and FeII having, respectively, 256 and 190 levels, as well as 2117 and 3443radiative transitions. Photoionization cross sections are from the IronProject. These atomic models were used to investigate non-LTE (NLTE)effects in iron abundances of late-type stars with different atmosphericparameters. We found that most Fe I lines in metal-poor stars are formedin conditions far from LTE. We derived metallicity corrections of about0.3 dex with respect to LTE values for the case of stars with[Fe/H]~-3.0. Fe II is found not to be affected by significant NLTEeffects. The main NLTE effect invoked in the case of Fe I isoverionization by ultraviolet radiation; thus classical ionizationequilibrium is far from being satisfied. An important consequence isthat surface gravities derived by LTE analysis are in error and shouldbe corrected before final abundance corrections. This apparently solvesthe observed discrepancy between spectroscopic surface gravities derivedby LTE analyses and those derived from Hipparcos parallaxes. A table ofNLTE [Fe/H] and log g values for a sample of metal-poor late-type starsis given.
| 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.
| Revised ages for stars in the solar neighbourhood
New ages are computed for the stars from the Edvardsson et al. (1993)data set. The revised values are systematically larger toward older ages(t>4 Gyr), while they are slightly lower for t<4 Gyr. A similar,but considerably smaller trend is present when the ages are computedwith the distances based on Hipparcos parallaxes. The resultingage-metallicity relation has a small, but distinct slope of ~ em0.07dex/Gyr. Tables 3\to8 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or WWW at URLhttp://cdsweb.u-strasbg.fr/Abstract.html
| 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
| Abundances of light elements in metal-poor stars. I. Atmospheric parameters and a new T_eff_ scale
We present atmospheric parameters for about 300 stars of differentchemical composition, whose spectra will be used to study the galacticenrichment of Fe and light elements. These parameters were derived usingan homogeneous iterative procedure, which considers new calibrations ofcolour-T_eff_ relations for F, G and K-type stars based on Infrared FluxMethod (IRFM) and interferometric diameters for population I stars, andthe Kurucz (1992) model atmospheres. We found that these calibrationsyield a self-consistent set of atmospheric parameters forT_eff_>4400K, representing a clear improvement over results obtainedwith older model atmospheres. Using this T_eff_ -scale and Feequilibrium of ionization, we obtained very low gravities (implyingluminosities incompatible with that expected for RGB stars) formetal-poor stars cooler than 4400K; this might be due either to amoderate Fe overionization (expected from statistical equilibriumcalculations) or to inadequacy of Kurucz models to describe theatmospheres of very cool giants. Our T_eff_ scale is compared with otherscales recently used for metal-poor stars; it agrees well with thoseobtained using Kurucz (1992) models, but it gives much larger T_eff_'sthan those obtained using OSMARCS models (Edvardsson et al. 1993). Thisdifference is attributed to the different treatment of convection in thetwo sets of models. For the Sun, the Kurucz (1992) model appears to bepreferable to the OSMARCS ones because it better predicts the solar limbdarkening; furthermore, we find that our photometric T_eff_ 's formetal-poor stars agree well with both direct estimates based on theIRFM, and with T_eff_'s derived from Hα wings when using Kuruczmodels.
| Analyses of Archival Data for Cool Dwarfs. VI. The 1993 Versions of the Temperature and [Fe/H] Catalogs
This paper contains supplements to two catalogs which have previouslybeen published by the author. One of the supplements updates a catalogof temperatures for FGK dwarfs (see Taylor 1994b). The other supplementupdates a catalog of averaged values of [Fe/H] for the same stars (seeTaylor 1994d). The combined catalogs and supplements include values of[Fe/H] published through the end of 1993. Detailed instructions aregiven for catalog users, including an algorithm for adding data to the[Fe/H] catalog. For recent sources of [Fe/H] which require comment,discussions of issues such as data editing and zero-pint corrections aregiven. A review is also given of a problem described by Gray (1994), whofinds that his spectroscopic temperatures and photometric temperaturesfrom Paper II scatter more around their mean relation than one wouldexpect from the contributing rms errors. It is found that near-Sunreddening (which is Gray's preferred solution of the problem) is not adefensible explanation for this scatter. For the moment, the real sourceof the scatter remains unknown. (SECTION: Stars)
| Carbon abundances in F and G dwarfs.
We have determined carbon abundances or upper limits from the forbidden[CI] line at 8727.13A for 85 F and early G type main sequence stars,with metallicities ranging between [Fe/H]=-1.0 and [Fe/H]=+0.25. The[CI] line has not been previously used for a study of this size. We findthat the C/Fe abundance ratio is slowly decreasing with time andincreasing metallicity in the disk, but with considerable observationalscatter. A discussion of results in the literature supports this result.Our data do not reveal any gradients with galactocentric birth distancefor our stars, but more accurate observations are desirable sincepossible gradients should be important to the question of the synthesisof C and Fe in the disk.
| The Chemical Evolution of the Galactic Disk - Part Two - Observational Data
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102..603E&db_key=AST
| The Chemical Evolution of the Galactic Disk - Part One - Analysis and Results
Not Available
| The extension of the MK spectral classification system to the intermediate population II F type stars
A grid of metal-weak spectral-classification standards is used tosystematically extend the MK spectral-classification system to F-typestars of the intermediate population II. The present method allowsmetal-weak program stars to be compared with standards of similarmetallicity and effective temperature. The results demonstrate that theintermediate population II is very homogeneous. Excellent agreement isobtained between the classifications of the present extended system anduvby-beta photometric results.
| Radial velocities of bright southern stars. V - 146 Population II F stars and related stars
This paper presents new radial velocity determinations, based on 20 A/mmcoude plates, for 146 southern stars brighter than m(v) = 8.3. Drawnfrom the catalog of uvby-beta photometry of southern A5-G0 stars byOlsen (1983), the program stars are certain or suspected Population IIstars. One triple-lined and 10 double-lined binaries have been detected,including HD 210737, for which a preliminary orbit has been derived.Notes on spectral peculiarities are given.
| The South Galactic Pole - Results from uvby-beta photometry of 572 O-F stars
Photometric observations on the uvby-beta system are presented for 572O-F stars within about 20 deg of the South Galactic Pole. Theinterstellar extinction near the pole is found to be zero out to 400 pcfrom the sun, in agreement with the H I maps of Burstein and Heiles(1982). Several evolved and Population II objects are identified.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Sculptor |
| Ascensió Recta: | 00h29m34.85s |
| Declinació: | -32°34'08.4" |
| Magnitud Aparent: | 7.623 |
| Distancia: | 75.758 parsecs |
| Moviment propi RA: | 86 |
| Moviment propi Dec: | 178.7 |
| B-T magnitude: | 8.152 |
| V-T magnitude: | 7.667 |
Catàlegs i designacions:
|
