Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Elemental abundances in the atmosphere of clump giants
Aims.The aim of this paper is to provide the fundamental parameters andabundances for a large sample of local clump giants with a highaccuracy. This study is a part of a big project, in which the verticaldistribution of the stars in the Galactic disc and the chemical anddynamical evolution of the Galaxy are being investigated. Methods:.The selection of clump stars for the sample group was made applying acolour-absolute magnitude window to nearby Hipparcos stars. Theeffective temperatures were estimated by the line depth ratio method.The surface gravities (log {g}) were determined by two methods (thefirst one was the method based on the ionization balance of iron and thesecond one was the method based on fitting of the wings of the Ca I6162.17 Å line). The abundances of carbon and nitrogen wereobtained from the molecular synthetic spectrum, and the Mg and Naabundances were derived using the non-LTE approximation. The "classical"models of stellar evolution without atomic diffusion androtation-induced mixing were employed. Results: .The atmosphericparameters ({T_eff}, log {g}, [Fe/H], {Vt}) and Li, C, N, O,Na, Mg, Si, Ca, and Ni abundances in 177 clump giants of the Galacticdisc were determined. The underabundance of carbon, overabundance ofnitrogen, and "normal" abundance of oxygen were detected. A small sodiumoverabundance was found. A possibility of a selection of the clumpgiants based on their chemical composition and the evolutionary trackswas explored. Conclusions: .The theoretical predictions based onthe classical stellar evolution models are in good agreement with theobserved surface variations of the carbon and nitrogen just after thefirst dredge-up episode. The giants show the same behaviour of thedependencies of O, Mg, Ca, and Si (α-elements) and Ni (iron-peakelement) abundances vs. [Fe/H] as dwarfs do. This allows us to use suchabundance ratios to study the chemical and dynamical evolution of theGalaxy.
| 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.
| The lithium content of the Galactic Halo stars
Thanks to the accurate determination of the baryon density of theuniverse by the recent cosmic microwave background experiments, updatedpredictions of the standard model of Big Bang nucleosynthesis now yieldthe initial abundance of the primordial light elements withunprecedented precision. In the case of ^7Li, the CMB+SBBN value issignificantly higher than the generally reported abundances for Pop IIstars along the so-called Spite plateau. In view of the crucialimportance of this disagreement, which has cosmological, galactic andstellar implications, we decided to tackle the most critical issues ofthe problem by revisiting a large sample of literature Li data in halostars that we assembled following some strict selection criteria on thequality of the original analyses. In the first part of the paper wefocus on the systematic uncertainties affecting the determination of theLi abundances, one of our main goal being to look for the "highestobservational accuracy achievable" for one of the largest sets of Liabundances ever assembled. We explore in great detail the temperaturescale issue with a special emphasis on reddening. We derive four sets ofeffective temperatures by applying the same colour {T}_eff calibrationbut making four different assumptions about reddening and determine theLTE lithium values for each of them. We compute the NLTE corrections andapply them to the LTE lithium abundances. We then focus on our "best"(i.e. most consistent) set of temperatures in order to discuss theinferred mean Li value and dispersion in several {T}_eff and metallicityintervals. The resulting mean Li values along the plateau for [Fe/H]≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075when the lowest effective temperature considered is taken equal to 5700K and 6000 K respectively. This is a factor of 2.48 to 2.81 (dependingon the adopted SBBN model and on the effective temperature range chosento delimit the plateau) lower than the CMB+SBBN determination. We findno evidence of intrinsic dispersion. Assuming the correctness of theCMB+SBBN prediction, we are then left with the conclusion that the Liabundance along the plateau is not the pristine one, but that halo starshave undergone surface depletion during their evolution. In the secondpart of the paper we further dissect our sample in search of newconstraints on Li depletion in halo stars. By means of the Hipparcosparallaxes, we derive the evolutionary status of each of our samplestars, and re-discuss our derived Li abundances. A very surprisingresult emerges for the first time from this examination. Namely, themean Li value as well as the dispersion appear to be lower (althoughfully compatible within the errors) for the dwarfs than for the turnoffand subgiant stars. For our most homogeneous dwarfs-only sample with[Fe/H] ≤ 1.5, the mean Li abundances are A(L)_NLTE = 2.177±0.071 and 2.215±0.074 when the lowest effective temperatureconsidered is taken equal to 5700 K and 6000 K respectively. This is afactor of 2.52 to 3.06 (depending on the selected range in {T}_eff forthe plateau and on the SBBN predictions we compare to) lower than theCMB+SBBN primordial value. Instead, for the post-main sequence stars thecorresponding values are 2.260±0.1 and 2.235±0.077, whichcorrespond to a depletion factor of 2.28 to 2.52. These results,together with the finding that all the stars with Li abnormalities(strong deficiency or high content) lie on or originate from the hotside of the plateau, lead us to suggest that the most massive of thehalo stars have had a slightly different Li history than their lessmassive contemporaries. In turn, this puts strong new constraints on thepossible depletion mechanisms and reinforces Li as a stellartomographer.
| 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.
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/419/167
| Lithium Abundance of Metal-poor Stars
High-resolution, high signal-to-noise ratio spectra have been obtainedfor 32 metal-poor stars. The equivalent widths of Li λ6708Åwere measured and the lithium abundances were derived. The averagelithium abundance of 21 stars on the lithium plateau is 2.33±0.02dex. The Lithium plateau exhibits a marginal trend along metallicity,dA(Li)/d[Fe/H] = 0.12±0.06, and no clear trend with the effectivetemperature. The trend indicates that the abundance of lithium plateaumay not be primordial and that a part of the lithium was produced inGalactic Chemical Evolution (GCE).
| A grid of synthetic spectra and indices Fe5270, Fe5335, Mgb and Mg2 as a function of stellar parameters and [alpha/Fe]
We have computed a grid of synthetic spectra in the wavelength rangelambda lambda 4600-5600 Å using revised model atmospheres, for arange of atmospheric parameters and values of [alpha -elements/Fe] = 0.0and +0.4. The Lick indices Fe5270, Fe5335, Mgb and Mg2 aremeasured on the grid spectra for FWHM = 2 to 8.3 Å. Relationsbetween the indices Fe5270, Fe5335 and Mg2 and the stellarparameters effective temperature Teff, log ; g, [Fe/H] and[alpha /Fe], valid in the range 4000 <= Teff <= 7000 K,are presented. These fitting functions are given for FWHM = 3.5 and 8.3Å. The indices were also measured for a list of 97 reference starswith well-known stellar parameters observed at ESO and OHP, and theseare compared to the computed indices. Finally, a comparison of theindices measured on the observed spectra and those derived from thefitting functions based on synthetic spectra is presented.Observations collected at the European Southern Observatory (ESO), LaSilla, Chile and at the Observatoire de Haute Provence (OHP), St-Michel,France.All Tables of Appendices A and B are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/404/661
| 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
| 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.
| 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.
| Abundances and Kinematics of Field Stars. II. Kinematics and Abundance Relationships
As an investigation of the origin of ``α-poor'' halo stars, weanalyze kinematic and abundance data for 73 intermediate-metallicitystars (-1>[Fe/H]>=-2) selected from Paper I of this series. We findevidence for a connection between the kinematics and the enhancement ofcertain element-to-iron ([X/Fe]) ratios in these stars. Statisticallysignificant correlations were found between [X/Fe] and galacticrest-frame velocities (vRF) for Na, Mg, Al, Si, Ca, and Ni,with marginally significant correlations existing for Ti and Y as well.We also find that the [X/Fe] ratios for these elements all correlatewith a similar level of significance with [Na/Fe]. Finally, we comparethe abundances of these halo stars against those of stars in nearbydwarf spheroidal (dSph) galaxies. We find significant differencesbetween the abundance ratios in the dSph stars and halo stars of similarmetallicity. From this result, it is unlikely that the halo stars in thesolar neighborhood, including even the ``α-poor'' stars, were oncemembers of disrupted dSph galaxies similar to those studied to date.
| Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,high signal-to-noise ratio survey of 168 stars, most of which aremetal-poor dwarfs. We follow a self-consistent LTE analysis technique todetermine the stellar parameters and abundances, and we estimate theeffects of random and systematic uncertainties on the resultingabundances. Element-to-iron ratios are derived for key α-, odd-Z,Fe-peak, and r- and s-process elements. Effects of non-LTE on theanalysis of Fe I lines are shown to be very small on average.Spectroscopically determined surface gravities are derived that arequite close to those obtained from Hipparcos parallaxes.
| Oxygen abundance in halo stars from O i triplet
Oxygen abundance for 14 halo stars through the O I 7774 Ätriplethave been derived from high resolution spectra (R = 25,000; S/N >100) obtained with echelle-spectrometer of 6-m telescope of SpecialAstrophysical Observatory of the Russian Academy of Sciences. Theeffective temperature, metallicity and other parameters have beenexamined. For example, the effective temperature was found from H_alphaline wings and photometric indices. The abundance analysis was carriedout using both LTE and non-LTE conceptions. For this aim, we havespecified the oxygen atomic model. The average [O/Fe] value appeared tobe 0.61 +/- 0.21 from the non-LTE determination. A trend of oxygenabundance increasing along with the iron abundance decreasing was found.The relation between [O/Fe] and [Fe/H] is linear:[O/Fe]=-0.370x[Fe/H]+0.047. In addition to the sample of our programstars, we also involved in the analysis, 24 targets from Cavallo et al.(\cite{Cav}). For their original results we have determined thenecessary non-LTE corrections. Our data are compared with the results ofother works (Tomkin et al., \cite{Tom}; King & Boesgaard,\cite{King2}; Boesgaard et al. \cite{BK2}).
| High resolution spectroscopy of selected faint stars
Not Available
| Lithium abundances in metal-poor stars. I. New observations
We present the lithium measurements of a continuing programme of lightelement abundances in metal-poor stars. New equivalent widths of the Lii lambda 670.8 nm resonance line in 67 metal-poor stars covering themetallicity range -3.5 <= [Fe/H] <= -0.4 are reported. For abouthalf of this sample, the observations presented here represent the firstmeasurement of the Li i line. The sample allowed a statisticalcomparison with previous measurements from other authors and a study ofthe consistency and reliability of the quoted error bars. This papershows that for most of the stars these error bars are good estimates ofthe true uncertainties associated with the determination of theequivalent widths of the Li i line. However, about 20% of the stars withtwo or more independent measurements show discrepancies in the Li iequivalent widths; in these cases, other sources of uncertainty notproperly taken into account (binarity effects, cosmic rays, imperfectflat-field correction, continuum determination, etc.) could also beimportant. Conclusions on the possible lithium abundance trends versuseffective temperature or metallicity and on any intrinsic scatter shouldbe treated cautiously until their robustness vis-a-vis these additionaluncertainties is proved. Based on observations made with the IsaacNewton and Nordic Optical Telescopes, which are operated on the islandof La Palma by the Isaac Newton Group and the NOT ScientificAssociation, respectively, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.
| Chromospheric CaII H and K emission among subdwarfs
Echelle spectra have been obtained of the CaII H and K lines for asample of metal-poor subdwarf stars as well as for a number of nearbyPopulation I dwarfs selected from among those included in the MountWilson HK survey. The main conclusion of this paper is that CaII H- andK-line emission does occur among subdwarfs. It is particularly notableamong those subdwarfs with colours of B-V>=0.75 all such starsobserved exhibit chromospheric emission, although emission is observedamong some subdwarfs bluer than this colour. The CaII K emission profilein most subdwarfs exhibits an asymmetry of V/R>1, similar to thatseen in the integrated light of the solar disc. Two quantitativeindicators of the contrast between the peaks in the emission profile andthe neighbouring photospheric line profile are introduced. Measurementsof these indicators show that the level of CaII emission among thesubdwarfs is similar to that among low-activity Population I dwarfs.
| Determination of the temperatures of selected ISO flux calibration stars using the Infrared Flux Method
Effective temperatures for 420 stars with spectral types between A0 andK3, and luminosity classes between II and V, selected for a fluxcalibration of the Infrared Space Observatory, ISO, have been determinedusing the Infrared Flux Method (IRFM). The determinations are based onnarrow and wide band photometric data obtained for this purpose, andtake into account previously published narrow-band measures oftemperature. Regression coefficients are given for relations between thedetermined temperatures and the photometric parameters (B2-V1), (b-y)and (B-V), corrected for interstellar extinction through use ofHipparcos parallaxes. A correction for the effect of metallicity on thedetermination of integrated flux is proposed. The importance of aknowledge of metallicity in the representation of derived temperaturesfor Class V, IV and III stars by empirical functions is discussed andformulae given. An estimate is given for the probable error of eachtemperature determination. Based on data from the ESA HipparcosAstrometry Satellite.
| 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
| Oxygen Abundances in Metal Poor Subgiant Stars from the O I Triplet
This paper examines the oxygen abundance of 24 main-sequence turn-offand subgiant stars through the permitted O I triplet near $\lambda$7774{\AA}. In agreement with prior investigators, we find that the Oabundance from the triplet is about 0.53 dex higher than is typicallyfound from other means such as the [O I] $\lambda$6300 {\AA} line or theUV and IR bands from OH. Our average [O/Fe] is +0.79 $\pm$ 0.29 (1$\sigma$), with the uncertainties in the individual methods beingdominated by systematic errors in our incomplete understanding of theformation of these high excitation lines under non-LTE conditions.Several methods of resolving the on-going discrepancy are discussed indetail. We find that the problem is not fixed by simply changing themodel atmosphere parameters, and that the best methods seem to require adetailed non-LTE analysis based on both the structure of the oxygen atomand the source function within and above the photosphere. The trend ofincreasing O with decreasing Fe is briefly explored and is found to bequalitatively consistent with other works. (SECTION: Stars)
| Subdwarfs: CNO abundances.
Not Available
| The atmospheric chemical composition of stars with large proper motions
Not Available
| The Average Oxygen Abundance in the Globular Cluster M13
Abstract image available at:http://adsabs.harvard.edu/abs/1996AJ....111.1175P
| The first results of Lithium content investigation at the 6m telescope
Not Available
| Spectroscopic investigation of stars with large proper motions.
Not Available
| Broad band JHK infrared photometry of an extended sample of late type dwarfs and subdwarfs.
The results of a long term programme of broad band JHK photometry, for asample of 360 late type stars, made at the Observatorio del Teide(Tenerife, Spain) are presented. Transformations between thesemagnitudes and those of several currently used systems (CIT (Elias etal. 1982 and Carney 1983), Johnson (Johnson 1966, and Lee 1970), and ESO(Bouchet et al. 1991)) are proposed. A comparison to the narrow-bandsystem of Selby et al. (1988) has been made, in order to check theaccuracy of the photometric system. A mean internal accuracy better than0.02mag in the three bands can be inferred from the comparison to thelarge number of stars in common with Carney (1983), and from thedispersion of the multiple measured stars. The list of standards, thefilter passbands and effective wavelengths, together with correlationsbetween the extinction coefficients, ultimately characterize thephotometric IR system of the Observatorio del Teide (TCS). Data ofcomparable quality previously published have been added in order tocomplete the sample. This way the final sample consists of 550 stars.From the analysis of optical and IR colour:colour diagrams, we maydeduce that the range F0-K0 is properly sampled for0.1>[Fe/H]>-3.0. In the range K0-M4, no reliable photometricestimates of metallicity can be assigned, and only a small number ofstars have spectroscopic determination of the metallicity. Nevertheless,after kinematical considerations, the stars in this spectral range arealso expected to sample the galactic populations of dwarfs. Themetallicity effects on the IR and optical colour:colour diagrams arebriefly discussed.
| A survey of proper motion stars. 12: an expanded sample
We report new photometry and radial velocities for almost 500 stars fromthe Lowell Proper Motion Catalog. We combine these results with ourprior sample and rederive stellar temperatures based on the photometry,reddening, metallicities (using chi squared matching of our 22,500 lowSignal to Noise (S/N) high resolution echelle spectra with a grid ofsynthetic spectra), distances, space motions, and Galactic orbitalparameters for 1269 (kinematics) and 1261 (metallicity) of the 1464stars in the complete survey. The frequency of spectroscopic binariesfor the metal-poor ((m/H) less than or equal to -1.2) stars with periodsshorter than 3000 days is at least 15%. The spectroscopic binaryfrequency for metal-rich stars ((m/H) greater than -0.5) appears to belower, about 9%, but this may be a selection effect. We also discussspecial classes of stars, including treatment of the double-linedspectroscopic binaries, and identification of subgiants. Four possiblenew members of the class of field blue stragglers are noted. We pointout the detection of three possible new white dwarfs, six broad-lined(binary) systems, and discuss briefly the three already knownnitrogen-rich halo dwarfs. The primary result of this paper will beavailable on CD-ROM, in the form of a much larger table.
| The primordial lithium abundance from extreme subdwarfs: New observations
High-resolution (R approximately equals 28,000), high signal-to-noise(S/N approximately equals 100) spectra of the Li I lambda-6707 regionhave been obtained for 90 halo dwarfs and main-sequence turnoff starswith (Fe/H) approximately less than or equal to -2.2. The mean lithiumabundance at 6300 K is found to be N(Li) = 12 + log (Li/H) = 2.32 +/-0.20 (95% confidence interval), where the quoted uncertainty reflectsthe error in the absolute abundance zero point from all known sources,random and systematic. Contrary to the findings of Spite and Spite(1982), these data show a larger lithium abundance dispersion than canbe explained by observational errors alone. The standard deviation ofdata points about the mean trend, excluding all upper limits, is 0.13dex, while the typical relative abundance error due to uncertainties inthe temperatures and equivalent widths is 0.08-0.09 dex. A formaldispersion analysis in the temperature-equivalent width plane rejectsthe null hypothesis (i.e., no intrinsic dispersion) at a greater than 6sigma confidence level (100% - 10-8%). In order for theobserved scatter to be consistent with noise, the relative equivalentwidth and temperature errors must both be increased by approximately 55%from their typical values of 3 mA and 100 K (1 sigma), respectively. Atrend of declining N(Li) with decreasing stellar metallicity isidentified as evidence of lithium production by Galactic sources. Allexcess scatter about the N(Li)-(Fe/H) relation is attributed to thecombination of lithium production and a approximately 2 Gyr dispersion(1 sigma) in the halo metallicity-to-age relation. This additionalsource of lithium abundance variations from star to star also accountsfor the observed intrinsic dispersion about the Spite plateau. Thedetection of Li-6 in HD 84937 (Smith, Lambert, and Nissen 1993c)suggests that Galactic cosmic-ray alpha + alpha reactions are thedominant source of lithium production in the early interstellar medium.The rate of Li-6 production inferred from the N(Li)-(Fe/H) trend canaccount for the current abundance of Li-6 observed in the interstellarmedium toward zeta Oph, zeta Per (Meyer, Hawkins, & Wright 1993),and rho Oph (Lemoine et al. 1993). The primordial lithium fraction isestimated from the surface lithium abundances of the hottest, mostmetal-poor stars in this program: 2.22 +/- 0.20 dex.
| The abundance of lithium in metal-poor subgiant stars
We have determined lithium abundances for a sample of 79 halo subgiants.The subgiant candidates were identified using uvby photometry fromseveral catalogs of metal-poor stars. The basic data werehigh-resolution, low-noise coude spectra in the 6700 A spectral region.Abundances of iron and calcium, derived from one Ca I and several Fe Ilines in our spectra, provided a metallicity discriminant for the starsin our sample. The subgiants with temperatures between 5500 and 4900 Kshow a steady decline of lithium abundance with advancing subgiantposition (and decreasing temperature). The observed trend is inqualitative agreement with recent theoretical models of lithium dilutionin metal-poor stars, especially if main-sequence diffusion is included.The initial lithium abundances in metal-poor stars may have beenslightly larger than that exhibited by stars near the main-sequenceturnoff. For stars with temperatures below 4900 K, the models predict nofurther dilution, but observed lithium abundances continue to declinewith decreasing temperature, indicating further lithium destruction onthe giant branch of metal-poor stars. In all postdilution subgiants, theobserved lithium abundances show more scatter than do stars at themain-sequence turnoff, suggesting variations in the main-sequencelithium destruction below the observable surface layers.
| A survey of proper motion stars. IX - The galactic halo's metallicity gradient
Using data already presented for a survey of proper motion stars and theBahcall, Schmidt, and Soneira (1983) model of the Galaxy, Galacticorbital parameters are computed, including planar and three-dimensionaleccentricities, apo- and perigalacticon distances, and maximum distancesreached above/below the plane, based on extreme values for R and theabsolute value of Z over 15 azimuthal periods. The orbital data are usedto bin the survey's stars by apogalacticon and maximum Z distances. Inan attempt to isolate a halo population sample, analyses are restrictedto those stars that lag behind the local standard of the rest's circularorbital velocity by 50, 100, 150, and 200 km/s. The mean metallicitiesof the stars in a variety of Rapo and Zmax bins are compared .
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Orsa Maggiore |
| Ascensione retta: | 11h58m00.08s |
| Declinazione: | +48°12'12.4" |
| Magnitudine apparente: | 8.355 |
| Distanza: | 180.18 parsec |
| Moto proprio RA: | -240.8 |
| Moto proprio Dec: | -125.5 |
| B-T magnitude: | 9.425 |
| V-T magnitude: | 8.444 |
Cataloghi e designazioni:
|
INHABITED SKY
