Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Chemical abundances of distant extremely metal-poor unevolved stars
Context. The old Galactic halo stars hold the fossil record of theinterstellar medium chemical composition at the time of their formation.Most of the stars studied so far are relatively near to the Sun, thisprompts the study of more distant stars, both to increase the size ofthe sample and to search for possible variations of abundance patternsat greater distances. Aims: The purpose of our study is todetermine the chemical composition of a sample of 16 candidate extremelymetal-poor (EMP) dwarf stars, extracted from the Sloan Digital SkySurvey (SDSS). There are two main purposes: in the first place to verifythe reliability of the metallicity estimates derived from the SDSSspectra; in the second place to see if the abundance trends found forthe brighter nearer stars studied previously also hold for this sampleof fainter, more distant stars. Methods: We used the UVES at theVLT to obtain high-resolution spectra of the programme stars. Theabundances were determined by an automatic analysis with the MyGIsFOScode, with the exception of lithium, for which the abundances weredetermined from the measured equivalent widths of the Li i resonancedoublet. Results: All candidates are confirmed to be EMP stars,with [Fe/H] ≤ -3.0. The chemical composition of the sample of starsis similar to that of brighter and nearer samples. We measured thelithium abundance for 12 stars and provide stringent upper limits forthree other stars, for a fourth star the upper limit is not significant,owing to the low signal-to noise ratio of the spectrum. The "meltdown"of the Spite plateau is confirmed, but some of the lowest metallicitystars of the sample lie on the plateau. Conclusions: Theconcordance of the metallicities derived from high-resolution spectraand those estimated from the SDSS spectra suggests that the latter maybe used to study the metallicity distribution of the halo. The abundancepattern suggests that the halo was well mixed for all probedmetallicities and distances. The fact that at the lowest metallicitieswe find stars on the Spite plateau suggests that the meltdown depends onat least another parameter, besides metallicity.Based on spectra obtained with UVES at the 8.2 m Kueyen ESO telescope,programmes 078.D-0217 and 081.D.0373.Table 1 is available in electronicform at http://www.aanda.org
| An extremely primitive star in the Galactic halo
The early Universe had a chemical composition consisting of hydrogen,helium and traces of lithium; almost all other elements weresubsequently created in stars and supernovae. The mass fraction ofelements more massive than helium, Z, is known as `metallicity'. Anumber of very metal-poor stars has been found, some of which have a lowiron abundance but are rich in carbon, nitrogen and oxygen. Fortheoretical reasons and because of an observed absence of stars withZ<1.5×10-5, it has been suggested that low-massstars cannot form from the primitive interstellar medium until it hasbeen enriched above a critical value of Z, estimated to lie in the range1.5×10-8 to 1.5×10-6 (ref. 8),although competing theories claiming the contrary do exist. (We use`low-mass' here to mean a stellar mass of less than 0.8 solar masses,the stars that survive to the present day.) Here we report the chemicalcomposition of a star in the Galactic halo with a very low Z(<=6.9×10-7, which is 4.5×10-5times that of the Sun) and a chemical pattern typical of classicalextremely metal-poor stars--that is, without enrichment of carbon,nitrogen and oxygen. This shows that low-mass stars can be formed atvery low metallicity, that is, below the critical value of Z. Lithium isnot detected, suggesting a low-metallicity extension of the previouslyobserved trend in lithium depletion. Such lithium depletion implies thatthe stellar material must have experienced temperatures above twomillion kelvin in its history, given that this is necessary to destroylithium.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Speckle interferometry of metal-poor stars in the solar neighborhood. II
The results of speckle interferometric observations of 115 metal-poorstars ([m/H] < ‑1) within 250 pc from the Sun and with propermotions µ ≳ 0.2″/yr, made with the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences,are reported. Close companions with separations ranging from0.034″ to 1″ were observed for 12 objects—G76-21,G59-1, G63-46, G135-16, G168-42, G141-47, G142-44, G190-10, G28-43,G217-8, G130-7, and G89-14—eight of them are astrometricallyresolved for the first time. The newly resolved systems include onetriple star—G190-10. If combined with spectroscopic and visualdata, our results imply a single:binary:triple:quadruple star ratio of147:64:9:1 for a sample of 221 primary components of halo and thick-diskstars.
| Beryllium in Ultra-Lithium-Deficient Halo Stars: The Blue Straggler Connection
There exists a small group of metal-deficient stars that have Liabundances well below the Li plateau that is defined by over 100unevolved stars with temperatures above 5800 K and values of[Fe/H]<-1.0. Abundances of Be have been determined for most of theseultra-Li-deficient stars in order to investigate the cause of the Lideficiencies. These Li-deficient stars have implications on the value ofprimordial Li. High-resolution and high signal-to-noise ratio spectrahave been obtained in the Be II spectral region near 3130 Å forsix ultra-Li-deficient stars with the Keck I telescope and its newUV-sensitive CCD on the upgraded HIRES. The spectrum synthesis techniquehas been used to determine Be abundances. All six stars are found tohave Be deficiencies also. Two have measurable but reduced Be, and fourhave only upper limits on Be. These results are consistent with the ideathat these Li- and Be-deficient stars are analogous to blue stragglers.The stars have undergone mass transfer events (or mergers) that destroyor dilute both Li and Be. The findings cannot be matched by the modelsthat predict that the deficiencies are due to extramixing in a subset ofhalo stars that were initially rapid rotators, with the possibleexception of one star, G139-8. Because the ultra-Li-deficient stars arealso Be-deficient, they appear to be genuine outliers in the populationof halo stars used to determine the Li plateau in that they no longerhave the Li in their atmospheres that was produced in the big bang.
| Mapping the Local Galactic Halo. I. Optical Photometry of Cool Subdwarf Candidates
Optical (BVRI) photometric measurements of a sample of 564 candidatecool subdwarfs in the nearby halo are presented. The stars generallyspan the color range 0.4<(B-V)<1.8 hence, the sample is composedof early F- through early M-type stars on the subdwarf sequence. Thesample is selected from the revised NLTT catalog of Gould and Salim andSalim and Gould via a reduced proper motion diagram. The photometry isprecise and accurate; in particular, for stars with 9
| 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.
| uvby-β photometry of high-velocity and metal-poor stars. XI. Ages of halo and old disk stars
New uvby-β data are provided for 442 high-velocity and metal-poorstars; 90 of these stars have been observed previously by us, and 352are new. When combined with our previous two photometric catalogues, thedata base is now made up of 1533 high-velocity and metal-poor stars, allwith uvby-β photometry and complete kinematic data, such as propermotions and radial velocities taken from the literature. Hipparcos, plusa new photometric calibration for Mv also based on theHipparcos parallaxes, provide distances for nearly all of these stars;our previous photometric calibrations give values for E(b-y) and [Fe/H].The [Fe/H], V(rot) diagram allows us to separate these stars intodifferent Galactic stellar population groups, such as old-thin-disk,thick-disk, and halo. The X histogram, where X is our stellar-populationdiscriminator combining V(rot) and [Fe/H], and contour plots for the[Fe/H], V(rot) diagram both indicate two probable components to thethick disk. These population groups and Galactic components are studiedin the (b-y)0, Mv diagram, compared to theisochrones of Bergbusch & VandenBerg (2001, ApJ, 556, 322), toderive stellar ages. The two thick-disk groups have the meancharacteristics: ([Fe/H], V(rot), Age, σW') ≈ (-0.7dex, 120 km s-1, 12.5 Gyr, 62.0 km s-1), and≈(-0.4, 160, 10.0, 45.8). The seven most metal-poor halo groups,-2.31 ≤ [Fe/H] ≤ -1.31, show a mean age of 13.0 ± 0.2(mean error) Gyr, giving a mean difference from the WMAP results for theage of the Universe of 0.7 ± 0.3 Gyr. These results for the agesand components of the thick disk and for the age of the Galactic halofield stars are discussed in terms of various models and ideas for theformation of galaxies and their stellar populations.
| 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.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| 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
| 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.
| Stellar Archaeology: A Keck Pilot Program on Extremely Metal-poor Stars from the Hamburg/ESO Survey. II. Abundance Analysis
We present a detailed abundance analysis of eight stars selected asextremely metal-poor candidates from the Hamburg/ESO Survey (HES). Forcomparison, we have also analyzed three extremely metal-poor candidatesfrom the HK survey, and three additional bright metal-poor stars. Withthis work, we have doubled the number of extremely metal-poor stars([Fe/H]<=3.0 dex) with high-precision abundance analyses. Based onthis analysis, our sample of extremely metal-poor candidates from theHES contains three stars with [Fe/H]<=-3.0 dex, three more with[Fe/H]<=-2.8 dex, and two stars that are only slightly moremetal-rich. Thus, the chain of procedures that led to the selection ofthese stars from the HES successfully provides a high fraction ofextremely metal-poor stars. We verify that our choices for stellarparameters, derived in Paper I and independently of the high-dispersionspectroscopic analysis, lead to acceptable ionization and excitationbalances for Fe. Substantial non-LTE effects in Fe appear to be ruledout by the above agreement, even at these extremely low metallicities.For the α-elements Mg, Si, Ca, and Ti, the light element Al, theiron-peak elements Sc, Cr, and Mn, and the neutron-capture elements Srand Ba, we find trends in abundance ratios [X/Fe] similar to those foundby previous investigations. These trends appear to be identical forgiants and for dwarfs. However, the scatter in most of these ratios,even at [Fe/H]<=-3.0 dex, is surprisingly small. Only Sr and Ba,among the elements we examined, show scatter larger than the expectederrors. Future work (the ``0Z Project'') will provide much strongerconstraints on the scatter (or lack thereof) in elemental abundances fora substantially greater number of stars. We discuss the implications ofthese results for the early chemical evolution of the Galaxy, includingsuch issues as the number of contributing supernovae and the sizes oftypical protogalactic fragments in which they were born. In addition, wehave identified a very metal-poor star in our sample that appears torepresent the result of the s-process chain, operating in a verymetal-poor environment, and exhibits extremely enhanced C, Ba, and Pband somewhat enhanced Sr. Based in large part on observations obtainedat the W. M. Keck Observatory, which is operated jointly by theCalifornia Institute of Technology, the University of California, andNASA.
| Stellar Archaeology: A Keck Pilot Program on Extremely Metal-poor Stars from the Hamburg/ESO Survey. I. Stellar Parameters
In this series of two papers, we present a high-dispersion spectroscopicanalysis of eight candidate extremely metal-poor stars selected from theHamburg/ESO Survey (HES) and of six additional very metal-poor stars. Wedemonstrate that with suitable vetting using moderate-resolutionspectra, the yield of this survey for stars with [Fe/H]<=-3.0 dex isvery high; three out of the eight stars observed thus far at highresolution from the HES are actually that metal-poor, three more have[Fe/H]<=-2.8 dex, and the remainder are only slightly moremetal-rich. In preparation for a large-scale effort to mine the HESdatabase for such stars about to get under way, we lay out in this paperthe basic principles we intend to use to determine in a uniform way thestellar parameters Teff, logg, and reddening. Based in largepart on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the California Institute of Technology, theUniversity of California, and NASA.
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
| On the possible existence of a self-regulating hydrodynamical process in slowly rotating stars. II. Lithium plateau in halo stars and primordial abundance
The lithium plateau observed in halo stars has long appeared as aparadox in the general context of the lithium abundance behavior instellar outer layers. First, the plateau is flat, second, the lithiumabundance dispersion is extremely small. This seems in contradictionwith the large lithium variations observed in younger stars. It is alsodifficult to understand theoretically: as lithium nuclei are destroyedby nuclear reactions at a relatively low temperature ( =~ 2.5 milliondegrees), the occurrence of macroscopic motions in the stellar outerlayers easily lead to lithium depletion at the surface. On the otherhand, if no macroscopic motions occur in the stellar gas, lithium issubject to microscopic diffusion which, in the case of halo stars,should also lead to depletion. Several ideas have been proposed toaccount for the lithium behavior in halo stars. The most promisingpossibilities were rotational-induced mixing, which could reduce lithiumin the same way for all the stars (Vauclair \cite{Vauclair88};Pinsonneault et al. \cite{Pinsonneault92} and \cite{Pinsonneault99}) andmass-loss, which could oppose the lithium settling (Vauclair &Charbonnel \cite{Vauclair95}, \cite{Vauclair98}). In both cases however,the parameters should be tightly adjusted to prevent any dispersion inthe final results. Vauclair (\cite{Vauclair99}) (Paper I) looked for aphysical process which could occur in slowly rotating stars and explainwhy the dispersion of the lithium abundances in the halo stars' plateauis so small. She pointed out that the displaystyle mu -gradient termswhich appear in the computations of the meridional circulation velocity(e.g. Mestel \cite{Mestel53}) were not introduced in previouscomputations of rotationally-induced mixing. This can lead to aself-regulating process which reduces the efficiency of the meridionalcirculation as well as the microscopic diffusion. Here we presentnumerical computations of this process and its influence on the lithiumabundance variations in halo stars. We show that in slowly rotatingstars, under some conditions, lithium can be depleted by a factor of upto two with a dispersion smaller than 0.1 dex in the middle part of thelithium plateau. We derive a primordial lithium abundance of 2.5+/- 0.1,consistent with the recent determinations of D/H and 4He/H.
| Catalogue of [Fe/H] determinations for FGK stars: 2001 edition
The catalogue presented here is a compilation of published atmosphericparameters (Teff, log g, [Fe/H]) obtained from highresolution, high signal-to-noise spectroscopic observations. This newedition has changed compared to the five previous versions. It is nowrestricted to intermediate and low mass stars (F, G and K stars). Itcontains 6354 determinations of (Teff, log g, [Fe/H]) for3356 stars, including 909 stars in 79 stellar systems. The literature iscomplete between January 1980 and December 2000 and includes 378references. The catalogue is made up of two tables, one for field starsand one for stars in galactic associations, open and globular clustersand external galaxies. The catalogue is distributed through the CDSdatabase. Access to the catalogue with cross-identification to othersets of data is also possible with VizieR (Ochsenbein et al.\cite{och00}). The catalogue (Tables 1 and 2) is only available inelectronic form at the 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/373/159 and VizieRhttp://vizier.u-strasbg.fr/.
| Ultra-Lithium-deficient Halo Stars and Blue Stragglers: A Common Origin?
We present data for four ultra-Li-deficient, warm, halo stars. The Lideficiency of two of these is a new discovery. Three of the four starshave effective temperatures Teff~6300 K, in contrast topreviously known Li-deficient halo stars, which spanned the temperaturerange of the Spite plateau. In this paper we propose that these andpreviously known ultra-Li-deficient halo stars may have had theirsurface lithium abundances reduced by the same mechanism as produceshalo field blue stragglers. Even though these stars have yet to revealthemselves as blue stragglers, they might be regarded as``blue-stragglers-to-be.'' In our proposed scenario, the surfaceabundance of Li in these stars could be destroyed (1) during the normalpre-main-sequence single-star evolution of their low-mass precursors,(2) during the post-main-sequence evolution of an evolved mass donor,and/or (3) via mixing during a mass-transfer event or stellar merger.The warmest Li-deficient stars at the turnoff would be regarded asemerging ``canonical'' blue stragglers, whereas cooler ones representsub-turnoff-mass blue-stragglers-to-be. The latter are presently hiddenon the main sequence, Li depletion being possibly the clearest signatureof their past history and future significance. Eventually, themain-sequence turnoff will reach down to their mass, exposing thoseLi-depleted stars as canonical blue stragglers when normal stars of thatmass evolve away. Arguing against this unified view is the observationthat the three Li-depleted stars at Teff~=6300 K are allbinaries, whereas very few of the cooler systems show evidence forbinarity; it is thus possible that two separate mechanisms areresponsible for the production of Li-deficient main-sequence halo stars.Based on observations obtained with the University College Londonéchelle spectrograph (UCLES) on the Anglo-Australian Telescope(AAT) and the Utrecht échelle spectrograph (UES) on the WilliamHerschel Telescope (WHT).
| Estimation of Stellar Metal Abundance. II. A Recalibration of the Ca II K Technique, and the Autocorrelation Function Method
We have recalibrated a method for the estimation of stellar metalabundance, parameterized as [Fe/H], based on medium-resolution (1-2Å) optical spectra (the majority of which cover the wavelengthrange 3700-4500 Å). The equivalent width of the Ca II K line (3933Å) as a function of [Fe/H] and broadband B-V color, as predictedfrom spectrum synthesis and model atmosphere calculations, is comparedwith observations of 551 stars with high-resolution abundances availablefrom the literature (a sevenfold increase in the number of calibrationstars that were previously available). A second method, based on theFourier autocorrelation function technique first described by Ratnatunga& Freeman, is used to provide an independent estimate of [Fe/H], ascalibrated by comparison with 405 standard-star abundances.Metallicities based on a combination of the two techniques for dwarfsand giants in the color range 0.30<=(B-V)_0<=1.2 exhibit anexternal 1 sigma scatter of approximately 0.10-0.20 dex over theabundance range -4.0<=[Fe/H]<=0.5. Particular attention has beengiven to the determination of abundance estimates at the metal-rich endof the calibration, where our previous attempt suffered from aconsiderable zero-point offset. Radial velocities, accurate toapproximately 10 km s^-1, are reported for all 551 calibration stars.
| Extremely Metal Poor Stars. VI. The Heterogeneous Class of Lithium-depleted Main-Sequence Turnoff Dwarfs
We present abundances of 14 elements in the metal-poor main-sequenceturnoff star G122-69 , which has at most one-tenth the Li abundanceobserved in most other stars of similar temperature and metallicity. Thedeficiency of Li is significant because of this element's role inconstraining primordial fnucleosynthesis and the baryon density of theuniverse. Although we have examined elements of intermediate atomicmass, in the iron peak, and heavy neutron capture species, we find noother abundance anomalies in G122-69. Nor do we find any evidence ofradial velocity variation at the 1 km s^-1 level. These data arecombined with our previous study of the three other known Li-deficientmain-sequence turnoff stars, thus permitting an analysis of the fullsample. The new data reinforce our earlier finding that there is noobvious common abundance abnormality that one might associate with theLi deficiency. Indeed, the four stars exhibit diverse abundance patternsand form a heterogeneous group. That said, the other three members ofthe group appear to have higher values of [Ba/Sr], but not necessarilyof [Ba/Fe] or [Sr/Fe], than most ``normal'' halo or even metal-deficientbarium stars. The higher than average [Ba/Sr] ratios may indicate thattheir envelope material underwent s-processing with a high neutronexposure near the limit of that identified in the metal-deficient bariumstars and Population I analogs, which are thought to originate throughcontamination by asymptotic giant branch star ejecta. However, we cannotfavor such an explanation any more than a normal r-process origin,especially in view of the unremarkable [Sr/Fe] and [Ba/Fe] values(except perhaps in G186-26). The most we can say is that the [Ba/Sr]ratios in three of these stars are at the high end of the rangeencountered for ``normal'' halo stars, but that no mechanism has beenunambiguously identified as responsible for their Li-depletion.
| Extremely Metal-Poor Stars. III. The Li-depleted Main-Sequence Turnoff Dwarfs
We present abundances of 14 elements for the metal-poor,near--main-sequence turnoff stars G66-30, G139-8, and G186-26, which arewell known to possess less than 1/5--1/10 the value of Li/H observed infield halo Spite Plateau dwarfs and are thought by many to have beenproduced in the currently accepted standard big bang cosmology. Thestars have [Fe/H] = -1.53, -2.24, and -2.68, respectively, and for thesevalues their collective abundance patterns are not abnormal. That is tosay, there is no common abundance abnormality that one might associatewith their Li deficiencies. In G186-26, we find an overabundance of theheavy neutron-capture elements that increases with atomic mass. Wemeasure [Ba/Fe] = +0.35, whereas most stars of this metallicity have[Ba/Fe] ~ -0.5, together with smaller enhancements for Sr and possiblyY. Such supersolar, neutron-capture, element abundances, however, areexhibited by ~25% of stars having [Fe/H] ~ -2.7. No such enhancement isfound in G66-30 or G139-8, and the simplest explanation for the data ofthe three stars is that there is no connection between Li depletion andthe abundance patterns of the heavy neutron-capture elements. G66-30 andG186-26 appear to be multiple systems, and we discuss the possible roleof binarity in producing the observed Li depletion. We concur with J. A.Thorburn that this group of objects is not the progeny of bluestragglers, and we discuss the alternative that some of the Li-depletedstars may be the Population II counterparts of the disk populationsubgiant CH, barium dwarf, and F str lambda 4077 stars. As suggested byothers, the most likely explanation for the abundance anomalies in theseobjects, which includes universal Li deficiency, involves mass transferacross a binary during the asymptotic giant branch evolutionary phase ofthe erstwhile primary of the system. We note that while such masstransfer might be expected to produce Li depletion, variations in C, N,and the heavy neutron-capture elements are possible but not allnecessary. Neither G66-30 nor G139-8 shows enhancement of C or theneutron-capture elements, and no information is available for N. Furtherwork is necessary to settle the issue. If mass transfer is responsiblefor Li depletions, such stars justifiably could be excluded frominvestigations of the primordial Li abundance. In the absence ofevidence for such mass transfer, however, the possibility remains thatsome of these objects, if not all, are extreme examples of a processthat has affected all Plateau stars.
| Empirical Calibration of Metallicity Indices for Single Stellar Populations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2218I&db_key=AST
| Lithium in Population II stars
Not Available
| 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.
| Two new lithium-deficient population II dwarfs
Observations are reported which show that G122-69 and G139-8 are normal,extremely metal-poor subdwarfs with no detectable lithium. This resultchallenges the contention that the most metal-deficient halo stars showno dispersion in surface lithium abundance beyond the observationalscatter; the ratio of extreme subdwarfs with unmeasurably low lithiumabundances may be close to 20 percent. Attempts to identify stellarN(Li) with the lithium fraction generated by big bang nucleosynthesismay therefore have been premature. Larger samples and a more completeunderstanding of the dispersion must play a key role in further studiesof this kind.
| Lithium-poor stars may question big bang.
Not Available
| Subdwarf Studies. III. The Halo Metallicity Distribution
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1991AJ....101.1865R&db_key=AST
| Subdwarf studies. II - Abundances and kinematics from medium resolution spectra. III - The halo metallicity distribution
Stars previously identified as having UV excesses are observed at 1-Aresolution in the Ca II K-line region. Comparisons of these data withother samples and with Monte Carlo simulations involving a singlecomponent halo have yielded estimates of halo velocity dispersions androtation velocity, corrected for the kinematic biases in the sample. Itis suggested that the data are not consistent with a model in which thehalo formed from star formation in a dissipating, collapsing cloud; theyare, however, reconcilable with the formation of the halo stars bynumerous, independently evolving gas clouds. The metallicitydistribution of a sample of 372 kinematically selected halo stars isthen constructed, with a view to selection effects in the data. Goodagreement is noted between the globular cluster metallicity distributionand a stochastic model with a mean of 10 enrichments/fragment.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Ercole |
| Ascensione retta: | 17h01m43.98s |
| Declinazione: | +16°09'03.3" |
| Magnitudine apparente: | 11.596 |
| Moto proprio RA: | -286.1 |
| Moto proprio Dec: | -244.3 |
| B-T magnitude: | 11.985 |
| V-T magnitude: | 11.629 |
Cataloghi e designazioni:
|
