目录
图像
上传图像
DSS Images Other Images
相关文章
Properties of stellar generations in globular clusters and relations with global parameters
We revise the scenario of the formation of Galactic globular clusters(GCs) by adding the observed detailed chemical composition of theirdifferent stellar generations to the set of their global parameters. Weexploit the unprecedented set of homogeneous abundances of more than1200 red giants in 19 clusters, as well as additional data fromliterature, to give a new definition of bona fide GCs, as the stellaraggregates showing the Na-O anticorrelation. We propose a classificationof GCs according to their kinematics and location in the Galaxy in threepopulations: disk/bulge, inner halo, and outer halo. We find that theluminosity function of GCs is fairly independent of their population,suggesting that it is imprinted by the formation mechanism onlymarginally affected by the ensuing evolution. We show that a largefraction of the primordial population should have been lost by theproto-GCs. The extremely low Al abundances found for the primordialpopulation of massive GCs indicate a very fast enrichment process beforethe formation of the primordial population. We suggest a scenario forthe formation of GCs that includes at least three main phases: i) theformation of a precursor population (likely due to the interaction ofcosmological structures similar to those that led to the formation ofdwarf spheroidals, but residing at smaller Galactocentric distances,with the early Galaxy or with other structures); ii) the triggering of along episode of star formation (the primordial population) from theprecursor population; and iii) the formation of the current GC, mainlywithin a cooling flow formed by the slow winds of a fraction of theprimordial population. The precursor population is very effective inraising the metal content in massive and/or metal-poor (mainly halo)clusters, while its rôle is minor in small and/or metal-rich(mainly disk) ones. Finally, we use principal component analysis andmultivariate relations to study the phase of metal enrichment from firstto second generation. We conclude that most of the chemical signaturesof GCs may be ascribed to a few parameters, the most important beingmetallicity, mass, and cluster age. Location within the Galaxy (asdescribed by the kinematics) also plays some rôle, whileadditional parameters are required to describe their dynamical status.Based on observations collected at ESO telescopes under programmes072.-D0507 and 073.D-0211.Appendix A is only available in electronicform at http://www.aanda.org
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| Lithium abundances of halo dwarfs based on excitation temperatures. II. Non-local thermodynamic equilibrium
Context. The plateau in the abundance of 7Li in metal-poorstars was initially interpreted as an observational indicator of theprimordial lithium abundance. However, this observational value is indisagreement with that deduced from calculations of Big Bangnucleosynthesis (BBN), when using the Wilkinson microwave anisotropyprobe (WMAP) baryon density measurements. One of the most importantfactors in determining the stellar lithium abundance is the effectivetemperature. In a previous study by the authors, new effectivetemperatures (Teff) for sixteen metal-poor halo dwarfs werederived using a local thermodynamic equilibrium (LTE) description of theformation of Fe lines. This new Teff scale reinforced thediscrepancy. Aims: For six of the stars from our previous studywe calculate revised temperatures using a non-local thermodynamicequilibrium (NLTE) approach. These are then used to derive a new meanprimordial lithium abundance in an attempt to solve the lithiumdiscrepancy. Methods: Using the code MULTI we calculate NLTEcorrections to the LTE abundances for the Fe i lines measured in the sixstars, and determine new Teff's. We keep other physicalparameters, i.e. log g, [Fe/H] and ?, constant at the valuescalculated in Paper I. With the revised Teff scale we derivenew Li abundances. We compare the NLTE values of Teff withthe photometric temperatures of Ryan et al. (1999, ApJ, 523, 654), theinfrared flux method (IRFM) temperatures of Meléndez &Ramírez (2004, ApJ, 615, L33), and the Balmer line wingtemperatures of Asplund et al. (2006, ApJ, 644, 229). Results: Wefind that our temperatures are hotter than both the Ryan et al. andAsplund et al. temperatures by typically ~110-160 K, but are stillcooler than the temperatures of Meléndez & Ramírez bytypically ~190 K. The temperatures imply a primordial Li abundance of2.19 dex or 2.21 dex, depending on the magnitude of collisions withhydrogen in the calculations, still well below the value of 2.72 dexinferred from WMAP + BBN. We discuss the effects of collisions on trendsof7Li abundances with [Fe/H] and Teff, as well asthe NLTE effects on the determination of log g through ionizationequilibrium, which imply a collisional scaling factor SH >1 for collisions between Fe and H atoms.
| Lithium in the globular cluster NGC 6397. Evidence for dependence on evolutionary status
Context: Most globular clusters are believed to host a single stellarpopulation. They can thus be considered a good place to study the Spiteplateau and to search for possible evolutionary modifications of the Licontent. Aims: We want to determine the Li content of subgiant(SG) and main sequence (MS) stars of the old, metal-poor globularcluster NGC 6397. This work was aimed not only at studying possible Liabundance variations but also to investigate the cosmological Lidiscrepancy. Methods: Here, we present FLAMES/GIRAFFE observationsof a sample of 84 SG and 79 MS stars in NGC 6397 selected in a narrowrange of B-V colour and, therefore, effective temperatures. We determineboth effective temperatures and Li abundances using three-dimensionalhydrodynamical model atmospheres for all the MS and SG stars of thesample. Results: We find a significant difference in the Liabundance between SG stars and MS stars, the SG stars having anabundance higher by almost 0.1 dex on average. We also find a decreasein the lithium abundance with decreasing effective temperature, both inMS and SG stars, albeit with a significantly different slope for the twoclasses of stars. This suggests that the lithium abundance in thesestars is, indeed, altered by some process, which istemperature-dependent. Conclusions: The lithium abundancepattern observed in NGC 6397 is different from what is found among fieldstars, casting some doubt on the use of globular cluster stars asrepresentative of Population II with respect to the lithium abundance.None of the available theories of Li depletion appears to satisfactorilydescribe our observations.Based on observations obtained with FLAMES/GIRAFFE at VLT Kueyen 8.2 mtelescope in programme 079.D-0399(A). Table and Figs. 3-10 are onlyavailable in electronic form at http://www.aanda.org Table 2 isavailable in electronic form at http://www.aanda.org and 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/505/L13
| 6Li/7Li estimates for metal-poor stars
Context: The presence of the lithium-6 isotope in some metal-poor starsis a matter of surprise because of the high values observed.Non-standard models of Big Bang nucleosynthesis and pre-Galactic cosmicray fusion and spallation have been proposed to explain these values.However, the observations of this light isotope are challenging whichmay make some detections disputable. Aims: The goal was todetermine ^6Li/^7Li for a sample of metal-poor stars; three of them havebeen previously studied and the remaining two are new for this type ofstudy. The purpose was to increase, if possible, the number of lithium-6detections and to confirm previously published results. Methods:Spectra of the resonance doublet line of neutral lithium Li I 670.78 nmwere taken with the high dispersion spectrograph at the Subaru 8.2m-telescope for a sample of five metal-poor stars({-3.12≤[Fe/H]≤-2.19}). The contribution of lithium-6 to the totalobserved line profile was estimated from the 1D-LTE analysis of the lineasymmetry. Results: Observed asymmetries could be reproducedassuming isotopic abundance ratios ^6Li/^7Li of the order of: 0.004 forBD +26° 3578, 0.010 for BD +02°3375 and G 64-37, 0.025 for BD+20° 3603 and 0.047 for BD -04°3208. We found that these results were very sensitive toseveral of the assumptions made in the analysis, in particular, thetreatment of the residual structure in the analysed spectra. Our finalestimates for the errors are respectively Δ^6Li/^7Li = ±0.028, 0.029, 0.039, 0.025 and 0.039. Conclusions: The ^6Li/^7Liratios for the sample are comparable to or even lower than these errorvalues, so that detections of lithium-6 can not safely be claimeddespite of the high resolving power (R ˜ 95 000) and S/N(400-600).
| Beryllium, Oxygen, and Iron Abundances in Extremely Metal-Deficient Stars
The abundance of beryllium in the oldest, most metal-poor stars acts asa probe of early star formation and Galactic chemical evolution. We haveanalyzed high-resolution, high signal-to-noise ratio Keck/HIRES spectraof 24 stars with [Fe/H] from -2.3 to -3.5 in order todetermine the history of Be abundance and explore the possibility of aBe plateau. We have determined stellar parameters of our samplespectroscopically, using equivalent widths of Fe I, Ti I, and Ti IIlines. We have determined O abundances from three OH features whichoccur in the same spectral region; this region is relatively uncrowdedand has a well determined continuum in these very/extremely metal-poorstars. We have supplemented this sample with reanalyzed spectra of 25stars from previous observations so that our total sample ranges in[Fe/H] from -0.5 to -3.5. Our results indicate that therelationship between Be and [Fe/H] continues to lower metallicities witha slope of 0.92 ± 0.04. Although there is no indication of aplateau with constant Be abundance, the four lowest metallicity stars(below [Fe/H] of -3.0) do show a Be enhancement relative to Fe atthe 1σ level. A single relationship between Be and [O/H] has aslope of 1.21 ± 0.08, but there is also a good fit with twoslopes: 1.59 above [O/H] = -1.6 and 0.74 for stars with [O/H]below -1.6. This change in slope could result from a change in thedominant production mechanism for Be. In the era of the formation of themore metal-poor stars, Be would be formed by acceleration of CNO atomsin the vicinity of SN II and in later times by high-energy cosmic-raysbombarding CNO in the ambient interstellar gas. We find an excellentcorrelation between [Fe/H] and [O/H] and show that [O/Fe] is near +1.0at [Fe/H] = -3.5 declining to 0 at [Fe/H] = 0.
| Statistical equilibrium of silicon in the atmospheres of metal-poor stars
Aims: The statistical equilibrium of neutral and ionized silicon in theatmospheres of metal-poor stars is discussed. Non-local thermodynamicequilibrium effects (NLTE) are investigated and the silicon abundancesin metal-poor stars determined. Methods: We have used highresolution, high signal to noise ratio spectra from the UVESspectragraph at the ESO VLT telescope. Line formation calculations of Sii and Si ii in the atmospheres of metal-poor stars are presented foratomic models of silicon including 174 terms and 1132 line transitions.Recent improved calculations of Si i and Si ii photoionizationcross-sections are taken into account, and the influence of thefree-free quasi-molecular absorption in the Ly? wing isinvestigated by comparing theoretical and observed fluxes of metal-poorstars. All abundance results are derived from LTE and NLTE statisticalequilibrium calculations and spectrum synthesis methods. Results:It is found that the extreme ultraviolet radiation is very important formetal-poor stars, especially for the high temperature, very metal-poorstars. The radiative bound-free cross-sections also play a veryimportant role for these stars. Conclusions: NLTE effects for Siare found to be important for metal-poor stars, in particular for warmmetal-poor stars. It is found that these effects depend on thetemperature. For warm metal-poor stars, the NLTE abundance correctionreaches ~0.2 dex relative to standard LTE calculations. Our resultsindicate that Si is overabundant for metal-poor stars.Based on observations obtained in the frame of the ESO programme ID165.N-0276(A).
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full 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/501/941
| Measurements of the Isotopic Ratio 6Li/7Li in Stars with Planets
High-resolution (R = 143,000), high signal-to-noise ratio (S/N =700-1100) Gemini-S bench-mounted High-Resolution Optical Spectrographspectra have been analyzed in a search for 6Li in five starswhich host extrasolar planets. The presence of detectable amounts of6Li in these mature, solar-type stars is a good monitor ofaccretion of planetary disk material, or solid bodies themselves, intothe outer layers of the parent stars. Detailed profile fitting of the LiI resonance doublet at ?6707.8 Å reveals no detectableamounts of 6Li in any star in our sample. The list of starsanalyzed includes HD 82943 for which 6Li has been previouslydetected at the level of 6Li/7Li = 0.05 ±0.02. The typical limits in the derived isotopic fraction are6Li/7Li <= 0.00-0.02. These upper limitsconstrain the amount of accreted material to less than ~0.02-0.5 Jovianmasses. The presence of detectable amounts of 6Li wouldmanifest itself as a red asymmetry in the Li I line profile and thederived upper limits on such asymmetries are discussed in light ofthree-dimensional hydrodynamic model atmospheres, where convectivemotions also give rise to slight red asymmetries in line profiles.
| Beryllium abundances and star formation in the halo and in the thick disk
Context: Beryllium is a pure product of cosmic ray spallation. Thisimplies a relatively simple evolution in time of the beryllium abundanceand suggests its use as a time-like observable. Aims: Our goal is toderive abundances of Be in a sample of 90 stars, the largest sample ofhalo and thick disk stars analyzed to date. We study the evolution of Bein the early Galaxy and its dependence on kinematic and orbitalparameters, and investigate its use as a cosmochronometer. Abundances ofBe, Fe, and α-elements of 73 stars are employed to study theformation of the halo and the thick disk of the Galaxy. Methods:Beryllium abundances are determined from high-resolution, highsignal-to-noise UVES spectra with spectrum synthesis. Atmosphericparameters and abundances of α-elements are adopted from theliterature. Lithium abundances are used to eliminate mixed stars fromthe sample. The properties of halo and thick disk stars are investigatedin diagrams of log(Be/H) vs. [ α/H] , log(Be/H) vs. [Fe/H], and [α/Fe] vs. log(Be/H) and with orbital and kinematic parameters. Results: We present our observational results in various diagrams. (i)In a log(Be/H) vs. [Fe/H] diagram we find a marginal statisticaldetection of a real scatter, above what is expected from measurementerrors, with a larger scatter among halo stars. The detection of thescatter is further supported by the existence of pairs of stars withidentical atmospheric parameters and different Be abundances; (ii) in alog(Be/H) vs. [ α/Fe] diagram, the halo stars separate into twocomponents; one is consistent with predictions of evolutionary models,while the other has too high α and Be abundances and is chemicallyindistinguishable from thick disk stars. This suggests that the halo isnot a single uniform population where a clear age-metallicity relationcan be defined; (iii) In diagrams of R_min vs. [ α/Fe] andlog(Be/H), the thick disk stars show a possible decrease in [α/Fe] with R_min, whereas no dependence of Be with R_min is seen.This anticorrelation suggests that the star formation rate was lower inthe outer regions of the thick disk, pointing towards an inside-outformation. The lack of correlation for Be indicates that it isinsensitive to the local conditions of star formation.Based on observations made with ESO VLT, at Paranal Observatory, underprograms 076.B-0133 and 077.B-0507, and on data obtained from theESO/ST-ECF Science Archive Facility and the UVES Paranal ObservatoryProject 266.D-5655.Tables 1-3, 6 and Appendices A-C are only available in electronic format http://www.aanda.org
| Calibration of Strömgren uvby-H? photometry for late-type stars - a model atmosphere approach
Context: The use of model atmospheres for deriving stellar fundamentalparameters, such as T_eff, log g, and [Fe/H], will increase as we findand explore extreme stellar populations where empirical calibrations arenot yet available. Moreover, calibrations for upcoming large satellitemissions of new spectrophotometric indices, similar to the uvby-H?system, will be needed. Aims: We aim to test the power oftheoretical calibrations based on a new generation of MARCS models bycomparisons with observational photomteric data. Methods: Wecalculated synthetic uvby-H? colour indices from synthetic spectra.A sample of 367 field stars, as well as stars in globular clusters, isused for a direct comparison of the synthetic indices versus empiricaldata and for scrutinizing the possibilities of theoretical calibrationsfor temperature, metallicity, and gravity. Results: We show thatthe temperature sensitivity of the synthetic (b-y) colour is very closeto its empirical counterpart, whereas the temperature scale based uponH? shows a slight offset. The theoretical metallicity sensitivityof the m1 index (and for G-type stars its combination withc_1) is somewhat higher than the empirical one, based upon spectroscopicdeterminations. The gravity sensitivity of the synthetic c1index shows satisfactory behaviour when compared to obervations of Fstars. For stars cooler than the sun, a deviation is significant in thec1-(b-y) diagram. The theoretical calibrations of (b-y),(v-y), and c1 seem to work well for Pop II stars and lead toeffective temperatures for globular cluster stars supporting recentclaims that atomic diffusion occurs in stars near the turnoff point ofNGC 6397. Conclusions: Synthetic colours of stellar atmospherescan indeed be used, in many cases, to derive reliable fundamentalstellar parameters. The deviations seen when compared to observationaldata could be due to incomplete linelists but are possibly also due tothe effects of assuming plane-parallell or spherical geometry and LTE.Model colours 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/498/527
| Extending the Virgo Stellar Stream with SEKBO Survey RR Lyrae Stars
A subset of the RR Lyrae (RRL) candidates identified from the SouthernEdgeworth-Kuiper Belt Object (SEKBO) survey data have been followed upphotometrically (n = 106) and spectroscopically (n = 51). Period andlight curve fitting reveals a 24% ± 7% contamination of SEKBOsurvey data by non-RRLs. This paper focuses on the region of the VirgoStellar Stream (VSS), particularly on its extension to the south of thedeclination limits of the Sloan Digital Sky Survey and of the QuasarEquatorial Survey Team RRL survey. The distribution of radial velocitiesin the Galactic standard of rest frame (V GSR) for the 11RRLs observed in the VSS region has two apparent peaks. The larger peakcoincides with the four RRLs having langV GSRrang = 127± 10 km s-1 and dispersion ? = 27 kms-1, marginally larger than that expected from theerrors alone. The two type ab RRLs in this group have lang[Fe/H]rang =-1.95 ± 0.1. Both the radial velocities and metalabundances are consistent with membership in the VSS. The secondvelocity peak, which occurs at langV GSRrang = -175± 10 km s-1 may indicate the presence of starsfrom the Sgr leading tidal tail, which is expected to have largenegative velocities in this region. We explore the spatial extent of theVSS by constructing luminosity functions from the SEKBO data andcomparing them to data synthesized with the Besançon Galacticmodel. Analysis of the excess over the model predictions reveals the VSSas a large (~760 deg2) overdensity centered at roughly (R.A.,decl.) ~ (186°, -4°), spanning a length of ~15 kpc inprojection, assuming a heliocentric distance of 19 kpc. The data revealfor the first time the more southern regions of the stream and trace itto decl. ?-15° and Galactic latitudes as low as b ?45°.
| Lithium abundances of halo dwarfs based on excitation temperature. I. Local thermodynamic equilibrium
Context: The discovery of the Spite plateau in the abundances of7Li for metal-poor stars led to the determination of anobservationally deduced primordial lithium abundance. However, after thesuccess of the Wilkinson Microwave Anisotropy Probe (WMAP) indetermining the baryon density, Ω_Bh2, there was adiscrepancy between observationally determined and theoreticallydetermined abundances in the case of 7Li. One of the mostimportant uncertain factors in the calculation of the stellar7Li abundance is the effective temperature, T_eff. Aims: We use sixteen metal-poor halo dwarfs to calculate new T_effvalues using the excitation energy method. With this temperature scalewe then calculate new Li abundances for this group of stars in anattempt to resolve the 7Li discrepancy. Methods: Usinghigh signal-to-noise (S/N ≈ 100) spectra of 16 metal-poor halodwarfs, obtained with the UCLES spectrograph on the AAT, measurements ofequivalent widths from a set of unblended Fe I lines are made. Theseequivalent widths are then used to calculate new T_eff values with theuse of the single line radiative transfer program WIDTH6, where we haveconstrained the gravity using either theoretical isochrones or theHipparcos parallax, rather than the ionization balance. The lithiumabundances of the stars are calculated with these temperatures. Results: The physical parameters are derived for the 16 programmestars, and two standards. These include T_eff, log g, [Fe/H],microturbulence and 7Li abundances. A comparison between thetemperature scale of this work and those adopted by others has beenundertaken. We find good consistency with the temperatures derived fromthe Hα line by Asplund et al. (2006, ApJ, 644, 229), but not withthe hotter scale of Meléndez & Ramírez (2004, ApJ,615, L33). We also present results of the investigation into whether anytrends between 7Li and metallicity or temperature are presentin these metal-poor stars.Appendix A 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/493/601
| Hydrodynamics and radiative transfer of 3D model atmospheres. Current status, limitations, and how to make headway
3D MHD models are important tools for advancing our understanding ofstellar atmospheres. A major computational challenge is the treatment ofradiative transfer; both to get a realistic treatment of the energytransfer in the 3D modelling and for the diagnostic problem ofcalculating the emergent spectrum in more detail from such models. Thecurrent status, limitations and future directions of 3D MHD atmosphericmodelling and the treatment of radiative transfer are here discussed.
| 6Li in the Atmosphere of GJ 117 Revisited
Detection of 6Li has been shown for energetic solar events,one chromospherically active binary, and several dwarf halo stars. Wehad previously found a 6Li/7Li=0.03+/-0.01 foractive K dwarf GJ 117 using VLT UVES observations. Here we present highsignal-to-noise (>1000) high spectral resolution observations takenwith the McDonald Observatory's 2.7 m and echelle spectrometer of GJ117. We have used the solar spectrum and template stars to eliminatepossible blends, such as Ti I, in the 6Li spectral region.Our new analysis, using an updated PHOENIX model atmosphere, finds6Li/7Li=0.05+/-0.02. In addition, bisectoranalysis showed no significant red asymmetries that would affect thelithium line profile. No changes above the statistical uncertainties arefound between the VLT and McDonald data. The amount of 6Liderived for GJ 117 is consistent with creation in spallation reactionson the stellar surface, but we caution that uncertainties in thecontinuum level may cause additional uncertainty in the 6Lifraction.
| A new stellar library in the region of the CO index at 2.3 μm. New index definition and empirical fitting functions
Context: The analysis of unresolved stellar populations demandsevolutionary synthesis models with realistic physical ingredients andextended wavelength coverage. Aims: We quantitatively describe thefirst CO bandhead at 2.3 μm to allow stellar population models toprovide improved predictions in this wavelength range. Methods: Weobserved a new stellar library with a better coverage of the stellaratmospheric parameter space than in earlier works. We performed adetailed analysis of the robustness of previous CO index definitionswith spectral resolution, wavelength calibration, signal-to-noise ratio,and flux calibration. Results: We define a new line-strength index forthe first CO bandhead at 2.3 μm, D_CO, better suited for stellarpopulation studies than previous index definitions. We derive empiricalfitting functions for the CO feature as a function of the stellarparameters (T_eff, log g and [Fe/H]), showing a detailed quantitativemetallicity dependence.
| Nitrogen abundance in turn-off stars of NGC 6397 and NGC 6752
Aims: Our goal is to measure the nitrogen abundance in 5 turn off (TO)stars of the two globular clusters NGC 6397 and NGC 6752, and to comparethe cluster abundances with those of field stars of comparablemetallicity. In addition, we investigate the variations in nitrogenabundance and its connections to variations in the abundances of otherlight elements. Methods: We determine the nitrogen abundancefrom the compact band-head of the (0-0) vibrational band of the NHA^3?-X^3?- system at 3360 Å, using spectra ofresolution R~ 45 000 acquired using the UVES spectrograph on the ESOKueyen-VLT 8.2 m telescope, analysed with spectrum synthesis based onplane parallel LTE model atmospheres. We apply a method previouslyapplied to field stars, to enable a direct comparison to be made betweenthe results. Results: The nitrogen abundances of two stars inNGC 6397 are found to be almost identical, even though there is adifference of one order of magnitude in their oxygen abundances. In athird star of the same cluster, the nitrogen abundance is slighty lower,although consistent, within the calculated margins of error, with theother two measurements. All stars in NGC 6397 are N-rich with respect tofield objects of similar metallicity. The two stars in NGC 6752 show adifference in nitrogen abundance of over one order of magnitude. Thesame stars differ in the abundances of other elements such as Na, O, andLi, although only by a factor ~3-4. Conclusions: NGC 6397 and NGC6752 are, at present, the only two globular clusters in which N has beenmeasured in TO stars in a way consistent with similar measurements forfield stars. The behaviour of N is different in the two clusters: novariation is observed between stars in NGC 6397, while a large variationis observed in NGC 6752. This is consistent with a picture in which thestars in NGC 6752 have been formed by a mixture of“pristine” material and material which has been processed byan early generation of stars, referred to as “polluters”.The N abundances reported will help to constrain the properties of thepolluters. In the case of NGC 6397, a simple pollution history isprobably not viable, since the observed variations in O abundances arenot accompanied by corresponding variations in N or Li.Based on observations collected at the ESO VLT, Paranal Observatory,Chile, program 075.D-0807(A).
| First stars XI. Chemical composition of the extremely metal-poor dwarfs in the binary CS 22876-032
Context: Unevolved metal-poor stars constitute a fossil record of theearly Galaxy, and can provide invaluable information on the propertiesof the first generations of stars. Binary systems also provide directinformation on the stellar masses of their member stars. Aims:The purpose of this investigation is a detailed abundance study of thedouble-lined spectroscopic binary CS 22876-032, which comprises the twomost metal-poor dwarfs known. Methods: We used high-resolution,high-S/N ratio spectra from the UVES spectrograph at the ESO VLTtelescope. Long-term radial-velocity measurements and broad-bandphotometry allowed us to determine improved orbital elements and stellarparameters for both components. We used OSMARCS 1D models and theturbospectrum spectral synthesis code to determine the abundances of Li,O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co and Ni. We also used theCO^5BOLD model atmosphere code to compute the 3D abundance corrections,notably for Li and O. Results: We find a metallicity of [Fe/H] ~-3.6 for both stars, using 1D models with 3D corrections of ~-0.1 dexfrom averaged 3D models. We determine the oxygen abundance from thenear-UV OH bands; the 3D corrections are large, -1 and -1.5 dex for thesecondary and primary respectively, and yield [O/Fe] ~ 0.8, close to thehigh-quality results obtained from the [OI] 630 nm line in metal-poorgiants. Other [ ?/Fe] ratios are consistent with those measured inother dwarfs and giants with similar [Fe/H], although Ca and Si aresomewhat low ([X/Fe] ? 0). Other element ratios follow those ofother halo stars. The Li abundance of the primary star is consistentwith the Spite plateau, but the secondary shows a lower abundance; 3Dcorrections are small. Conclusions: The Li abundance in theprimary star supports the extension of the Spite Plateau value at thelowest metallicities, without any decrease. The low abundance in thesecondary star could be explained by endogenic Li depletion, due to itscooler temperature. If this is not the case, another, yet unknownmechanism may be causing increased scatter in A(Li) at the lowestmetallicities.
| Line shift, line asymmetry, and the ^6Li/^7Li isotopic ratio determination
Context: Line asymmetries are generated by convective Doppler shifts instellar atmospheres, especially in metal-poor stars, where convectivemotions penetrate to higher atmospheric levels. Such asymmetries areusually neglected in abundance analyses. The determination of the^6Li/^7Li isotopic ratio is prone to suffering from such asymmetries, asthe contribution of ^6Li is a slight blending reinforcement of the redwing of each component of the corresponding ^7Li line, with respect toits blue wing. Aims: The present paper studies the halo star HD74000 and estimates the impact of convection-related asymmetries on theLi isotopic ratio determination. Methods: Two methods are usedto meet this aim. The first, which is purely empirical, consists inderiving a template profile from another element that can be assumed tooriginate in the same stellar atmospheric layers as Li I, producingabsorption lines of approximately the same equivalent width asindividual components of the ^7Li I resonance line. The second methodconsists in conducting the abundance analysis based on NLTE lineformation in a 3D hydrodynamical model atmosphere, taking into accountthe effects of photospheric convection. Results: The results ofthe first method show that the convective asymmetry generates an excessabsorption in the red wing of the ^7Li absorption feature that mimicsthe presence of ^6Li at a level comparable to the hitherto publishedvalues. This opens the possibility that only an upper limit on ^6Li/^7Lihas thus far been derived. The second method confirms these findings. Conclusions: From this work, it appears that a systematicreappraisal of former determinations of ^6Li abundances in halo stars iswarranted.Based on observations carried out at the European Southern Observatory(ESO), under prog. ID 75.D-0600. Tables 1-3, and additional referencesare only available in electronic form at http://www.aanda.org
| Is ^6Li in metal-poor halo stars produced in situ by solar-like flares?
The high ^6Li abundances recently measured in metal-poor halo stars arefar above the value predicted by Big Bang nucleosynthesis. They cannotbe explained by galactic cosmic-ray interactions in the interstellarmedium either. Various pre-galactic sources of ^6Li have been proposedin the literature. We study the possibility that the observed ^6Li wasproduced by repeated solar-like flares on the main sequence of theselow-metallicity stars. The time-dependent flaring activity of theseobjects is estimated from the observed evolution of rotation-inducedactivity in Pop I dwarf stars. As in solar flares, ^6Li could be mainlycreated in interactions of flare-accelerated ^3He with stellaratmospheric ^4He, via the reaction ^4He(^3He, p)^6Li. Stellar dilutionand destruction of flare-produced ^6Li are evaluated from theevolutionary models of metal-poor stars developed by Richard andco-workers. Stellar depletion should be less important for ^6Li atomssynthesized in flares than for those of protostellar origin. Theoreticalfrequency distributions of ^6Li/^7Li ratios are calculated using aMonte-Carlo method and compared with the observations. Excellentagreement is found with the measured ^6Li/^7Li distribution, when takinginto account the contribution of protostellar ^6Li originating fromgalactic cosmic-ray nucleosynthesis. We propose as an observational testof the model to seek for a positive correlation between ^6Li/^7Li andstellar rotation velocity. We also show that the amounts of ^7Li, Be andB produced in flares of metal-poor halo stars are negligible as comparedwith the measured abundances of these species. ^6Li in low-metallicitystars may be a unique evidence of the nuclear processes occuring instellar flares.
| Measuring the Balmer Jump and the Effective Gravity in FGK Stars
It is difficult to accurately measure the effective gravity (logg) inlate-type stars using broadband (e.g., UBV or SDSS) or intermediate-band(uvby) photometric systems, especially when the stars can cover a rangeof metallicities and reddenings. However, simple spectroscopicobservational and data reduction techniques can yield accurate valuesfor logg through comparison of the Balmer jumps of low-resolutionspectra with recent grids of synthetic flux spectra.
| A Search for Nitrogen-enhanced Metal-poor Stars
Theoretical models of very metal-poor intermediate-mass asymptotic giantbranch (AGB) stars predict a large overabundance of primary nitrogen.The very metal-poor, carbon-enhanced, s-process-rich stars, which arethought to be the polluted companions of now extinct AGB stars, providedirect tests of the predictions of these models. Recent studies of thecarbon and nitrogen abundances in metal-poor stars have focused on themost carbon-rich stars, leading to a potential selection bias againststars that have been polluted by AGB stars that produced large amountsof nitrogen and hence have small [C/N] ratios. We call these starsnitrogen-enhanced metal-poor (NEMP) stars and define them as having[N/Fe]>+0.5 and [C/N]<-0.5. In this paper we report on the [C/N]abundances of a sample of 21 carbon-enhanced stars, all but three ofwhich have [C/Fe]<+2.0. If NEMP stars were made as easily ascarbon-enhanced metal-poor (CEMP) stars, then we expected to findbetween two and seven NEMP stars. Instead, we found no NEMP stars in oursample. Therefore, this observational bias is not an importantcontributor to the apparent dearth of N-rich stars. Our [C/N] values arein the same range as values reported previously in the literature (-0.5to +2.0), and all stars are in disagreement with the predicted [C/N]ratios for both low- and high-mass AGB stars. We suggest that thedecrease in [C/N] from the low-mass AGB models is due to enhancedextramixing, while the lack of NEMP stars may be caused by unfavorablemass ratios in binaries or the difficulty of mass transfer in binarysystems with large mass ratios.Based on observations obtained at Cerro Tololo Inter-AmericanObservatory and Kitt Peak National Observatory, a division of theNational Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation.
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Medium-resolution Isaac Newton Telescope library of empirical spectra
A new stellar library developed for stellar population synthesismodelling is presented. The library consists of 985 stars spanning alarge range in atmospheric parameters. The spectra were obtained at the2.5-m Isaac Newton Telescope and cover the range λλ3525-7500 Å at 2.3 Å (full width at half-maximum) spectralresolution. The spectral resolution, spectral-type coverage,flux-calibration accuracy and number of stars represent a substantialimprovement over previous libraries used in population-synthesis models.
| Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars
Aims.Parameters for 55 nearby metal-poor stars are determined usinghigh-resolution spectroscopy. Together with similar data taken from arecent analysis, they are used to show trends of their Galacticevolution with stellar [Fe/H] or [Mg/H] abundances. The separation ofabundance ratios between disk and halo stars is used as a basiccriterion for population membership. Methods.After carefulselection of a clean subsample free of suspected or known binaries andpeculiar stars, abundances of Mg, Na and Al are based on NLTE kineticequilibrium calculations applied to spectrum synthesis methods. Results.The relation between [Na/Mg] and [Fe/H] is a continuousenrichment through all three Galactic populations spanning a range ofvalues between a metal-poor plateau at [ Na/Mg] = -0.7 and solar values.[Al/Mg] displays a step-like difference between stars of the Galactichalo with overline[Al/Mg] -0.45 and the two disk populations withoverline[Al/Mg] +0.10. [Al/Mg] ratios, together with the [Mg/Fe]ratios, asymmetric drift velocities V, and stellar evolutionary ages,make possible the individual discrimination between stars of the thickdisk and the halo. At present, this evidence is limited by the smallnumber of stars, and by the theoretical and empirical uncertainties ofstellar age determinations, but it achieves a high significance. Conclusions.While the stellar sample is not complete with respect tospace volume, the resulting abundances indicate the necessity to revisecurrent models of chemical evolution to allow for an adequate productionof Al in early stellar generations.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| 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.
| Chemical abundances of very metal-poor stars
High-resolution and high signal-to-noise ratio spectra of 32 verymetal-poor stars were obtained with the Coudé echellespectrograph mounted on the 2.16-m telescope at the NationalAstronomical Observatories (Xinglong, China). Equivalent widths of FeI,FeII, OI, NaI, MgI, AlI, SiI, SiII, KI, CaI, ScII, TiI, VI, CrI, MnI,NiI, CuI and BaII lines were measured. Stellar effective temperatureswere determined by colour indices. Stellar surface gravities werecalculated from Hipparcos parallaxes and stellar evolutionary tracks.Photospheric abundances of 16 elements were derived by localthermodynamical equilibrium analysis. Stellar space motions (U, V, W)and Galactic orbital parameters were calculated. Based on kinematics,sample stars were separated into dissipative collapse and accretioncomponents of halo population. The global kinematics of the twocomponents were analysed. Element abundances were discussed as functionsof metallicities. The results of oxygen and α-elements abundanceconfirmed the previous works. The [K/Fe] shows a gradual systematicincrease toward a lower metallicity, such as in the case ofα-elements. The [Ba/Fe] trend suggests that the s-processdominated Ba production at least for the metal-poor stars with[Fe/H]> -2.0.
| 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.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
