Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
The non-LTE line formation of neutral carbon in late-type stars
Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE)line formation of neutral carbon in late-type stars in order to removesome of the potential systematic errors in stellar abundance analysesemploying C i features. Methods: .The statistical equilibrium codeMULTI was used on a grid of plane-parallel 1D MARCS atmosphericmodels. Results: .Within the parameter space explored, thehigh-excitation C i lines studied are stronger in non-LTE due to thecombined effect of line source function drop and increased line opacitydue to overpopulation of the lower level for the transitions considered;the relative importance of the two effects depends on the particularcombination of T{eff}, log g, [Fe/H] and [C/Fe] and on theanalysed C i line. As a consequence, the non-LTE abundance correctionsare negative and can be substantially so, for example -0.4 dex inhalo turn-off stars at [Fe/H] -3. The magnitude of the non-LTEcorrections is rather insensitive to whether inelastic H collisions areincluded or not. Conclusions: .Our results have implications onstudies of nucleosynthetic processes and on Galactic chemical evolutionmodels. When applying our calculated corrections to recent observationaldata, the upturn in [C/O] at low metallicity might still be present(thus apparently still necessitating contributions from massive Pop. IIIstars for the carbon production), but at a lower level and possibly witha rather shallow trend of -0.2 dex/dex below [O/H] -1.
| 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.
| Permitted Oxygen Abundances and the Temperature Scale of Metal-poor Turnoff Stars
We use high-quality VLT/UVES published data of the permitted O I tripletand Fe II lines to determine oxygen and iron abundances in unevolved(dwarfs, turnoff, subgiants) metal-poor halo stars. The calculationshave been performed both in LTE and non-LTE (NLTE), employing effectivetemperatures obtained with the new infrared flux method (IRFM)temperature scale by Ramírez & Meléndez, and surfacegravities from Hipparcos parallaxes and theoretical isochrones. A newlist of accurate transition probabilities for Fe II lines, tied to theabsolute scale defined by laboratory measurements, has been used.Interstellar absorption has been carefully taken into account byemploying reddening maps, stellar energy distributions andStrömgren photometry. We find a plateau in the oxygen-to-iron ratioover more than 2 orders of magnitude in iron abundance(-3.2<[Fe/H]<-0.7), with a mean [O/Fe]=0.5 dex (σ=0.1 dex),independent of metallicity, temperature, and surface gravity. The flat[O/Fe] ratio is mainly due to the use of adequate NLTE corrections andthe new IRFM temperature scale, which, for metal-poor F/early G dwarfsis hotter than most Teff scales used in previous studies ofthe O I triplet. According to the new IRFM Teff scale, thetemperatures of turnoff halo stars strongly depend on metallicity, aresult that is in excellent qualitative and quantitative agreement withstellar evolution calculations, which predict that the Teffof the turnoff at [Fe/H]=-3 is about 600-700 K higher than that at[Fe/H]=-1. Recent determinations of Hα temperatures in turnoffstars are in excellent relative agreement with the new IRFMTeff scale in the metallicity range -2.7<[Fe/H]<-1,with a zero-point difference of only 61 K.
| Chemical abundances of 32 mildly metal-poor stars
Context: .The formation scenario of the Galactic thick disk is anunresolved problem. Chemical abundances in long-lived dwarf stars of thethin and thick disks provide information of the Galactic diskformation.Aims.We present photospheric abundances of the O, Na, Mg, Al,Si, Ca, Sc, Ti, V, Cr, Mn, Ni, and Ba elements for 32 mildly metal-poorstars with [Fe/H] -0.7. According to their kinematics, age, and [α/Fe] , sample stars are identified to thin disk, thick disk, andhalo population memberships. Element abundances for sample stars arediscussed as a function of metallicity.Methods.High resolution and highsignal-to-noise ratio spectra were obtained with the CoudéEchelle Spectrograph mounted on the 2.16 m telescope at the NationalAstronomical Observatories (Xinglong, China). Effective temperatureswere estimated from colour indices, and surface gravities from Hipparcosparallaxes. Stellar abundances were determined from a differential LTEanalysis. The kinematics parameters were calculated from the parallax,proper motion, and radial velocity. Stellar ages were determined fromtheoretical stellar evolution tracks.Results.The average age of thethick disk stars is older than the thin disk stars. Our elementabundance results extend and confirm previous works. The oxygen andother α-elements (Mg, Si, Ca, and Ti) abundances of thin and thickdisk stars show distinct trends at [Fe/H]≤-0.60. The [Al/Fe]behaviour is exactly as an α-element, although the separation for[Na/Fe] of thin and thick disk stars is not clear. The elements V, Cr,and Ni follow Fe very closely, and there is no offset between thin andthick disk stars, but the Sc and Mn abundance trends of the thin andthick disk stars are different, and [Ba/Fe] of thin disk and thick diskstars shows different behaviour.
| 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.
| CCD Photometry of M92
We present Johnson B and V photometry for the galactic globular clusterM92 (NGC 6341). Photometric results for a total of 30,000 starsare obtained and are plotted on a V versus (B-V) diagram. We fittheoretical isochrones to this diagram in order to get an estimate forthe age of M92. The age which we find is 16×109years with the following values for the metallicity and He-abundance:[Fe/H] = -2.03, Y = 0.235. The distance modulus to this cluster turnsout to be m - M = 14.6 in accordance with that obtained by Stetson &Harris (1988). We also perform stellar counts in order to produce aluminosity function which is successfully fitted by the same theoreticalmodels fitted to the colour-magnitude diagram.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| Chemical abundances in 43 metal-poor stars
We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe,Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood,most of them subgiants or turn-off-point stars with iron abundances[Fe/H] ranging from -0.4 to -3.0. About half of this sample has not beenspectroscopically analysed in detail before. Effective temperatures wereestimated from uvby photometry, and surface gravities primarily fromHipparcos parallaxes. The analysis is differential relative to the Sun,and was carried out with plane-parallel MARCS models. Various sources oferror are discussed and found to contribute a total error of about0.1-0.2 dex for most elements, while relative abundances, such as[Ca/Fe], are most probably more accurate. For the oxygen abundances,determined in an NLTE analysis of the 7774 Å triplet lines, theerrors may be somewhat larger. We made a detailed comparison withsimilar studies and traced the reasons for the, in most cases,relatively small differences. Among the results we find that [O/Fe]possibly increases beyond [Fe/H] = -1.0, though considerably less sothan in results obtained by others from abundances based on OH lines. Wedid not trace any tendency toward strong overionization of iron, andfind the excesses, relative to Fe and the Sun, of the α elementsMg, Si, and Ca to be smaller than those of O. We discuss someindications that also the abundances of different α elementsrelative to Fe vary and the possibility that some of the scatter aroundthe trends in abundances relative to iron may be real. This may supportthe idea that the formation of Halo stars occurred in smaller systemswith different star formation rates. We verify the finding by Gratton etal. (2003b, A&A, 406, 131) that stars that do not participate in therotation of the galactic disk show a lower mean and larger spread in [α/Fe] than stars participating in the general rotation. The latterstars also seem to show some correlation between [ α/Fe] androtation speed. We trace some stars with peculiar abundances, amongthese two Ba stars, HD 17072 and HD196944, the second already known to be rich in s elements.Finally we advocate that a spectroscopic study of a larger sample ofhalo stars with well-defined selection criteria is very important, inorder to add to the very considerable efforts that various groups havealready made.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| Carbon, Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem
Eleven models of Galactic chemical evolution, differing in the carbon,nitrogen, and oxygen yields adopted, have been computed to reproduce theGalactic O/H values obtained from H II regions. All the models fit theoxygen gradient, but only two models also fit the carbon gradient, thosebased on carbon yields that increase with metallicity owing to stellarwinds in massive stars (MSs) and decrease with metallicity owing tostellar winds in low- and intermediate-mass stars (LIMSs). Thesuccessful models also fit the C/O versus O/H evolution history of thesolar vicinity obtained from stellar observations. We also compare thepresent-day N/H gradient and the N/O versus O/H and the C/Fe, N/Fe, O/Feversus Fe/H evolution histories of the solar vicinity predicted by ourtwo best models with those derived from H II regions and from stellarobservations. While our two best models fit the C/H and O/H gradients,as well as the C/O versus O/H history, only model 1 fits well the N/Hgradient and the N/O values for metal-poor stars but fails to fit theN/H values for metal-rich stars. Therefore, we conclude that our twobest models solve the C enrichment problem but that further work needsto be done on the N enrichment problem. By adding the C and O productionsince the Sun was formed predicted by models 1 and 2 to the observedsolar values, we find an excellent agreement with the O/H and C/H valuesof the solar vicinity derived from H II region O and C recombinationlines. Our results are based on an initial mass function (IMF) steeperthan Salpeter's a Salpeter-like IMF predicts C/H, N/H, and O/H ratioshigher than observed. One of the most important results of this paper isthat the fraction of carbon due to MSs and LIMSs in the interstellarmedium is strongly dependent on time and on the galactocentric distance;at present about half of the carbon in the interstellar medium of thesolar vicinity has been produced by MSs and half by LIMSs.
| Abundance correlations in mildly metal-poor stars. II. Light elements (C to Ca)
Accurate relative abundances have been obtained for carbon, oxygen,sodium, aluminium, silicon, and calcium in a sample of mildly metal-poorstars. This analysis complements a previous study carried out by Jehinet al. ([CITE], A&A, 341, 241), which provided the basis for theEASE scenario. This scenario postulates that field metal-poor stars wereborn in self-enriched proto-globular cluster clouds. By furtherinvestigating the correlations between the different α-elementabundances, we propose a modified scenario for the formation ofintermediate metallicity stars, in which the stars exhibiting lower thanaverage α/Fe abundance ratios would form in low mass clouds,unable to sustain the formation of very massive stars (M 30~M_ȯ). Moreover, the carbon-to-iron ratio is found to decrease asone climbs the so-called Population IIb branch, i.e. when the s-elementabundance increases. In the framework of the EASE scenario, we interpretthis anticorrelation between the carbon and the s-element abundances asa signature of a hot bottom burning process in the metal-poor AGB starswhich expelled the matter subsequently accreted by our Population IIbstars.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO Programmes 56.E-0384, 57.E-0400 and 59.E-0257).
| The Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| Heavy elements and chemical enrichment in globular clusters
High resolution (R 40 000) and high S/N spectra have been acquiredwith UVES on the VLT-Kueyen (Paranal Observatory, ESO Chile) for severalmain sequence turnoff stars (V 17 mag) and subgiants at the baseof the Red Giant Branch (V 16 mag) in three globular clusters (NGC6397, NGC 6752 and 47 Tuc/NGC 104) at different metallicities(respectively [Fe/H] ≃ -2.0; -1.5; -0.7). Spectra for a sample of25 field halo subdwarves have also been taken with equal resolution, buthigher S/N. These data have been used to determine the abundances ofseveral neutron-capture elements in these three clusters: strontium,yttrium, barium and europium. This is the first abundance determinationof these heavy elements for such unevolved stars in these three globularclusters. These values, together with the [Ba/Eu] and [Sr/Ba] abundanceratios, have been used to test the self-enrichment scenario. Acomparison is done with field halo stars and other well known Galacticglobular clusters in which heavy elements have already been measured inthe past, at least in bright giants (V 11-12 mag). Our resultsshow clearly that globular clusters have been uniformly enriched by r-and s-process syntheses, and that most of them seem to follow exactlythe same abundance patterns as field halo stars, which discards the``classical'' self-enrichment scenario for the origin of metallicitiesand heavy elements in globular clusters.Based on data collected at the European Southern Observatory with theVLT-UT2, Paranal, Chile (ESO-LP 165.L-0263).
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| Cu and Zn in the early Galaxy
We present Cu and Zn abundances for 38 FGK stars, mostly dwarfs,spanning a metallicity range between solar and [Fe/H] = -3. Theabundances were obtained using Kurucz's local thermal equilibrium (LTE)model atmospheres and the near-UV lines of Cu I 3273.95 Å and Zn I3302.58 Å observed at high spectral resolution. The trend of[Cu/Fe] versus [Fe/H] is almost solar for [Fe/H] > -1 and thendecreases to a plateau <[Cu/Fe]> = -0.98 at [Fe/H] < -2.5,whereas the [Zn/Fe] trend is essentially solar for [Fe/H] > -2 andthen slightly increases at lower metallicities to an average value of<[Zn/Fe]> = +0.18. We compare our results with previous work onthese elements, and briefly discuss them in terms of nucleosynthesisprocesses. Predictions of halo chemical evolution fairly reproduce thetrends, especially the [Cu/Fe] plateau at very low metallicities, but toa lesser extent the higher [Zn/Fe] ratios at low metallicities,indicating possibly missing yields.
| Galactic evolution of nitrogen
We present detailed spectroscopic analysis of nitrogen abundances in 31unevolved metal-poor stars analysed by spectral synthesis of the near-UVNH band at 3360 Å observed at high resolution with varioustelescopes. We found that [N/Fe] scales with that of iron in themetallicity range -3.1 <[Fe/H]<0 with the slope 0.01±0.02.Furthermore, we derive uniform and accurate (N/O) ratios using oxygenabundances from near-UV OH lines obtained in our previous studies. Wefind that a primary component of nitrogen is required to explain theobservations. The NH lines are discovered in the VLT/UVES spectra of thevery metal-poor subdwarfs G64-12 and LP815-43 indicating that thesestars are N rich. The results are compared with theoretical models andobservations of extragalactic H II regions and Damped Lyα systems.This is the first direct comparison of the (N/O) ratios in these objectswith those in Galactic stars.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| The Luminosity Function and Color-Magnitude Diagram of the Globular Cluster M12
In this paper we present the V and I luminosity functions andcolor-magnitude diagrams derived from wide-field(23'×23') BVI photometry of theintermediate-metallicity ([Fe/H]~-1.3) Galactic globular cluster M12.Using observed values (and ranges of values) for the clustermetallicity, reddening, distance modulus, and age, we compare these datawith recent α-enhanced stellar evolution models for low-massmetal-poor stars. We describe several methods of making comparisonsbetween theoretical and observed luminosity functions to isolate theevolutionary timescale information that the luminosity functionscontain. We find no significant evidence of excesses of stars on the redgiant branch, although the morphology of the subgiant branch in theobserved luminosity function does not match theoretical predictions in asatisfactory way. Current uncertainties in Teff-colortransformations (and possibly also in other physics inputs to themodels) make more detailed conclusions about the subgiant branchmorphology impossible. Given the recent constraints on cluster ages fromthe WMAP experiment, we find that good-fitting models that do notinclude He diffusion (both color-magnitude diagrams and luminosityfunctions) are too old (by ~1-2 Gyr) to adequately represent the clusterluminosity function. The inclusion of helium diffusion in the modelsprovides an age reduction (compared with nondiffusive models) that isconsistent with the age of the universe being 13.7+/-0.2 Gyr.
| On the correlation of elemental abundances with kinematics among galactic disk stars
We have performed the detailed analysis of 174 high-resolution spectraof FGK dwarfs obtained with the ELODIE echelle spectrograph at theObservatoire de Haute-Provence. Abundances of Fe, Si and Ni have beendetermined from equivalent widths under LTE approximation, whereasabundances of Mg have been determined under NLTE approximation usingequivalent widths of 4 lines and profiles of 5 lines. Spatial velocitieswith an accuracy better than 1 km s-1, as well as orbits,have been computed for all stars. They have been used to define 2subsamples kinematically representative of the thin disk and the thickdisk in order to highlight their respective properties. A transitionoccurs at [Fe/H] =-0.3. Stars more metal-rich than this value have aflat distribution with Zmax;<1 kpc and σW<20 km s-1, and a narrow distribution of [α/Fe].There exist stars in this metallicity regime which cannot belong to thethin disk because of their excentric orbits, neither to the thick diskbecause of their low scale height. Several thin disk stars areidentified down to [Fe/H] =-0.80. Their Mg enrichment is lower thanthick disk stars with the same metallicity. We confirm from a largersample the results of Feltzing et al. (\cite{felt03}) and Bensby et al.(\cite{ben03}) showing a decrease of [α/Fe] with [Fe/H] in thethick disk interpreted as the signature of the SNIa which haveprogressively enriched the ISM with iron. However our data suggest thatthe star formation in the thick disk stopped when the enrichment was[Fe/H] =-0.30, [Mg/Fe] =+0.20, [Si/Fe] =+0.17. A vertical gradient in[α/Fe] may exist in the thick disk but should be confirmed with alarger sample. Finally we have identified 2 new candidates of the HR1614moving group.Based on spectra collected with the ELODIE spectrograph at the 1.93-mtelescope of the Observatoire de Haute Provence (France).Tables 3 and 8 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/418/551
| Stars of Extragalactic Origin in the Solar Neighborhood
For 77 main-sequence F-G stars in the solar neighborhood with publishediron, magnesium, and europium abundances determined from high-dispersionspectra and with the ages estimated from theoretical isochrones, wecalculated the spatial velocities using Hipparcos data and the Galacticorbital elements. A comparison with the orbital elements of the globularclusters that are known to have been accreted by our Galaxy in the pastreveals stars of extragalactic origin. We show that the abundance ratiosof r- and alpha-elements in all the accreted stars differ sharply fromthose in the stars that are genetically associated with the Galaxy.According to current theoretical models, europium is produced mainly inlow-mass type-II supernovae (SNe II), while magnesium is synthesized inlarge quantities in high-mass SN II progenitors. Since all the oldaccreted stars of our sample exhibit a significant Eu overabundancerelative to Mg, we conclude that the maximum masses of the SN IIprogenitors outside the Galaxy were much lower than those inside it. Onthe other hand, only a small number of young accreted stars exhibit lownegative ratios [Eu/Mg] < 0. This can be explained by the delay ofprimordial star formation and the explosions of high-mass SNe II in arelatively small part of extragalactic space. We provide evidence thatthe interstellar medium was weakly mixed at the early evolutionarystages of the Galaxy formed from a single protogalactic cloud, and thatthe maximum mass of the SN II progenitors increased in it with timesimultaneously with the increase in mean metallicity.
| Neutron-Capture Elements in Halo, Thick-Disk, and Thin-Disk Stars: Neodymium
We have derived the LTE neodymium abundances in 60 cool stars withmetallicities [Fe/H] from 0.25 to -1.71 by applying a synthetic-spectrumanalysis to spectroscopic observations of NdII lines with a resolutionof λ/Δλ⋍60 000 and signal-to-noise ratios of100 200. We have improved the atomic parameters of NdII and blendinglines by analyzing the corresponding line pro files in the solarspectrum. Neodymium is overabundant with respect to iron in halo stars,[Nd/Fe]=0.33±0.09, with the [Nd/Fe] ratio decreasingsystematically with metallicity when [Fe/H]>-1. This reflects anonset of efficient iron production in type I supernovae during theformation of the thick disk. The [Nd/Ba] and [Nd/Eu] abundance ratiosbehave differently in halo, thick-disk, and thin-disk stars. Theobserved abundance ratios in halo stars, [Nd/Ba]=0.34±0.08 and[Nd/Eu]=-0.27±0.05, agree within the errors with the ratios ofthe elemental yields for the r-process. These results support theconclusion of other authors based on analyses of other elements that ther-process played the dominant role in the synthesis of heavy elementsduring the formation of the halo. The [Nd/Ba] and [Nd/Eu] ratios forthick-disk stars are almost independent of metallicity([Nd/Ba]=0.28(±0.03)-0.01(±0.04) [Fe/H] and[Nd/Eu]=-0.13(±0.03)+0.05(±0.04) [Fe/H]) but are smallerin absolute value than the corresponding ratios for halo stars,suggesting that the synthesis of s-process nuclei started during theformation of the thick disk. The s-process is estimated to havecontributed ⋍30% of the neodymium produced during this stage ofthe evolution of the Galaxy. The [Nd/Ba] ratio decreases abruptly by0.17 dex in the transition from the thick to the thin disk. Thesystematic decrease of [Nd/Ba] and increase of [Nd/Eu] with increasingmetallicity of thin-disk stars point toward a dominant role of thes-process in the synthesis of heavy elements during this epoch.
| Sulphur and zinc abundances in Galactic stars and damped Lyα systems
High resolution spectra of 34 halo population dwarf and subgiant starshave been obtained with VLT/UVES and used to derive sulphur abundancesfrom the λ λ 8694.0, 8694.6 and λ λ 9212.9,9237.5 S I lines. In addition, iron abundances have been determined from19 Fe II lines and zinc abundances from the λ λ 4722.2,4810.5 lines. The abundances are based on a classical 1D, LTE modelatmosphere analysis, but effects of 3D hydrodynamical modelling on the[S/Fe], [Zn/Fe] and [S/Zn] ratios are shown to be small. We find thatmost halo stars with metallicities in the range -3.2 < [Fe/H] <-0.8 have a near-constant [S/Fe] ≃ +0.3; a least square fit to[S/Fe] vs. [Fe/H] shows a slope of only -0.04 ± 0.01. Among halostars with -1.2 < [Fe/H] < -0.8 the majority have [S/Fe] ≃+0.3, but two stars (previously shown to have low α/Fe ratios)have [S/Fe] ≃ 0.0. For disk stars with [Fe/H] > -1, [S/Fe]decreases with increasing [Fe/H] . Hence, sulphur behaves like othertypical α-capture elements, Mg, Si and Ca. Zinc, on the otherhand, traces iron over three orders of magnitude in [Fe/H], althoughthere is some evidence for a small systematic Zn overabundance ([Zn/Fe]≃ +0.1) among metal-poor disk stars and for halo stars with [Fe/H]< -2.0. Recent measurements of S and Zn in ten damped Lyαsystems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances inthe range -2.1 < [Zn/H] < -0.15 show an offset relative to the[S/Zn] - [Zn/H] relation in Galactic stars. Possible reasons for thisoffset are discussed, including low and intermittent star formationrates in DLAs.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).Table A1 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/415/993
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| The evolution of the C/O ratio in metal-poor halo stars
We report new measurements of carbon and oxygen abundances in 34 F and Gdwarf and subgiant stars belonging to the halo population and spanning arange of metallicity from [Fe/H] = -0.7 to -3.2 . The survey is based onobservations of four permitted lines of C I near 9100 Å and the OI,λ 7774 triplet, all recorded at high signal-to-noise ratioswith the UVES echelle spectrograph on the ESO VLT. The line equivalentwidths were analysed with the 1D, LTE, MARCS model atmosphere code todeduce C and O abundances; corrections due to non-LTE and 3D effects arediscussed. When combined with similar published data for disk stars, ourresults confirm the metallicity dependence of the C/O ratio known fromprevious stellar and interstellar studies: C/O drops by a factor of3-4 as O/H decreases from solar to 1/10 solar. Analysed withinthe context of standard models for the chemical evolution of the solarvicinity, this drop results from the metallicity dependence of the Cyields from massive stars with mass loss, augmented by the delayedrelease of C from stars of low and intermediate mass. The former is,however, always the dominant factor. Our survey has also uncoveredtentative evidence to suggest that, as the oxygen abundance decreasesbelow [O/H] = -1, [C/O] may not remain constant at [C/O] = -0.5, aspreviously thought, but increase again, possibly approaching near-solarvalues at the lowest metallicities ([O/H] ≲ -3). With the currentdataset this is no more than a 3σ effect and it may be due tometallicity-dependent non-LTE corrections to the [C/O] ratio which havenot been taken into account. However, its potential importance as awindow on the nucleosynthesis by Population III stars is a strongincentive for future work, both observational and theoretical, to verifyits reality.Based on observations collected at the European Southern Observatory,Chile (ESO No. 67.D-0106).
| High-Precision Near-Infrared Photometry of a Large Sample of Bright Stars Visible from the Northern Hemisphere
We present the results of 8 yr of infrared photometric monitoring of alarge sample of stars visible from Teide Observatory (Tenerife, CanaryIslands). The final archive is made up of 10,949 photometric measuresthrough a standard InSb single-channel photometer system, principally inJHK, although some stars have measures in L'. The core of this list ofstars is the standard-star list developed for the Carlos SánchezTelescope. A total of 298 stars have been observed on at least twooccasions on a system carefully linked to the zero point defined byVega. We present high-precision photometry for these stars. The medianuncertainty in magnitude for stars with a minimum of four observationsand thus reliable statistics ranges from 0.0038 mag in J to 0.0033 magin K. Many of these stars are faint enough to be observable with arraydetectors (42 are K>8) and thus to permit a linkage of the bright andfaint infrared photometric systems. We also present photometry of anadditional 25 stars for which the original measures are no longeravailable, plus photometry in L' and/or M of 36 stars from the mainlist. We calculate the mean infrared colors of main-sequence stars fromA0 V to K5 V and show that the locus of the H-K color is linearlycorrelated with J-H. The rms dispersion in the correlation between J-Hand H-K is 0.0073 mag. We use the relationship to interpolate colors forall subclasses from A0 V to K5 V. We find that K and M main-sequence andgiant stars can be separated on the color-color diagram withhigh-precision near-infrared photometry and thus that photometry canallow us to identify potential mistakes in luminosity classclassification.
| Abundances for metal-poor stars with accurate parallaxes. I. Basic data
We present element-to-element abundance ratios measured from highdispersion spectra for 150 field subdwarfs and early subgiants withaccurate Hipparcos parallaxes (errors <20%). For 50 stars new spectrawere obtained with the UVES on Kueyen (VLT UT2), the McDonald 2.7 mtelescope, and SARG at TNG. Additionally, literature equivalent widthswere taken from the works by Nissen & Schuster, Fulbright, andProchaska et al. to complement our data. The whole sample includes boththick disk and halo stars (and a few thin disk stars); most stars havemetallicities in the range -2<[Fe/H]<-0.6. We found our data, thatof Nissen & Schuster, and that of Prochaska to be of comparablequality; results from Fulbright scatter a bit more, but they are stillof very good quality and are extremely useful due to the large size ofhis sample. The results of the present analysis will be used inforthcoming papers to discuss the chemical properties of thedissipational collapse and accretion components of our Galaxy.Based in part on data collected at the European Southern Observatory,Chile, at the MacDonald Observatory, Texas, USA, and at the TelescopioNazionale Galileo, Canary Island, INAF,Italy-Spain.}\fnmsep\thanks{Table 1 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia\resizebox{8.8cm}{2.2mm}htpp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/187}
| Sodium Abundances in Stellar Atmospheres with Differing Metallicities
The non-LTE sodium abundances of 100 stars with metallicities-3<[Fe/H]<0.3 are determined using high-dispersion spectra withhigh signal-to-noise ratios. The sodium abundances [Na/Fe] obtained areclose to the solar abundance and display a smaller scatter than valuespublished previously. Giants (logg<3.8) with [Fe/H]<-1 do notdisplay overabundances of sodium, and their sodium abundances do notshow an anticorrelation with the oxygen abundance, in contrast toglobular-cluster giants. They likewise do not show sodium-abundancevariations with motion along the giant branch. No appreciable decreasein the sodium abundance was detected for dwarfs (logg>3.8) withmetallicities -2<[Fe/H]<-1. The observed relation between [Na/Fe]and [Fe/H] is in satisfactory agreement with the theoreticalcomputations of Samland, which take into account the metallicitydependence of the sodium yield and a number of other factors affectingthe distribution of elements in the Galaxy during the course of itsevolution.
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are only available in electronic form, at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| 3D kinematics of white dwarfs from the SPY project
We present kinematics of a sample of 107 DA white dwarfs from the SPYproject (ESO SN Ia Progenitor surveY) and discuss kinematic criteria fora distinction of thin disk, thick disk, and halo populations. This isthe first homogeneous sample of white dwarfs for which 3D space motionshave been determined.Since the percentage of old stars among white dwarfs is higher thanamong main-sequence stars, they are presumably valuable tools in studiesof old populations such as the halo and the thick disk. Studies of whitedwarf kinematics can help to determine the fraction of the total mass ofour Galaxy contained in the form of thick disk and halo white dwarfs, anissue which is still under discussion.Radial velocities and spectroscopic distances obtained by the SPYproject are combined with our measurements of proper motions to derive3D space motions. Galactic orbits and further kinematic parameters arecomputed. We calculate individual errors of kinematic parameters bymeans of a Monte Carlo error propagation code. Our kinematic criteriafor assigning population membership are deduced from a sample of F and Gstars taken from the literature for which chemical criteria can be usedto distinguish between thin disk, thick disk and halo.Candidates for thick disk and halo members are selected in a first stepfrom the classical U-V-velocity diagram. Our final assignment ofpopulation membership is based on orbits and position in theJz-eccentricity diagram. We find four halo and twelve thickdisk white dwarfs.We also present a systematic study of the effects of ignoring the radialvelocity in kinematic investigations.Based on observations obtained at the Paranal Observatory of theEuropean Southern Observatory for programs 165.H-0588 and 167.D-0407.Tables 1 and 2 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/400/877Figures 8, 10-12, 14-16, 19, 20 are only available in electronic form athttp://www.edpsciences.org
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Дельфин |
| Прямое восхождение: | 20h26m11.92s |
| Склонение: | +09°27'00.4" |
| Видимая звёздная величина: | 8.331 |
| Расстояние: | 55.741 парсек |
| Собственное движение RA: | 117.9 |
| Собственное движение Dec: | -547.9 |
| B-T magnitude: | 8.911 |
| V-T magnitude: | 8.379 |
Каталоги и обозначения:
|
