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Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Oxygen abundances in metal-poor subgiants as determined from [O I], O I and OH lines
The debate on the oxygen abundances of metal-poor stars has its originin contradictory results obtained using different abundance indicators.To achieve a better understanding of the problem we have acquired highquality spectra with the Ultraviolet and Visual Echelle Spectrograph atVLT, with a signal-to-noise of the order of 100 in the near ultravioletand 500 in the optical and near infrared wavelength range. Threedifferent oxygen abundance indicators, OH ultraviolet lines around 310.0nm, the [O i] line at 630.03 nm and the O i lines at 777.1-5 nm wereobserved in the spectra of 13 metal-poor subgiants with-3.0≤[Fe/H]≤-1.5. Oxygen abundances were obtained from theanalysis of these indicators which was carried out assuming localthermodynamic equilibrium and plane-parallel model atmospheres.Abundances derived from O i were corrected for departures from localthermodynamic equilibrium. Stellar parameters were computed usingT_eff-vs.-color calibrations based on the infrared flux method andBalmer line profiles, Hipparcos parallaxes and Fe II lines. [O/Fe]values derived from the forbidden line at 630.03 nm are consistent withan oxygen/iron ratio that varies linearly with [Fe/H] as[O/Fe]=-0.09(±0.08)[Fe/H]+0.36(±0.15). Values based on theO i triplet are on average 0.19±0.22 dex(s.d.) higher than thevalues based on the forbidden line while the agreement between OHultraviolet lines and the forbidden line is much better with a meandifference of the order of -0.09±0.25 dex(s.d.). In general, ourresults follow the same trend as previously published results with theexception of the ones based on OH ultraviolet lines. In that case ourresults lie below the values which gave rise to the oxygen abundancedebate for metal-poor stars.
| Li and Be depletion in metal-poor subgiants
A sample of metal-poor subgiants has been observed with the UVESspectrograph at the Very Large Telescope and abundances of Li and Behave been determined. Typical signal-to-noise per spectral bin valuesfor the co-added spectra are of the order of 500 for the ion{Li}{i} line(670.78 nm) and 100 for the ion{Be}{ii} doublet lines (313.04 nm). Thespectral analysis of the observations was carried out using the Uppsalasuite of codes and marcs (1D-LTE) model atmospheres with stellarparameters from photometry, parallaxes, isochrones and Fe ii lines.Abundance estimates of the light elements were corrected for departuresfrom local thermodynamic equilibrium in the line formation. Effectivetemperatures and Li abundances seem to be correlated and Be abundancescorrelate with [O/H]. Standard models predict Li and Be abundancesapproximately one order of magnitude lower than main-sequence valueswhich is in general agreement with the observations. On average, ourobserved depletions seem to be 0.1 dex smaller and between 0.2 and 0.4dex larger (depending on which reference is taken) than those predictedfor Li and Be, respectively. This is not surprising since the initial Liabundance, as derived from main-sequence stars on the Spite plateau, maybe systematically in error by 0.1 dex or more, and uncertainties in thespectrum normalisation and continuum drawing may affect our Beabundances systematically.
| Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman
We develop and test a method for the estimation of metallicities([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhancedmetal-poor (CEMP) stars based on the application of artificial neuralnetworks, regressions, and synthesis models to medium-resolution (1-2Å) spectra and J-K colors. We calibrate this method by comparisonwith metallicities and carbon abundance determinations for 118 starswith available high-resolution analyses reported in the recentliterature. The neural network and regression approaches make use of apreviously defined set of line-strength indices quantifying the strengthof the Ca II K line and the CH G band, in conjunction with J-K colorsfrom the Two Micron All Sky Survey Point Source Catalog. The use ofnear-IR colors, as opposed to broadband B-V colors, is required becauseof the potentially large affect of strong molecular carbon bands onbluer color indices. We also explore the practicality of obtainingestimates of carbon abundances for metal-poor stars from the spectralinformation alone, i.e., without the additional information provided byphotometry, as many future samples of CEMP stars may lack such data. Wefind that although photometric information is required for theestimation of [Fe/H], it provides little improvement in our derivedestimates of [C/Fe], and hence, estimates of carbon-to-iron ratios basedsolely on line indices appear sufficiently accurate for most purposes.Although we find that the spectral synthesis approach yields the mostaccurate estimates of [C/Fe], in particular for the stars with thestrongest molecular bands, it is only marginally better than is obtainedfrom the line index approaches. Using these methods we are able toreproduce the previously measured [Fe/H] and [C/Fe] determinations withan accuracy of ~0.25 dex for stars in the metallicity interval-5.5<=[Fe/H]<=-1.0 and with 0.2<=(J-K)0<=0.8. Athigher metallicity, the Ca II K line begins to saturate, especially forthe cool stars in our program, and hence, this approach is not useful insome cases. As a first application, we estimate the abundances of [Fe/H]and [C/Fe] for the 56 stars identified as possibly carbon-rich, relativeto stars of similar metal abundance, in the sample of ``strong G-band''stars discussed by Beers, Preston, and Shectman.
| 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.
| 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.
| 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.
| 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.
| 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.
| Comparing Deep Mixing in Globular Cluster and Halo Field Giants: Carbon Abundance Data from the Literature
The behavior of carbon abundance as a function of luminosity is used tocompare the rates of deep mixing within red giants of four globularclusters and the Galactic halo field population. Measurements of [C/Fe]for the clusters M92, NGC 6397, M3, and M13 have been compiled from theliterature, together with the Gratton et al. data for halo field stars.Plots of [C/Fe] versus absolute visual magnitude show that forMV<+1.6 the rate of decline of carbon abundance withincreasing luminosity on the red giant branch isd[C/Fe]/dMV~0.22+/-0.03 among the field stars, as well as inM92, NGC 6397, and M3. Among giants fainter than MV=+1.6 thevariation of [C/Fe] with absolute magnitude is much less. The dataindicate that the rate at which deep mixing introduces carbon-depletedmaterial into the convective envelopes of field halo stars during theupper red giant branch phase of evolution is similar to that of manyglobular cluster giants. The notable exception appears to be M13, inwhich stars exhibit deep mixing at a greater rate; this may account forthe high incidence of very low oxygen abundances among the most luminousgiants in M13 in comparison to M3.
| 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 Abundances in Metal-poor Stars
We present oxygen abundances derived from both the permitted andforbidden oxygen lines for 55 subgiants and giants with [Fe/H] valuesbetween -2.7 and solar with the goal of understanding the discrepancy inthe derived abundances. A first attempt, using Teff valuesfrom photometric calibrations and surface gravities from luminositiesobtained agreement between the indicators for turn-off stars, but thedisagreement was large for evolved stars. We find that the difference inthe oxygen abundances derived from the permitted and forbidden lines ismost strongly affected by Teff, and we derive a newTeff scale based on forcing the two sets of lines to give thesame oxygen abundances. These new parameters, however, do not agree withother observables, such as theoretical isochrones or Balmer-line profilebased Teff determinations. Our analysis finds thatone-dimensional, LTE analyses (with published non-LTE corrections forthe permitted lines) cannot fully resolve the disagreement in the twoindicators without adopting a temperature scale that is incompatiblewith other temperature indicators. We also find no evidence ofcircumstellar emission in the forbidden lines, removing such emission asa possible cause for the discrepancy.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Mapping the Galactic Halo. VI. Spectroscopic Measures of Luminosity and Metallicity
We present our calibration of spectroscopic measures of luminosity andmetallicity for halo giant candidates and give metallicities anddistances for our first sample of spectroscopically confirmed giants.These giants have distances ranging from 15 to 83 kpc. As surveys reachfarther into the Galaxy's halo with K giant samples, identification ofgiants becomes more difficult. This is because the numbers of foregroundhalo K dwarfs rise for V magnitudes of 19-20, typical for halo giants at~100 kpc. Our photometric survey uses the strength of the Mg b/H featurenear 5170 Å to weed K dwarfs out of the disk and thick disk, butwe need spectroscopic measures of the strength of the Ca II K, Ca Iλ4227, and Mg b/H features to distinguish between the verymetal-poor dwarfs and halo giants. Using a full error analysis of ourspectroscopic measures, we show why a signal-to-noise ratio of ~15pixel-1 at Ca I λ4227 and ~10 at Ca II K is needed forreliable luminosity discrimination. We use the Ca II K and Mg b featuresto measure metallicity in our halo giants, with typical errors (randomplus systematic) of 0.3 dex for [Fe/H] values from -0.8 to -3.0.
| Spectroscopic Binaries, Velocity Jitter, and Rotation in Field Metal-poor Red Giant and Red Horizontal-Branch Stars
We summarize 2007 radial velocity measurements of 91 metal-poor fieldred giants. Excluding binary systems with orbital solutions, ourcoverage averages 13.7 yr per star, with a maximum of 18.0 yr. We reportfour significant findings. (1) Sixteen stars are found to bespectroscopic binaries, and we present orbital solutions for 14 of them.The spectroscopic binary frequency of the metal-poor red giants, with[Fe/H]<=-1.4, for periods less than 6000 days, is 16%+/-4%, which isnot significantly different from that of comparable-metallicity fielddwarfs, 17%+/-2%. The two CH stars in our program, BD -1°2582 and HD135148, are both spectroscopic binaries. (2) Velocity jitter is presentamong about 40% of the giants with MV<=-1.4. The twobest-observed cases, HD 3008 and BD +22°2411, showpseudoperiodicities of 172 and 186 days, longer than any knownlong-period variable in metal-poor globular clusters. Photometricvariability seen in HD 3008 and three other stars showing velocityjitter hints that starspots are the cause. However, the phasing of thevelocity data with the photometry data from Hipparcos is not consistentwith a simple starspot model for HD 3008. We argue against orbitalmotion effects and radial pulsation, so rotational modulation remainsthe best explanation. The implied rotational velocities for HD 3008 andBD +22°2411, both with MV<=-1.4 and R~50Rsolar, exceed 12 km s-1. (3) Including HD 3008and BD +22°2411, we have found signs of significant excess linebroadening in eight of the 17 red giants with MV<=-1.4,which we interpret as rotation. In three cases, BD +30°2034, CD-37°14010, and HD 218732, the rotation is probably induced by tidallocking between axial rotation and the observed orbital motion with astellar companion. But this cannot explain the other five stars in oursample that display signs of significant rotation. This high frequencyof elevated rotational velocities does not appear to be caused bystellar mass transfer or mergers: there are too few main-sequencebinaries with short enough periods. We also note that the lack of anynoticeable increase in mean rotation at the magnitude level of the redgiant branch luminosity function ``bump'' argues against the rapidrotation's being caused by the transport of internal angular momentum tothe surface. Capture of a planetary-mass companion as a red giantexpands in radius could explain the high rotational velocities. (4) Wealso find significant rotation in at least six of the roughly 15 (40%)red horizontal-branch stars in our survey. It is likely that theenhanced rotation seen among a significant fraction of both blue and redhorizontal-branch stars arose when these stars were luminous red giants.Rapid rotation alone therefore appears insufficient cause to populatethe blue side of the horizontal branch. While the largest projectedrotational velocities seen among field blue and red horizontal-branchstars are consistent with their different sizes, neither are consistentwith the large values we find for the largest red giants. This suggeststhat some form of angular momentum loss (and possibly mass loss) hasbeen at work. Also puzzling is the apparent absence of rotation seen infield RR Lyrae variables. Angular momentum transfer and conservation inevolved metal-poor field stars thus pose many interesting questions forthe evolution of low-mass stars.
| 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
| IRFM temperature calibrations for the Vilnius, Geneva, RI(C) and DDO photometric systems
We have used the infrared flux method (IRFM) temperatures of a largesample of late type dwarfs given by Alonso et al. (\cite{alonso:irfm})to calibrate empirically the relations Teff=f (colour,[Fe/H]) for the Vilnius, Geneva, RI(C) (Cousins) and DDOphotometric systems. The resulting temperature scale and intrinsiccolour-colour diagrams for these systems are also obtained. From thisscale, the solar colours are derived and compared with those of thesolar twin 18 Sco. Since our work is based on the same Teffand [Fe/H] values used by Alonso et al. (\cite{alonso:escala}) tocalibrate other colours, we now have an homogeneous calibration for alarge set of photometric systems.Based on data from the GCPD.
| Abundances and Kinematics of Field Stars. II. Kinematics and Abundance Relationships
As an investigation of the origin of ``α-poor'' halo stars, weanalyze kinematic and abundance data for 73 intermediate-metallicitystars (-1>[Fe/H]>=-2) selected from Paper I of this series. We findevidence for a connection between the kinematics and the enhancement ofcertain element-to-iron ([X/Fe]) ratios in these stars. Statisticallysignificant correlations were found between [X/Fe] and galacticrest-frame velocities (vRF) for Na, Mg, Al, Si, Ca, and Ni,with marginally significant correlations existing for Ti and Y as well.We also find that the [X/Fe] ratios for these elements all correlatewith a similar level of significance with [Na/Fe]. Finally, we comparethe abundances of these halo stars against those of stars in nearbydwarf spheroidal (dSph) galaxies. We find significant differencesbetween the abundance ratios in the dSph stars and halo stars of similarmetallicity. From this result, it is unlikely that the halo stars in thesolar neighborhood, including even the ``α-poor'' stars, were oncemembers of disrupted dSph galaxies similar to those studied to date.
| Abundances of Cu and Zn in metal-poor stars: Clues for Galaxy evolution
We present new observations of copper and zinc abundances in 90metal-poor stars, belonging to the metallicity range -3<[Fe/H]<-0.5. The present study is based on high resolutionspectroscopic measurements collected at the Haute Provence Observatoire(R= 42 000, S/N>100). The trend of Cu and Zn abundances as a functionof the metallicity [Fe/H] is discussed and compared to that of otherheavy elements beyond iron. We also estimate spatial velocities andgalactic orbital parameters for our target stars in order to disentanglethe population of disk stars from that of halo stars using kinematiccriteria. In the absence of a firm a priori knowledge of thenucleosynthesis mechanisms controlling Cu and Zn production, and of therelative stellar sites, we derive constraints on these last from thetrend of the observed ratios [Cu/Fe] and [Zn/Fe] throughout the historyof the Galaxy, as well as from a few well established properties ofbasic nucleosynthesis processes in stars. We thus confirm that theproduction of Cu and Zn requires a number of different sources (neutroncaptures in massive stars, s-processing in low and intermediate massstars, explosive nucleosynthesis in various supernova types). We alsoattempt a ranking of the relative roles played by different productionmechanisms, and verify these hints through a simple estimate of thegalactic enrichment in Cu and Zn. In agreement with suggestionspresented earlier, we find evidence that type Ia Supernovae must play arelevant role, especially for the production of Cu. Based on the spectracollected with the 1.93-m telescope of Haute Provence Observatory.
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
| Heavy element abundances in cool dwarf stars: An implication for the evolution of the Galaxy
We present revised strontium, barium and europium abundances for 63 coolstars with metallicities [Fe/H] ranging from -2.20 to 0.25. The stellarsample has been extracted from Fuhrmann's lists (\cite{Fuhr3, Fuhr5}).It is confined to main-sequence and turnoff stars. The results are basedon NLTE line formation obtained in differential model atmosphereanalyses of spectra that have a typical S/N of 200 and a resolution of40 000 to 60 000. The element abundance ratios reveal a distinctchemical history of the halo and thick disk compared with that of thethin disk. Europium is overabundant relative to iron and barium in haloand thick disk stars suggesting that during the formation of thesegalactic populations high-mass stars exploding as SNe II dominatednucleosynthesis on a short time scale of the order of 1 Gyr. We note theimportance of [Eu/Mg] determinations for halo stars. Our analysis leadsto the preliminary conclusion that Eu/Mg ratios found in halo stars donot support current theoretical models of the r-process based onlow-mass SNe; instead they seem to point at a halo formation time muchshorter than 1 Gyr. A steep decline of [Eu/Fe] and a slight decline of[Eu/Ba] with increasing metallicity have been first obtained for thickdisk stars. This indicates the start of nucleosynthesis in the lowermass stars, in SN I and AGB stars, which enriched the interstellar gaswith iron and the most abundant s-process elements. From a decrease ofthe Eu/Ba ratio by ~ 0.10 ... 0.15 dex the time interval correspondingto the thick disk formation phase can be estimated. The step-like changeof element abundance ratios at the thick to thin disk transition foundin our previous analysis (Mashonkina & Gehren \cite{euba}) isconfirmed in this study: [Eu/Ba] and [Eu/Fe] are reduced by ~ 0.25 dexand ~ 0.15 dex, respectively; [Ba/Fe] increases by ~ 0.1 dex. This isindicative of an intermediate phase before the early stage of the thindisk developed, during which only evolved middle and low mass (<8M_sun) stars contributed to nucleosynthesis. Our data provide anindependent method to calculate the duration of this phase. The mains-process becomes dominant in the production of heavy elements beyondthe iron group during the thin disk evolution. We find that in the thindisk stars Ba/Fe ratios increase with time from [Ba/Fe] = -0.06 in starsolder than 8 Gyr to [Ba/Fe] = 0.06 in stars that are between 2 and 4 Gyrold. Based on observations collected at the German Spanish AstronomicalCenter, Calar Alto, Spain.
| Analysis of neutron capture elements in metal-poor stars
We derived model atmosphere parameters (Teff, log g, [Fe/H],Vt) for 90 metal-deficient stars (-0.5<[Fe/H]<-3),using echelle spectra from the ELODIE library (Soubiran et al.\cite{soubet98}). These parameters were analyzed and compared withcurrent determinations by other authors. The study of the followingelements was carried out: Mg, Si, Ca, Sr, Y, Ba, La, Ce, Nd, and Eu. Therelative contributions of s- and r-processes were evaluated andinterpreted through theoretical computations of the chemical evolutionof the Galaxy. The chemical evolution models (Pagel &Tautvaišienė \cite{pagta95}; Timmes et al. \cite{timet95})depict quite well the behaviour of [Si/Fe], [Ca/Fe] with [Fe/H]. Thetrend of [Mg/Fe] compares more favourably with the computations of Pagel& Tautvaišienė (\cite{pagta95}) than those of Timmes etal. (\cite{timet95}). The runs of n-capture elements vs. metallicity aredescribed well both by the model of Pagel & Tautvaišienė(\cite{pagta95}, \cite{pagta97}) and by the model of Travaglio et al.(\cite{travet99}) at [Fe/H]>-1.5, when the matter of the Galaxy issufficiently homogeneous. The analysis of n-capture element abundancesconfirms the jump in [Ba/Fe] at [Fe/H]=-2.5. Some stars from our sampleat [Fe/H]<-2.0 show a large scatter of Sr, Ba, Y, Ce. This scatter isnot caused by the errors in the measurements, and may reflect theinhomogeneous nature of the prestellar medium at early stages ofgalactic evolution. The matching of [Ba/Fe], [Eu/Fe] vs. [Fe/H] with theinhomogeneous model by Travaglio et al. (\cite{travet01a}) suggests thatat [Fe/H]<-2.5, the essential contribution to the n-rich elementabundances derives from the r-process. The main sources of theseprocesses may be low mass SN II. The larger dispersion of s-processelement abundances with respect to alpha -rich elements may arise bothfrom the birth of metal-poor stars in globular clusters with followingdifferent evolutionary paths and (or) from differences in s-elementenrichment in Galaxy populations. Based on spectra collected at theObservatoire de Haute-Provence (OHP), France
| On the stellar content of the open clusters Melotte 105, Hogg 15, Pismis 21 and Ruprecht 140
CCD observations in the B, V and I passbands have been used to generatecolour-magnitude diagrams reaching down to V ~ 19 mag for two slightlycharacterized (Melotte 105 and Hogg 15) and two almost unstudied (Pismis21 and Ruprecht 140) open clusters. The sample consists of about 1300stars observed in fields of about 4arcmin x4arcmin . Our analysis showsthat neither Pismis 21 nor Ruprecht 140 are genuine open clusters sinceno clear main sequences or other meaningful features can be seen intheir colour-magnitude diagrams. Melotte 105 and Hogg 15 are openclusters affected by E(B-V) = 0.42 +/- 0.03 and 0.95 +/- 0.05,respectively. Their distances to the Sun have been estimated as 2.2 +/-0.3 and 2.6 +/- 0.08 kpc, respectively, while the corresponding agesestimated from empirical isochrones fitted to the Main Sequence clustermembers are ~ 350 Myr and 300 Myr, respectively. The present data arenot consistent with the membership of the WN6 star HDE 311884 to Hogg15. Tables 2 to 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.793.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/370/931
| Abundances and Kinematics of Field Halo and Disk Stars. I. Observational Data and Abundance Analysis
We describe observations and abundance analysis of a high-resolution,high signal-to-noise ratio survey of 168 stars, most of which aremetal-poor dwarfs. We follow a self-consistent LTE analysis technique todetermine the stellar parameters and abundances, and we estimate theeffects of random and systematic uncertainties on the resultingabundances. Element-to-iron ratios are derived for key α-, odd-Z,Fe-peak, and r- and s-process elements. Effects of non-LTE on theanalysis of Fe I lines are shown to be very small on average.Spectroscopically determined surface gravities are derived that arequite close to those obtained from Hipparcos parallaxes.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Age-luminosity relations for low-mass metal-poor stars
We present a grid of evolutionary calculations for metal-poor low-massstars for a variety of initial helium and metal abundances. Theintention is mainly to provide a database for deriving directly stellarages of halo and globular cluster stars for which basic stellarparameters are known, but the tracks can also be used for isochrone orluminosity function construction, since they extend to the tip of thered giant branch. Fitting formulae for age-luminosity relations areprovided as well. The uncertainties of the evolutionary ages due toinherent shortcomings in the models and due to the unclear effectivenessof diffusion are discussed. A first application to field single stars ispresented. Appendix and Tables A1 to A24 are only available inelectronic form at http://www.edpsciences.org
| Barium and europium abundances in cool dwarf stars and nucleosynthesis of heavy elements
We revise barium abundances in 29 cool stars with metallicities [Fe/H]ranging from -2.20 to 0.07 and europium abundances in 15 stars with[Fe/H] from -1.52 to 0.07. The sample has been extracted from Fuhrmann'slists (\cite{Fuhr3, Fuhr4}) and confined to main-sequence and turnoffstars with only one subgiant added. The results are based ondifferential NLTE model atmosphere analyses of spectra that have atypical S/N of 200 and a resolution of 40000 or 60000. The statisticalequilibrium of Eu Ii is first investigated with a model atom containing32 levels of Eu Ii plus the ground state of Eu Iii. NLTE effectsdecrease the equivalent widths of the Eu Ii lines compared with LTEresulting in positive NLTE abundance corrections which are below 0.08dex for all the stars investigated. The solar barium abundance eps{Ba,sun} = 2.21 and the europium abundance eps {Eu,sun} = 0.53 are foundfrom the Ba Ii and Eu Ii solar flux line profile fitting, and theycoincide within error bars with meteoritic abundances of Grevesse et al.(\cite{met96}). Here the usual scale with eps {H} = 12 is used. Theisotopic ratio \iso{151}{Eu}: \iso{153}{Eu} = 55: 45 is obtained fromsolar disk center intensity profile fitting of the Eu Ii lambda 4129Åline. We report here for the first time that the elemental ratios[Ba/Fe], [Eu/Fe] and [Eu/Ba] show a different behaviour for stars ofdifferent Galactic populations. For the halo stars the [Ba/Fe] ratiosare approximately solar, europium is overabundant relative to iron andbarium with the mean values [Eu/Fe] = 0.62 and [Eu/Ba] = 0.64. For thickdisk stars it is found that a) barium is slightly underabundant relativeto iron by about 0.1 dex; b) europium is overabundant relative to ironwith the [Eu/Fe] ratios between 0.30 and 0.44; and c) europium isoverabundant relative to barium with a mean value of [Eu/Ba] = 0.49 +/-0.03. A step-like change in the [Eu/Ba] and [Ba/Fe] ratios occurs at thethick to thin disk transition; so, nearly solar elemental ratios[Ba/Fe], [Eu/Fe] and [Eu/Ba] are found for the thin disk stars. Thesedata suggest that a) the halo and thick disk stellar population formedquickly during an interval comparable with the evolution time of an AGBprogenitor of 3 to 4 M_sun, and the r-process dominated heavy elementproduction at that epoch; b) there was a hiatus in star formation beforethe early stage of the thin disk developed. The even-to-odd Ba isotoperatios estimated from hyperfine structure (HFS) affecting the Ba Iiresonance line in the halo and thick disk stars favour a significantcontribution of \iso{138}{Ba} to barium for a pure r-process, and thisis supported by the recent data of Arlandini et al. (\cite{rs99}). Basedon observations at the German Spanish Astronomical Center, Calar Alto,Spain
| Mixing along the red giant branch in metal-poor field stars
We have determined Li, C, N, O, Na, and Fe abundances, and12C/13C isotopic ratios for a sample of 62 fieldmetal-poor stars in the metallicity range -2<=[Fe/H]<= -1. Starswere selected in order to have accurate luminosity estimates from theliterature, so that evolutionary phases could be clearly determined foreach star. We further enlarged this dataset by adding 43 more starshaving accurate abundances for some of these elements and similarly welldefined luminosities from the literature. This large sample was used toshow that (small mass) lower-RGB stars (i.e. stars brighter than thefirst dredge-up luminosity and fainter than that of the RGB bump) haveabundances of light elements in agreement with predictions fromclassical evolutionary models: only marginal changes occur for CNOelements, while dilution within the convective envelope causes thesurface Li abundance to decrease by a factor of ~ 20. A second, distinctmixing episode occurs in most (perhaps all) small mass metal-poor starsjust after the RGB bump, when the molecular weight barrier left by themaximum inward penetration of the convective shell is canceled by theoutward expansion of the H-burning shell, in agreement with recenttheoretical predictions. In field stars, this second mixing episode onlyreaches regions of incomplete CNO burning: it causes a depletion of thesurface 12C abundance by about a factor of 2.5, and acorresponding increase in the N abundance by about a factor of 4. The12C/13C is lowered to about 6 to 10 (close to butdistinctly higher than the equilibrium value of 3.5), while practicallyall remaining Li is burnt. However an O-Na anti-correlation such astypically observed amongst globular cluster stars, is not present infield stars. None of the 29 field stars more evolved than the RGB bump(including 8 RHB stars) shows any sign of an O depletion or Naenhancement. This means that the second mixing episode is not deepenough to reach regions were ON-burning occurs in field stars. Based inpart on observations made at the ESO La Silla ObservatoryTables 1, 2, 3, 5 and 6 are available in electronic form only at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
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| Csillagkép: | Egyszarvú |
| Rektaszcenzió: | 06h26m40.77s |
| Deklináció: | +03°25'29.8" |
| Vizuális fényesség: | 8.03 |
| Távolság: | 136.24 parszek |
| RA sajátmozgás: | 4.8 |
| Dec sajátmozgás: | -91.9 |
| B-T magnitude: | 8.848 |
| V-T magnitude: | 8.098 |
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