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Публикации по объекту
Medium-resolution Isaac Newton Telescope library of empirical spectra
A new stellar library developed for stellar population synthesismodelling is presented. The library consists of 985 stars spanning alarge range in atmospheric parameters. The spectra were obtained at the2.5-m Isaac Newton Telescope and cover the range λλ3525-7500 Å at 2.3 Å (full width at half-maximum) spectralresolution. The spectral resolution, spectral-type coverage,flux-calibration accuracy and number of stars represent a substantialimprovement over previous libraries used in population-synthesis models.
| 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.
| Sulphur abundances in metal-poor stars
We investigate the debated "sulphur discrepancy" found among metal-poorstars of the Galactic halo with [Fe/H] < -2. This discrepancy stemsin part from the use of two different sets of sulphur lines, the veryweak triplet at 8694-95 Å and the stronger triplet lines at9212-9238 Å. For three representative cases of metal-poor dwarf,turnoff and subgiant stars, we argue that the abundances from theλλ8694-95 lines have been overestimated which has led to acontinually rising trend of [S/Fe] as metallicity decreases. Given thatthe near-IR region is subject to CCD fringing, these weak lines becomeexcessively difficult to measure accurately in the metallicity regime of[Fe/H] < -2. Based on homogeneously determined spectroscopic stellarparameters, we also present updated [S/Fe] ratios from theλλ9212-9238 lines which suggest a plateau-like behavioursimilar to that seen for other alpha elements.
| 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.
| 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.
| Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra
The LTE abundances of sulfur (S) of 21 metal-poor stars and one normalstar were explored in the metallicity range of -3 < [Fe/H] ≤ 0,based on the equivalent widths of the S I (1) 9212, 9237Å and S I(6) 8693, 9894Å lines measured on high-resolution spectra, whichwere observed by the OAO 1.88-m telescope equipped with HIDES. Our mainresults are: (1) The abundances derived from the S I (6) lines areconsistent with those from the S I (1) lines among our sample stars inthe range of [Fe/H] > -2 with an average difference of +0.03 ±0.05 dex, whereas a significant discrepancy is observed in the range of[Fe/H] ≤ -2. (2) The behavior of [S(6)/Fe], versus [Fe/H] of ourhalo sample stars exhibits a nearly flat trend with an average of +0.62± 0.09 dex in the range of -3 < [Fe/H] < -1.25, and shows adistribution around +0.29 dex in -1.25 ≤ [Fe/H] ≤ -0.7. Oursample stars with -1.25 ≤ [Fe/H] ≤ -0.5 follow an increasingtrend with decreasing [Fe/H]. The behavior of [S(1)/Fe] of our samplestars also shows essentially the same trend as [S(6)/Fe], though it isquantitatively different. (3) The S behavior in the range of -3 <[Fe/H] ≤ 0 inferred from the abundances of multiplets 6 and 1 arequalitatively consistent with each other, and may be represented by acombination of a nearly flat trend and a linearly increasing trend withdecreasing [Fe/H]. A transition of the trend is likely to occur at[Fe/H] -1.5 dex.
| 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.
| 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.
| 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.
| 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.
| On the Galactic chemical evolution of sulfur
Sulfur abundances have been determined for ten stars to resolve a debatein the literature on the Galactic chemical evolution of sulfur in thehalo phase of the Milky Way. Our analysis is based on observations ofthe S I lines at 9212.9, 9228.1, and 9237.5 Å for stars for whichthe S abundance was obtained previously from much weaker S I lines at8694.0 and 8694.6 Å. In contrast to the previous results showing[S/Fe] to rise steadily with decreasing [Fe/H], our results show that[S/Fe] is approximately constant for metal-poor stars ([Fe/H] -1)at [S/Fe] ≃ +0.3. Thus, sulfur behaves in a similar way to theother \alpha elements, with an approximately constant [S/Fe] formetallicities lower than [Fe/H]≃ -1. We suggest that the reasonfor the earlier claims of a rise of [S/Fe] is partly due to the use ofthe weak S I 8694.0 and 8694.6 Å lines and partly uncertainties inthe determination of the metallicity when using Fe I lines. The S I9212.9, 9228.1, and 9237.5 Å lines are preferred for an abundanceanalysis of sulfur for metal-poor stars.
| First stars IV. CS 29497-030: Evidence for operation of the s-process at very low metallicity
We present an abundance analysis of the very metal-poor, carbon-enhancedstar CS 29497-030. Our results indicate that this unusually hot turnoffstar (Teff = 6650 K, log g = 3.5) has a metallicity [Fe/H] =-2.8, and exhibits large overabundances of carbon ([C/Fe] = +2.38),nitrogen ([N/Fe] = +1.88), and oxygen ([O/Fe] = +1.67). This star alsoexhibits a large enhancement in its neutron-capture elements; thepattern follows that expected to arise from the s-process. Inparticular, the Pb abundance is found to be very high with respect toiron ([Pb/Fe] = +3.5), and also with respect to the second peaks-process elements (e.g., Ba, La, Ce, Nd), which fits into the newlyintroduced classification of lead (Pb) stars. The known spectroscopicbinary status of this star, along with the observed s-process abundancepattern, suggest that it has accreted matter from a companion, whichformerly was an Asymptotic Giant-Branch (AGB) star. In a preliminaryanalysis, we have also identified broad absorption lines of metallicspecies that suggest a large axial rotational velocity for this star,which may be the result of spin-up associated with the accretion ofmaterial from its previous AGB companion. In addition, this star isclearly depleted in the light element Li. When considered along with itsrather high inferred temperature, these observations are consistent withthe expected properties of a very low metallicity halo blue straggler.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (program ID 165.N-0276(A)).Table \ref{tab6} is only available in electronic form athttp://www.edpsciences.org
| 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.
| 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.
| 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.
| An automated system to classify stellar spectra - I
Analyses of stellar spectra often begin with the determination of anumber of parameters that define a model atmosphere. This work presentsa prototype for an automated spectral classification system that uses a150-Å-wide region around Hβ, and applies to stars of spectraltypes A-K with normal (scaled solar) chemical composition. The new toolexploits synthetic spectra based on plane-parallel flux-constant modelatmospheres. The input data are high signal-to-noise ratio spectra witha resolution greater than approximately 1 Å. The output parametersare forced to agree with an external scale of effective temperatures,based on the infrared flux method. The system is fast - a spectrum isclassified in a few seconds - and well suited for implementation on aweb server. We estimate upper limits to the 1σ random error in theretrieved effective temperatures, surface gravities and metallicities as100 K, 0.3 and 0.1 dex, respectively.
| Keck NIRSPEC Infrared OH Lines: Oxygen Abundances in Metal-poor Stars down to [Fe/H] = -2.9
Infrared OH lines at 1.5-1.7 μm in the H band were obtained with theNIRSPEC high-resolution spectrograph at the 10 m Keck Telescope for asample of seven metal-poor stars. Detailed analyses have been carriedout, based on optical high-resolution data obtained with the Fiber-fedExtended Range Optical Spectrograph at ESO. Stellar parameters werederived by adopting infrared flux method effective temperatures,trigonometric and/or evolutionary gravities, and metallicities from FeII lines. We obtain that the sample stars with metallicities[Fe/H]<-2.2 show a mean oxygen abundance [O/Fe]~0.54 for a solaroxygen abundance of ɛ(O)=8.87, or [O/Fe]~0.64 ifɛ(O)=8.77 is assumed. Observations carried out with the KeckTelescope within the Gemini-Keck agreement, and at the European SouthernObservatory.
| Determination of accurate stellar radial-velocity measures
Wavelength measurements in stellar spectra cannot readily be interpretedas true stellar motion on the sub-km s-1 accuracy level dueto the presence of many other effects, such as gravitational redshiftand stellar convection, which also produce line shifts. Following arecommendation by the IAU, the result of an accurate spectroscopicradial-velocity observation should therefore be given as the``barycentric radial-velocity measure'', i.e. the absolute spectralshift as measured by an observer at zero gravitational potential locatedat the solar-system barycentre. Standard procedures for reducingaccurate radial-velocity observations should be reviewed to take intoaccount this recommendation. We describe a procedure to determineaccurate barycentric radial-velocity measures of bright stars, based ondigital cross-correlation of spectra obtained with the ELODIEspectrometer (Observatoire de Haute-Provence) with a synthetic templateof Fe I lines. The absolute zero point of the radial-velocity measuresis linked to the wavelength scale of the Kurucz (1984) Solar Flux Atlasvia ELODIE observations of the Moon. Results are given for the Sun and42 stars, most of them members of the Hyades and Ursa Major clusters.The median internal standard error is 27 m s-1. The externalerror is estimated at around 120 m s-1, mainly reflecting theuncertainty in the wavelength scale of the Solar Flux Atlas. For the Sunwe find a radial-velocity measure of +257+/- 11 m s-1referring to the full-disk spectrum of the selected Fe I lines. Based onobservations made at Observatoire de Haute-Provence
| 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.
| Kurucz Model Energy Distributions: A Comparison With Real Stars. II. Metal-Deficient Stars
Energy distributions of synthetic spectra for Kurucz model atmospheresare compared with observed energy distributions of metal-deficient starsof the blue horizontal-branch (BHB), F--G--K subdwarf (SD) and G--Kgiant (MDG) types. The best coincidence of the synthetic and observedenergy curves is found for BHB stars. The largest differences are foundin the ultraviolet wavelengths for subdwarfs and cool MDGs. Theinfluence of errors of effective temperatures, gravities andmetallicities is estimated.
| 12C/13C in Metal-poor Field Halo Giants
We have estimated 12C/13C in 15 metal-poor(-2.4<=[Fe/H]<=-1.0) field halo giant stars from spectra of the13CO v=3-1 and v=2-0 band heads and surrounding12CO and 13CO R-branch lines. Our isotope ratiosare consistent with previous measurements for stars in our sample with12C/13C determined either from the infraredfirst-overtone bands of CO or from optical G-band spectra of CH and redsystem bands of CN. We have also compiled carbon isotope ratios from theliterature for a much larger sample of field and cluster red giantbranch (RGB) stars spanning a wide range of metallicities(-2.4<=[Fe/H]<=solar). Combining these data, we confirm thedecline of the isotope ratio as stars evolve up the RGB and we haveidentified a trend toward higher levels of mixing in more metal-poorstars. Standard RGB first dredge-up models do not predict the carbonisotope ratios that we observe in the more evolved (higher luminosity)metal-poor stars, but more recent models that account for other mixingmechanisms can explain these data; even for very metal-poor stars suchas those that we have observed in the Galactic halo.
| Sulphur Abundance in Very Metal-poor Stars
We have obtained high-resolution high signal-to-noise ratio spectra ofthe S I near-infrared doublet at 8694 Å in eight metal-poor starswith metallicities in the range -0.6<=[Fe/H]<=-3.0. Elementalsulphur abundances were derived for six targets, and upper limits wereset for two of the most metal-poor stars in the sample. The sulphurdoublet at 8694.62 Å has been detected in three stars (HD 19445,HD 2665, and HD 2796) with [Fe/H]<=-1.9. Our observations, combinedwith those available in the literature, indicate a monotonic increase ofthe [S/Fe] ratio as [Fe/H] decreases, reaching values of [S/Fe]~0.7-0.8below [Fe/H]=-2. We discuss plausible scenarios for the interpretationof these results. Based on observations obtained with the WilliamHerschel Telescope (WHT), operated on the island of La Palma by theIsaac Newton Group in the Spanish Observatorio del Roque de losMuchachos (ORM) of the Instituto de Astrofísica de Canarias.
| Oxygen Abundances in Metal-poor Stars (-2.2<[Fe/H]<-1.2) from Infrared OH Lines
Infrared OH lines at 1.55-1.56 μm in the H band were obtained withthe Phoenix high-resolution spectrograph at the 2.1 m telescope of theKitt Peak National Observatory for a sample of 14 metal-poor stars.Detailed analyses of the sample stars have been carried out, derivingstellar parameters based on two methods: (a) spectroscopic parametersand (b) infrared flux method (IRFM) effective temperatures,trigonometric gravities, and metallicities from Fe II lines. The Fe Ilines present in the H-band region observed were well fitted by thestellar parameters within Δ[Fe/H]<=0.15 dex. The oxygenabundances were derived from fits of spectrum synthesis calculations tothe infrared OH lines. CO lines in the H and K bands were obtained for asubsample in order to determine their carbon abundances. Adopting thespectroscopic parameters, a mean oxygen-to-iron ratio of [O/Fe]~0.52 isobtained, whereas using the IRFM temperatures, Hipparcos gravities, and[Fe II/H], [O/Fe]~0.25 is found. A mean of the two methods gives a finalvalue of [O/Fe]~0.4 for the metallicity range -2.2<[Fe/H]<-1.2 ofthe sample metal-poor stars.
| Diverse Supernova Sources for the R-Process and Abundances in Metal-poor Stars
The dispersion and mean trends of r-process abundances in metal-poorstars are discussed based on a model of diverse supernova sources forthe r-process. This model is unique in that its key parameters areinferred from solar system data independent of stellar observations atlow metallicities. It is shown that this model provides a goodexplanation for the observed dispersion and mean trend of Eu abundancesover -3<~[Fe/H]<~-1. It is also shown that this model provides ameans to discuss r-abundances in general. For example, the Ag abundancein any metal-poor star with observed Eu and Fe abundances can becalculated from the model. This approach is demonstrated with successfor two stars and can be further tested by future Ag data. Thedispersion and mean trend of Ag abundances in metal-poor stars are alsocalculated for comparison with future observations.
| A database of high and medium-resolution stellar spectra
We present a database of 908 spectra of 709 stars obtained with theELODIE spectrograph at the Observatoire de Haute-Provence. 52 orders ofthe echelle spectra have been carefully fitted together to providecontinuous, high-resolution spectra in the wavelength range lambdalambda = 410-680 nm. The archive provides a large coverage of the spaceof atmospheric parameters: T_eff from 3700 K to 13 600 K, log g from0.03 to 5.86 and [Fe/H] from -2.8 to +0.7. At the nominal resolution,R=42 000, the mean signal-to-noise ratio is 150 per pixel. The spectragiven at this resolution are normalized to their pseudo-continuum andare intended to serve for abundance studies, spectral classification andtests of stellar atmosphere models. A lower resolution version of thearchive, at R=10 000, is calibrated in physical flux with a broad-bandphotometric precision of 2.5% and narrow-band precision of 0.5%. It iswell suited to stellar population synthesis of galaxies and clusters,and to kinematical investigations of stellar systems. The archive isdistributed in FITS format through the HYPERCAT and CDS databases. Basedon observations made on the 193 cm telescope at the Haute-ProvenceObservatory, France. Table 1 is only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/1048
| Lead: Asymptotic Giant Branch Production and Galactic Chemical Evolution
The enrichment of Pb in the Galaxy is followed in the framework of adetailed model of Galactic chemical evolution that already provedadequate to reproduce the chemical enrichment of O and of the elementsfrom Ba to Eu. The stellar yields are computed through nucleosynthesiscalculations in the asymptotic giant branch (AGB) phase of low- andintermediate-mass stars covering a wide range of metallicities. Thephysical parameters of the stellar structure were derived from fullstellar evolutionary models computed previously. We show that low-massAGB stars are the main producers of Pb in the Galaxy, with a complexdependence on metallicity and a maximum efficiency at [Fe/H]~-1. Ourcalculations succeed in reproducing the abundances of Pb isotopes in thesolar system: the role attributed by the classical analysis of thes-process to the strong component, in order to explain more than 50% ofsolar 208Pb, is actually played by the high production of Pbin low-mass and low-metallicity AGB stars. We then follow the Galacticchemical evolution of Pb isotopes and give our expectations on thes-process contribution to each of them at the epoch of the solar systemformation. Finally, we present new spectroscopic estimates of Pbabundance on a sample of field stars and compare them, together with afew other determinations available, with the predicted trend of [Pb/Fe]in the Galaxy.
| Detection of Metal-poor Stars in the Direction of the North Galactic Pole
A simple approach to detecting metal-poor stars is to measure amagnesium index, which depends on the Mg H band plus the three nearby Mgb lines and is derived through intermediate-band interference filters.An empirically established line of demarcation in the Mg index versusB-V diagram separates metal-poor stars from solar-abundance stars. Afurther separation between metal-poor dwarfs and giants depends on B-Vprimarily dwarfs for B-V<0.55, giants for B-V>0.7, with both dwarfsand giants falling in the transition region. For the metal-poor giantsthe distance from the demarcation line correlates well with [Fe/H],permitting estimates of stellar abundances. Stars in two regions on thesky in the vicinity of the north Galactic pole have been observed withsuch a set of filters. Eighteen stars (6% of the population of 299) inthe sample covering the V range 8.7 to 15.6 and 48 stars (31% of thepopulation of 163) in a deeper probe to V=19.9 found through thisprocess are suspected metal-poor stars according to their Mg indices.Twenty-three are specifically deemed giants, with<[Fe/H]><=-1.5.
| Neutron-Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal-poor Giants
New abundances for neutron-capture (n-capture) elements in a largesample of metal-poor giants from the Bond survey are presented. Thespectra were acquired with the KPNO 4 m echelle and coudé feedspectrographs, and have been analyzed using LTE fine-analysis techniqueswith both line analysis and spectral synthesis. Abundances of eightn-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, and Dy) in 43 stars havebeen derived from blue (λλ4070-4710, R~20,000, S/Nratio~100-200) echelle spectra and red (λλ6100-6180,R~22,000, S/N ratio~100-200) coudé spectra, and the abundance ofBa only has been derived from the red spectra for an additional 27stars. Overall, the abundances show clear evidence for a largestar-to-star dispersion in the heavy element-to-iron ratios. Thiscondition must have arisen from individual nucleosynthetic events inrapidly evolving halo progenitors that injected newly manufacturedn-capture elements into an inhomogeneous early Galactic halointerstellar medium. The new data also confirm that at metallicities[Fe/H]<~-2.4, the abundance pattern of the heavy (Z>=56) n-captureelements in most giants is well-matched to a scaled solar systemr-process nucleosynthesis pattern. The onset of the main r-process canbe seen at [Fe/H]~-2.9 this onset is consistent with the suggestion thatlow mass Type II supernovae are responsible for the r-process.Contributions from the s-process can first be seen in some stars withmetallicities as low as [Fe/H]~-2.75 and are present in most stars withmetallicities [Fe/H]>-2.3. The appearance of s-process contributionsas metallicity increases presumably reflects the longer stellarevolutionary timescale of the (low-mass) s-process nucleosynthesissites. The lighter n-capture elements (Sr-Y-Zr) are enhanced relative tothe heavier r-process element abundances. Their production cannot beattributed solely to any combination of the solar system r- and mains-processes, but requires a mixture of material from the r-process andfrom an additional n-capture process that can operate at early Galactictime. This additional process could be the weak s-process in massive(~25 Msolar) stars, or perhaps a second r-process site, i.e.,different from the site that produces the heavier (Z>=56) n-captureelements.
| 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.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Кассиопея |
| Прямое восхождение: | 00h30m45.45s |
| Склонение: | +57°03'53.6" |
| Видимая звёздная величина: | 7.737 |
| Расстояние: | 473.934 парсек |
| Собственное движение RA: | 40.9 |
| Собственное движение Dec: | -65.3 |
| B-T magnitude: | 8.649 |
| V-T magnitude: | 7.813 |
Каталоги и обозначения:
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