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Публикации по объекту
Oxygen Abundances in Low- and High-α Field Halo Stars and the Discovery of Two Field Stars Born in Globular Clusters
Oxygen abundances of 67 dwarf stars in the metallicity range -1.6 <[Fe/H] < -0.4 are derived from a non-LTE analysis of the 777 nm O Itriplet lines. These stars have precise atmospheric parameters measuredby Nissen and Schuster, who find that they separate into three groupsbased on their kinematics and α-element (Mg, Si, Ca, Ti)abundances: thick disk, high-α halo, and low-α halo. We findthe oxygen abundance trends of thick-disk and high-α halo starsvery similar. The low-α stars show a larger star-to-star scatterin [O/Fe] at a given [Fe/H] and have systematically lower oxygenabundances compared to the other two groups. Thus, we find the behaviorof oxygen abundances in these groups of stars similar to that of theα elements. We use previously published oxygen abundance data ofdisk and very metal-poor halo stars to present an overall view (-2.3< [Fe/H] < +0.3) of oxygen abundance trends of stars in the solarneighborhood. Two field halo dwarf stars stand out in their O and Naabundances. Both G53-41 and G150-40 have very low oxygen and very highsodium abundances, which are key signatures of the abundance anomaliesobserved in globular cluster (GC) stars. Therefore, they are likelyfield halo stars born in GCs. If true, we estimate that at least 3%± 2% of the local field metal-poor star population was born inGCs.
| Two distinct halo populations in the solar neighborhood. IV. Lithium abundances
Context. A previous study of F and G main-sequence stars in the solarneighborhood has revealed the existence of two distinct halo populationswith a clear separation in [?/Fe] for the metallicity range -1.4< [Fe/H] < -0.7. Taking into account the kinematics and ages ofthe stars, some Galactic formation models suggest that the "high-alpha"halo stars were formed in situ, whereas the "low-alpha" stars have beenaccreted from satellite galaxies. Aims: In this paper weinvestigate if there is a systematic difference in the lithiumabundances of stars belonging to the high- and low-alpha halopopulations. Methods: Equivalent widths of the Li i 6707.8Å resonance line are measured from high resolution VLT/UVES andNOT/FIES spectra and used to derive Li abundances on the basis of MARCSmodel atmospheres. Furthermore, masses of the stars are determined fromthe log Teff - log g diagram by interpolating betweenevolutionary tracks based on Yonsei-Yale models. Results: Thereis no significant systematic difference in the lithium abundances ofhigh- and low-alpha stars. For the large majority of stars with masses0.7 < M/M&sun; < 0.9 and heavy-element mass fractions0.001 ? Z < 0.006, the lithium abundance is well fitted by arelation A(Li) = a0 + a1 M + a2 Z +a3 M Z, where a0, a1, a2,and a3 are constants. Extrapolating this relation to Z = 0leads to a lithium abundance close to the primordial value predictedfrom standard Big Bang nucleosynthesis calculations and the WMAP baryondensity. The relation, however, does not apply to stars withmetallicities below [Fe/H] ? - 1.5. Conclusions: We suggestthat metal-rich halo stars were formed with a lithium abundance close tothe primordial value, and that lithium in their atmospheres has beendepleted in time with an approximately linear dependence on stellar massand Z. The lack of a systematic difference in the Li abundances of high-and low-alpha stars indicates that an environmental effect is notimportant for the destruction of lithium.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility (programs 65.L-0507, 67.D-0086, 67.D-0439,68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585,76.B-0133 and 77.B-0507).Table 1 is available in electronic form at http://www.aanda.org
| Non-LTE effects on the lead and thorium abundance determinations for cool stars
Context. Knowing accurate lead abundances of metal-poor stars providesconstraints on the Pb production mechanisms in the early Galaxy.Accurately deriving thorium abundances permits a nucleo-chronometric agedetermination of the star. Aims: We aim to improve thecalculation of the Pb i and Th ii lines in stellar atmospheres based onnon-local thermodynamic equilibrium (non-LTE) line formation, and toevaluate the influence of departures from LTE on Pb and Th abundancedeterminations for a range of stellar parameters by varying themetallicity from the solar value down to [Fe/H] = -3. Methods: Wepresent comprehensive model atoms for Pb i and Th ii and describecalculations of the Pb i energy levels and oscillator strengths. Results: The main non-LTE mechanism for Pb i is the ultravioletoverionization. We find that non-LTE leads to systematically depletedtotal absorption in the Pb i lines and accordingly, positive abundancecorrections. The departures from LTE increase with decreasingmetallicity. Using the semi-empirical model atmosphere HM74, wedetermine the lead non-LTE abundance for the Sun to be log?Pb, &sun; = 2.09, in agreement with the meteoriticlead abundance. We revised the Pb and Eu abundances of the two stronglyr-process enhanced stars CS 31082-001 and HE1523-0901 and the metal-poor stellar sample. Our new resultsprovide strong evidence of universal Pb-to-Eu relative r-process yieldsduring the course of Galactic evolution. The stars in the metallicityrange -2.3 < [Fe/H] < -1.4 have Pb/Eu abundance ratios that are,on average, 0.51 dex higher than those of strongly r-process enhancedstars. We conclude that the s-process production of lead started asearly as the time when Galactic metallicity had reached [Fe/H] = -2.3.The average Pb/Eu abundance ratio of the mildly metal-poor stars, with-1.4 ? [Fe/H] ? -0.59, is very close to the corresponding SolarSystem value, in line with the theoretical predictions that AGB starswith [Fe/H] ? -1 provided the largest contribution to the solarabundance of s-nuclei of lead. The departures from LTE for Th ii arecaused by the pumping transitions from the levels with Eexc< 1 eV. Non-LTE leads to weakened Th ii lines and positive abundancecorrections. Overall, the abundance correction does not exceed 0.2 dexwhen collisions with H i atoms are taken into account in statisticalequilibrium calculations.
| Stellar population models in the UV. I. Characterisation of the New Generation Stellar Library
Context. The spectral predictions of stellar population models are notas accurate in the ultra-violet (UV) as in the optical wavelengthdomain. One of the reasons is the lack of high-quality stellarlibraries. The New Generation Stellar Library (NGSL), recently released,represents a significant step towards the improvement of this situation. Aims: To prepare NGSL for population synthesis, we determined theatmospheric parameters of its stars, we assessed the precision of thewavelength calibration and characterised its intrinsic resolution. Wealso measured the Galactic extinction for each of the NGSL stars. Methods: For our analyses we used ULySS, a full spectrum fittingpackage, fitting the NGSL spectra against the MILES interpolator. Results: We find that the wavelength calibration is precise up to 0.1px, after correcting a systematic effect in the optical range. Thespectral resolution varies from 3 Å in the UV to 10 Å in thenear-infrared (NIR), corresponding to a roughly constant reciprocalresolution R = ?/?? ? 1000 and an instrumentalvelocity dispersion ?ins ? 130 km s-1. Wederived the atmospheric parameters homogeneously. The precision for theFGK stars is 42 K, 0.24 and 0.09 dex for Teff, log g and[Fe/H], respectively. The corresponding mean errors are 29 K, 0.50 and0.48 dex for the M stars, and for the OBA stars they are 4.5 percent,0.44 and 0.18 dex. The comparison with the literature shows that ourresults are not biased.Table A1 is only available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/538/A143
| Two distinct halo populations in the solar neighborhood. III. Evidence from stellar ages and orbital parameters
Context. In Papers I and II of this series, we have found clearindications of the existence of two distinct populations of stars in thesolar neighborhood belonging to the metal-rich end of the halometallicity distribution function. Based on high-resolution, high S/Nspectra, it is possible to distinguish between "high-alpha" and"low-alpha" components using the [?/Fe] versus [Fe/H] diagram. Aims: Precise relative ages and orbital parameters are determined for67 halo and 16 thick-disk stars having metallicities in the range -1.4< [Fe/H] < -0.4 to better understand the context of the two halopopulations in the formation and evolution of the Galaxy. Methods: Ages are derived by comparing the positions of stars in the logTeff-log g diagram with isochrones from the Y2models interpolated to the exact [Fe/H] and [?/Fe] values of eachstar. The stellar parameters have been adopted from the precedingspectroscopic analyses, but possible systematic errors inTeff and log g are considered and corrected. With spacevelocities from Paper I as initial conditions, orbital integrations havebeen carried out using a detailed, observationally constrainedMilky Way model including a bar and spiral arms. Results: The "high-alpha" halo stars have ages 2-3 Gyr larger thanthe "low-alpha" ones, with some probability that the thick-disk starshave ages intermediate between these two halo components. The orbitalparameters show very distinct differences between the "high-alpha" and"low-alpha" halo stars. The "low-alpha" ones have rmax's to30-40 kpc, zmax's to ?18 kpc, and emax'sclumped at values greater than 0.85, while the "high-alpha" ones,rmax's to about 16 kpc, zmax's to 6-8 kpc, andemax values more or less uniformly distributed over 0.4-1.0. Conclusions: A dual in situ-plus-accretion formation scenariobest explains the existence and characteristics of these two metal-richhalo populations, but one remaining defect is that this model is notconsistent regarding the rmax's obtained for the in situ"high-alpha" component; the predicted values are too small. It appearsthat ? Cen may have contributed in asignificant way to the existence of the "low-alpha" component; recentmodels, including dynamical friction and tidal stripping, have producedresults consistent with the present mass and orbital characteristics of? Cen, while at the same time includingextremes in the orbital parameters as great as those of the "low-alpha"component.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility (programmes 65.L-0507, 67.D-0086, 67.D-0439,68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585,76.B-0133 and 77.B-0507).Tables 1 and 4 are available in electronic format http://www.aanda.org.
| Beryllium and Alpha-element Abundances in a Large Sample of Metal-poor Stars
The light elements, Li, Be, and B, provide tracers for many aspects ofastronomy including stellar structure, Galactic evolution, andcosmology. We have made observations of Be in 117 metal-poor starsranging in metallicity from [Fe/H] = -0.5 to -3.5 with KeckI/HIRES. Our spectra are high resolution (~42,000) and high signal tonoise (the median is 106 per pixel). We have determined the stellarparameters spectroscopically from lines of Fe I, Fe II, Ti I, and Ti II.The abundances of Be and O were derived by spectrum synthesistechniques, while abundances of Fe, Ti, and Mg were found from manyspectral line measurements. There is a linear relationship between[Fe/H] and A(Be) with a slope of +0.88 ± 0.03 over three ordersof magnitude in [Fe/H]. We find that Be is enhanced relative to Fe;[Be/Fe] is +0.40 near [Fe/H] ~-3.3 and drops to 0.0 near [Fe/H]~-1.7. For the relationship between A(Be) and [O/H], we find agradual change in slope from 0.69 ± 0.13 for the Be-poor/O-poorstars to 1.13 ± 0.10 for the Be-rich/O-rich stars. Inasmuch asthe relationship between [Fe/H] and [O/H] seems robustly linear (slope =+0.75 ± 0.03), we conclude that the slope change in Be versus Ois due to the Be abundance. Much of the Be would have been formed in thevicinity of Type II supernova (SN II) in the early history of the Galaxyand by Galactic cosmic-ray (GCR) spallation in the later eras. AlthoughBe is a by-product of CNO, we have used Ti and Mg abundances asalpha-element surrogates for O in part because O abundances are rathersensitive to both stellar temperature and surface gravity. We find thatA(Be) tracks [Ti/H] very well with a slope of 1.00 ± 0.04. Italso tracks [Mg/H] very well with a slope of 0.88 ± 0.03. We havekinematic information on 114 stars in our sample and they divide equallyinto dissipative and accretive stars. Almost the full range of [Fe/H]and [O/H] is covered in each group. There are distinct differences inthe relationships of A(Be) and [Fe/H] and of A(Be) and [O/H] for thedissipative and the accretive stars. It is likely that the formation ofBe in the accretive stars was primarily in the vicinity of SN II, whilethe Be in the dissipative stars was primarily formed by GCR spallation.We find that Be is not as good a cosmochronometer as Fe. We have found aspread in A(Be) that is valid at the 4? level between [O/H] =-0.5 and -1.0, which corresponds to -0.9 and-1.6 in [Fe/H].
| The non-local thermodynamic equilibrium barium abundance in dwarf stars in the metallicity range of -1 < [Fe/H] < +0.3
We present the results of determination of the barium abundanceconsidering the non-local thermodynamic equilibrium (NLTE) effects in172 dwarf stars in the metallicity range of -1 < [Fe/H] < +0.3,assigned to different Galactic substructures by kinematic criteria. Weused a model of the Ba atom with 31 levels of Ba I and 101 levels of BaII. The atmosphere models for the investigated stars were computed usingthe ATLAS9 code modified by new opacity distribution functions byCastelli & Kurucz. The NLTE profiles of the unblended Ba II (4554,5853, 6496 Å) were computed and then compared to those observed.The line 6141 Å was also used, but with an allowance for itscorrelation with the iron line. The average barium abundances in thethin and thick discs are 0.01 ± 0.08 and -0.03 ± 0.07,respectively. The comparison to the calculations of the Galacticchemical evolution by Serminato et al. was conducted. The trend obtainedfor the Ba abundance versus [Fe/H] suggests a complex barium productionprocess in the thin and thick discs.
| The copper and zinc abundances in stars of galactic sub-structures
We have determined abundances of copper, zinc, sodium, and aluminum inthe atmospheres of 172 F, G, and K dwarf stars (-1.0 < [Fe/H] <0.3) belonging to the Galaxy's thin and thick disks and to the Herculesmoving group. Our observations were performed with the ELODIEéchelle spectrometer on the 1.93-m telescope of the HauteProvence Observatory, with a resolving power of R = 42 000 andsignal-to-noise ratio S/N > 100. The Na, Al, Cu, and Zn abundanceswere derived in an LTE approximation; the synthetic spectrum for thecopper lines was calculated taking into account super-fine structure ofthe lines. We analyzed the abundances of these elements as a function ofmetallicity [Fe/H] for stars of the thin and thick disks of the Galaxyand the Hercules moving group. The Cu abundances and their trends withmetallicity are essentially the same in the three studiedsub-structures. The mean Al and Zn abundances for stars of the thin andthick disks differ significantly.
| The atmospheric parameters and spectral interpolator for the MILES stars
Context. Empirical libraries of stellar spectra are used to classifystars and synthetize stellar populations. MILES is a mediumspectral-resolution library in the optical domain covering a wide rangeof temperatures, surface gravities and metallicities. Aims: Weredetermine the atmospheric parameters of these stars in order toimprove the homogeneity and accuracy. We build an interpolating functionthat returns a spectrum as a function of the three atmosphericparameters, and finally we characterize the precision of the wavelengthcalibration and stability of the spectral resolution. Methods: Weused the ULySS program with the ELODIE library as a reference andcompared the results with those in the literature. Results: Weobtain precisions of 60 K, 0.13, and 0.05 dex, respectively, forTeff, log g, and [Fe/H] for the FGK stars. For the M stars,the mean errors are 38 K, 0.26, and 0.12 dex and 3.5%, 0.17, and 0.13dex for the OBA. We construct an interpolator that we test against theMILES stars themselves. We test it also by measuring the atmosphericparameters of the CFLIB stars with MILES as reference and find it to bemore reliable than the ELODIE interpolator for the evolved hot stars,like those of the blue horizontal branch in particular.FITS files are only and Table 1 also available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/531/A165
| Element abundances in the stars of the MILES spectral library: the Mg/Fe ratio
We have obtained [Mg/Fe] measurements for 76.3 per cent of the stars inthe Mid-resolution Isaac Newton Telescope Library of Empirical Spectra(MILES) spectral library used for understanding stellar atmospheres andstellar populations in galaxies and star clusters. These abundanceratios were obtained through (1) a compilation of values from theliterature using abundances from high-resolution (HR) spectroscopicstudies and (2) a robust spectroscopic analysis using the MILESmid-resolution (MR) optical spectra. All the [Mg/Fe] values werecarefully calibrated to a single uniform scale, by using an extensivecontrol sample with results from HR spectra. The small averageuncertainties in the calibrated [Mg/Fe] values [respectively 0.09 and0.12 dex with methods (1) and (2)] and the good coverage of the starswith [Mg/Fe] over stellar atmospheric parameter space of the librarywill permit the building of new simple stellar populations (SSPs) withempirical ?-enhancements. These will be available for a range of[Mg/Fe], including both sub-solar and super-solar values, and forseveral metallicities and ages. These models will open up new prospectsfor testing and applications of evolutionary stellar populationsynthesis.
| Single-lined Spectroscopic Binary Star Candidates in the RAVE Survey
Repeated spectroscopic observations of stars in the RAdial VelocityExperiment (RAVE) database are used to identify and examine single-linedbinary (SB1) candidates. The RAVE latest internal database (VDR3)includes radial velocities, atmospheric parameters, and other parametersfor approximately a quarter of a million different stars with slightlyless than 300,000 observations. In the sample of ~20,000 stars observedmore than once, 1333 stars with variable radial velocities wereidentified. Most of them are believed to be SB1 candidates. The fractionof SB1 candidates among stars with several observations is between 10%and 15% which is the lower limit for binarity among RAVE stars. Due tothe distribution of time spans between the re-observation that is biasedtoward relatively short timescales (days to weeks), the periods of theidentified SB1 candidates are most likely in the same range. Because ofthe RAVE's narrow magnitude range most of the dwarf candidates belong tothe thin Galactic disk while the giants are part of the thick disk withdistances extending to up to a few kpc. The comparison of the list ofSB1 candidates to the VSX catalog of variable stars yielded severalpulsating variables among the giant population with radial velocityvariations of up to few tens of km s-1. There are 26 matchesbetween the catalog of spectroscopic binary orbits (S_{B^9}) and thewhole RAVE sample for which the given periastron time and the time ofRAVE observation were close enough to yield a reliable comparison. RAVEmeasurements of radial velocities of known spectroscopic binaries areconsistent with their published radial velocity curves.
| Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba
Context. Current models of galaxy formation predict that the Galactichalo was assembled hierarchically. By measuring abundance ratios instars it may be possible to identify substructures in the halo resultingfrom this process. Aims: A previous study of 94 dwarf stars with-1.6 < [Fe/H] < -0.4 in the solar neighborhood has revealed theexistence of two distinct halo populations with a systematic differencein [?/Fe] at a given metallicity. In continuation of that work,abundances of Mn, Cu, Zn, Y, and Ba are determined for the same sampleof stars. Methods: Equivalent widths of atomic lines are measuredfrom high resolution VLT/UVES and NOT/FIES spectra and used to deriveabundance ratios from an LTE analysis based on MARCS model atmospheres.The analysis is made relative to two thick-disk stars, HD22879 and HD 76932, such that very precisedifferential values are obtained. Results: Systematic differencesbetween the "high-?" and "low-?" halo populations are foundfor [Cu/Fe], [Zn/Fe], and [Ba/Y], whereas there is no significantdifference in the case of [Mn/Fe]. At a given metallicity, [Cu/Fe] showsa large scatter that is closely correlated with a corresponding scatterin [Na/Fe] and [Ni/Fe]. Conclusions: The metallicity trends of[Cu/Fe], [Zn/Fe], and [Ba/Y] can be explained from existingnucleosynthesis calculations if the high-? stars formed in regionswith such a high star formation rate that only massive stars and type IIsupernovae contributed to the chemical enrichment. The low-?stars, on the other hand, most likely originate from systems with aslower chemical evolution, characterized by additional enrichment fromtype Ia supernovae and low-mass AGB stars.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility (programs 65.L-0507, 67.D-0086, 67.D-0439,68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585,76.B-0133 and 77.B-0507).Tables 1, 2, and excerpt of Table 3 areavailable in electronic form at http://www.aanda.orgTables 1, 2, andfull Table 3 are also available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A15
| Coudé-feed stellar spectral library - atmospheric parameters
Context. Empirical libraries of stellar spectra play an important rolein different fields. For example, they are used as reference for theautomatic determination of atmospheric parameters, or for buildingsynthetic stellar populations to study galaxies. The CFLIB(Coudé-feed library, Indo-US) database is at present one of themost complete libraries, in terms of its coverage of the atmosphericparameters space (T{eff}, log g and [Fe/H]) and wavelengthcoverage 3460-9464 Å at a resolution of 1 Å FWHM. Althoughthe atmospheric parameters of most of the stars were determined fromdetailed analyses of high-resolution spectra, for nearly 300 of the 1273stars of the library at least one of the three parameters is missing.For the others, the measurements, compiled from the literature, areinhomogeneous. Aims: In this paper, we re-determine theatmospheric parameters, directly using the CFLIB spectra, and comparethem to the previous studies. Methods: We use the ULySS programto derive the atmospheric parameters, using the ELODIE library as areference. Results: Based on comparisons with several previousstudies we conclude that our determinations are unbiased. For the 958 F,G, and K type stars the precision on T{eff}, log g, and[Fe/H] is respectively 43 K, 0.13 dex and 0.05 dex. For the 53 M starsthey are 82 K, 0.22 dex and 0.28 dex. And for the 260 OBA type stars therelative precision on T{eff} is 5.1%, and on log g, and[Fe/H] the precision is respectively 0.19 dex and 0.16 dex. Theseparameters will be used to re-calibrate the CFLIB fluxes and to producesynthetic spectra of stellar populations.Tables 2 and 3 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/525/A71
| The Spitzer Atlas of Stellar Spectra (SASS)
We present the Spitzer Atlas of Stellar Spectra, which includes 159stellar spectra (5-32 ?m R ~ 100) taken with the InfraredSpectrograph on the Spitzer Space Telescope. This Atlas gathersrepresentative spectra of a broad section of the Hertzsprung-Russelldiagram, intended to serve as a general stellar spectral reference inthe mid-infrared. It includes stars from all luminosity classes, as wellas Wolf-Rayet (WR) objects. Furthermore, it includes some objects ofintrinsic interest, such as blue stragglers and certain pulsatingvariables. All of the spectra have been uniformly reduced, and all areavailable online. For dwarfs and giants, the spectra of early-typeobjects are relatively featureless, characterized by the presence ofhydrogen lines in A spectral types. Besides these, the most noticeablephotospheric features correspond to water vapor and silicon monoxide inlate-type objects and methane and ammonia features at the latestspectral types. Most supergiant spectra in the Atlas present evidence ofcircumstellar gas and/or dust. The sample includes five M supergiantspectra, which show strong dust excesses and in some cases polycyclicaromatic hydrocarbon features. Sequences of WR stars present thewell-known pattern of lines of He I and He II, as well as forbiddenlines of ionized metals. The characteristic flat-top shape of the [NeIII] line is evident even at these low spectral resolutions. SeveralLuminous Blue Variables and other transition stars are present in theAtlas and show very diverse spectra, dominated by circumstellar gas anddust features. We show that the [8]-[24] Spitzer colors (IRAC and MIPS)are poor predictors of spectral type for most luminosity classes.
| The Ubiquity of the Rapid Neutron-capture Process
To better characterize the abundance patterns produced by the r-process,we have derived new abundances or upper limits for the heavy elementszinc (Zn, Z= 30), yttrium (Y, Z= 39), lanthanum (La, Z= 57), europium(Eu, Z= 63), and lead (Pb, Z= 82). Our sample of 161 metal-poor starsincludes new measurements from 88 high-resolution and highsignal-to-noise spectra obtained with the Tull Spectrograph on the 2.7 mSmith Telescope at the McDonald Observatory, and other abundances areadopted from the literature. We use models of the s-process inasymptotic giant branch stars to characterize the high Pb/Eu ratiosproduced in the s-process at low metallicity, and our new observationsthen allow us to identify a sample of stars with no detectable s-processmaterial. In these stars, we find no significant increase in the Pb/Euratios with increasing metallicity. This suggests that s-processmaterial was not widely dispersed until the overall Galactic metallicitygrew considerably, perhaps even as high as [Fe/H] =-1.4, in contrastwith earlier studies that suggested a much lower mean metallicity. Weidentify a dispersion of at least 0.5 dex in [La/Eu] in metal-poor starswith [Eu/Fe] <+0.6 attributable to the r-process, suggesting thatthere is no unique "pure" r-process elemental ratio among pairs of rareearth elements. We confirm earlier detections of an anti-correlationbetween Y/Eu and Eu/Fe bookended by stars strongly enriched in ther-process (e.g., CS 22892-052) and those with deficiencies of the heavyelements (e.g., HD 122563). We can reproduce the range of Y/Eu ratiosusing simulations of high-entropy neutrino winds of core-collapsesupernovae that include charged-particle and neutron-capture componentsof r-process nucleosynthesis. The heavy element abundance patterns inmost metal-poor stars do not resemble that of CS 22892-052, but thepresence of heavy elements such as Ba in nearly all metal-poor starswithout s-process enrichment suggests that the r-process is a commonphenomenon.This paper includes data taken at The McDonald Observatory of TheUniversity of Texas at Austin.
| Forming the first planetary systems: debris around Galactic thick disc stars
The thick disc contains stars formed within the first Gyr of Galactichistory, and little is known about their planetary systems. The SpitzerMIPS instrument was used to search 11 of the closest of these oldlow-metal stars for circumstellar debris, as a signpost that bodies atleast as large as planetesimals were formed. A total of 22 thick discstars has now been observed, after including archival data, but dust isnot found in any of the systems. The data rule out a high incidence ofdebris among star systems from early in the Galaxy's formation. However,some stars of this very old population do host giant planets, atpossibly more than the general incidence among low-metal Sun-like stars.As the Solar system contains gas giants but little cometary dust, thethick disc could host analogue systems that formed many Gyr before theSun.
| The ability of intermediate-band Strömgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities
Context. Several large scale photometric and spectroscopic surveys arebeing undertaken to provide a more detailed picture of the Milky Way.Given the necessity of generalisation in the determination of, e.g.,stellar parameters when tens and hundred of thousands of stars areconsidered it remains important to provide independent, detailed studiesto verify the methods used in the surveys. Aims: Our first aim isto critically evaluate available calibrations for deriving [M/H] fromStrömgren photometry. Secondly, we develop the standard sequencesfor dwarf stars to reflect their inherent metallicity dependence.Finally, we test how well metallicities derived from ugriz photometryreproduce metallicities derived from the well-tested system ofStrömgren photometry. Methods: We evaluate availablemetallicity calibrations based on Strömgren uvby photometry fordwarf stars using a catalogue of stars with both uvby photometry andspectroscopically determined iron abundances ([Fe/H]). The catalogue wascreated for this project. Using this catalogue, we also evaluateavailable calibrations that determine log g. A larger catalogue, inwhich metallicity is determined directly from uvby photometry, is usedto trace metallicity-dependent standard sequences for dwarf stars. Wealso perform comparisons, for both dwarf and giant stars, ofmetallicities derived from ugriz photometry with metallicities derivedfrom Strömgren photometry. Results: We provide a homogenisedcatalogue of 451 dwarf stars with 0.3 < (b-y)0 < 1.0.All stars in the catalogue have uvby photometry and [Fe/H] determinedfrom spectra with high resolution and high signal-to-noise ratios (S/N).Using this catalogue, we test how well various photometric metallicitycalibrations reproduce the spectroscopically determined [Fe/H]. Usingthe preferred metallicity calibration for dwarf stars, we derive newstandard sequences in the c1,0 versus (b-y)0 planeand in the c1,0 versus (v-y)0 plane for dwarfstars with 0.40 < (b-y)0 < 0.95 and 1.10 <(v-y)0 < 2.38. Conclusions: We recommend thecalibrations of Ramírez & Meléndez (2005) in derivingmetallicities from Strömgren photometry and find that intermediateband photometry, such as Strömgren photometry, more accurately thanbroad band photometry reproduces spectroscopically determined [Fe/H].Strömgren photometry is also better at differentiating betweendwarf and giant stars. We conclude that additional investigations of thedifferences between metallicities derived from ugriz photometry andintermediate-band photometry, such as Strömgren photometry, arerequired.Appendix A is only available in electronic form at http://www.aanda.org, Appendix B isavailable in electronic form at http://www.aanda.org and at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/521/A40
| A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups
Context. Nearby late-type stars are excellent targets for seeking youngobjects in stellar associations and moving groups. The origin of thesestructures is still misunderstood, and lists of moving group membersoften change with time and also from author to author. Most members ofthese groups have been identified by means of kinematic criteria,leading to an important contamination of previous lists by old fieldstars. Aims: We attempt to identify unambiguous moving groupmembers among a sample of nearby-late type stars by studying theirkinematics, lithium abundance, chromospheric activity, and otherage-related properties. Methods: High-resolution echelle spectra(R ~ 57 000) of a sample of nearby late-type stars are used to deriveaccurate radial velocities that are combined with the precise Hipparcosparallaxes and proper motions to compute galactic-spatial velocitycomponents. Stars are classified as possible members of the classicalmoving groups according to their kinematics. The spectra are also usedto study several age-related properties for young late-type stars, i.e.,the equivalent width of the lithium Li i 6707.8 Å line or theR'HK index. Additional information like X-ray fluxes from theROSAT All-Sky Survey or the presence of debris discs is also taken intoaccount. The different age estimators are compared and the moving groupmembership of the kinematically selected candidates are discussed. Results: From a total list of 405 nearby stars, 102 have beenclassified as moving group candidates according to their kinematics.i.e., only ~25.2% of the sample. The number reduces when age estimatesare considered, and only 26 moving group candidates (25.5% of the 102candidates) have ages in agreement with the star having the same age asan MG member.Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofísica deAndalucía (CSIC) and observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundación Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canarias.Appendices and Tables 1,5-15 are available in electronic form at http://www.aanda.orgTable 1 is alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/521/A12
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood
Debris discs - analogous to the asteroid and Kuiper-Edgeworth belts inthe Solar system - have so far mostly been identified and studied inthermal emission shortward of 100?m. The Herschel space observatoryand the Submillimetre Common-User Bolometer Array-2 (SCUBA-2) camera onthe James Clerk Maxwell Telescope will allow efficient photometricsurveying at 70 to 850?m, which allows for the detection of coolerdiscs not yet discovered, and the measurement of disc masses andtemperatures when combined with shorter wavelength photometry. TheSCUBA-2 Unbiased Nearby Stars survey (SUNS) and the Disc Emission via aBias-free Reconnaissance in the Infrared/Submillimetre (DEBRIS) HerschelOpen Time Key Project are complementary legacy surveys observing samplesof ~500 nearby stellar systems. To maximize the legacy value of thesesurveys, great care has gone into the target selection process. Thispaper describes the target selection process and presents the targetlists of these two surveys.
| A Non-local Thermodynamic Equilibrium Analysis of Boron Abundances in Metal-poor Stars
The non-local thermodynamic equilibrium (NLTE) line formation of neutralboron in the atmospheres of cool stars are investigated. Our resultsconfirm that NLTE effects for the B I resonance lines, which are due toa combination of overionization and optical pumping effects, are mostimportant for hot, metal-poor, and low-gravity stars; however, theamplitude of departures from local thermodynamic equilibrium (LTE) foundby this work is smaller than that of previous studies. In addition, ourcalculation shows that the line formation of B I will get closer to LTEif the strength of collisions with neutral hydrogen increases, which iscontrary to the result of previous studies. The NLTE line formationresults are applied to the determination of boron abundances for asample of 16 metal-poor stars with the method of spectrum synthesis ofthe B I 2497 Å resonance lines using the archived HST/GHRSspectra. Beryllium and oxygen abundances are also determined for thesestars with the published equivalent widths of the Be II 3131 Åresonance and O I 7774 Å triplet lines, respectively. Theabundances of the nine stars which are not depleted in Be or B showthat, no matter what the strength of collisions with neutral hydrogenmay be, both Be and B increase with O quasilinearly in the logarithmicplane, which confirms the conclusions that Be and B are mainly producedby the primary process in the early Galaxy. The most noteworthy resultof this work is that B increases with Fe or O at a very similar speedas, or a bit faster than, Be does, which is in accord with thetheoretical models. The B/Be ratios remain almost constant over themetallicity range investigated here. Our average B/Be ratio falls in theinterval [13 ± 4, 17 ± 4], which is consistent with thepredictions of the spallation process. The contribution of B from the?-process may be required if the 11B/10Bisotopic ratios in metal-poor stars are the same as the meteoric value.An accurate measurement of the 11B/10B ratios inmetal-poor stars is crucial to understanding the production history ofboron.Based on observations made with ESO telescopes and NASA/ESA Hubble SpaceTelescope obtained from the ESO/ST-ECF Science Archive Facility; basedon spectral data retrieved from the ELODIE archive at Observatoire deHaute-Provence (OHP).
| StarCAT: A Catalog of Space Telescope Imaging Spectrograph Ultraviolet Echelle Spectra of Stars
StarCAT is a catalog of high resolution ultraviolet spectra of objectsclassified as "stars," recorded by Space Telescope Imaging Spectrograph(STIS) during its initial seven years of operations (1997-2004). StarCATis based on 3184 echelle observations of 545 distinct targets, with atotal exposure duration of 5.2 Ms. For many of the objects, broadultraviolet coverage has been achieved by splicing echellegrams taken intwo or more FUV (1150-1700 Å) and/or NUV (1600-3100 Å)settings. In cases of multiple pointings on conspicuously variablesources, spectra were separated into independent epochs. Otherwise,different epochs were combined to enhance the signal-to-noise ratio(S/N). A post-facto correction to the {\sf calstis} pipeline data setscompensated for subtle wavelength distortions identified in a previousstudy of the STIS calibration lamps. An internal "fluxing" procedureyielded coherent spectral energy distributions (SEDs) for objects withbroadly overlapping wavelength coverage. The best StarCAT materialachieves 300 m s-1 internal velocity precision;absolute accuracy at the 1 km s-1 level; photometricaccuracy of order 4%; and relative flux precision several times better(limited mainly by knowledge of SEDs of UV standard stars). WhileStarCAT represents a milestone in the large-scale post-processing ofSTIS echellegrams, a number of potential improvements in the underlying"final" pipeline are identified.
| The ALHAMBRA Photometric System
This paper presents the characterization of the optical range of theALHAMBRA photometric system, a 20 contiguous, equal-width, medium-bandCCD system with wavelength coverage from 3500 Å to 9700 Å.The photometric description of the system is done by presenting the fullresponse curve as a product of the filters, CCD, and atmospherictransmission curves, and using some first- and second-order moments ofthis response function. We also introduce the set of standard stars thatdefines the system, formed by 31 classic spectrophotometric standardstars which have been used in the calibration of other known photometricsystems, and 288 stars, flux calibrated homogeneously, from the NextGeneration Spectral Library (NGSL). Based on the NGSL, we determine thetransformation equations between Sloan Digital Sky Survey ugrizphotometry and the ALHAMBRA photometric system, in order to establishsome relations between both systems. Finally, we develop and discuss astrategy to calculate the photometric zero points of the differentpointings in the ALHAMBRA project.
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
Aims: Precise abundance ratios are determined for 94 dwarf starswith Teff K, -1.6 < [Fe/H] < -0.4, and distances D? 335 pc. Most of them have halo kinematics, but 16 thick-disk starsare included. Methods: Equivalent widths of atomic lines aremeasured from VLT/UVES and NOT/FIES spectra with resolutions R? 55000 and R ? 40 000, respectively. An LTE abundance analysis basedon MARCS models is applied to derive precise differential abundanceratios of Na, Mg, Si, Ca, Ti, Cr, and Ni with respect to Fe. Results: The halo stars fall into two populations, clearly separated in[?/Fe], where ? refers to the average abundance of Mg, Si,Ca, and Ti. Differences in [Na/Fe] and [Ni/Fe] are also present with aremarkably clear correlation between these two abundance ratios. Conclusions: The “high-?” stars may be ancient disk orbulge stars “heated” to halo kinematics by merging satellitegalaxies or they could have formed as the first stars during thecollapse of a proto-Galactic gas cloud. The kinematics of the“low-?” stars suggest that they have been accretedfrom dwarf galaxies, and that some of them may originate from the? Cen progenitor galaxy.Based on observations made with the Nordic Optical Telescope on LaPalma, and on data from the European Southern Observatory ESO/ST-ECFScience Archive Facility.Tables 3 and 4 are also available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/L10Figures5-8 and Tables 1-4 are only available in electronic form at http://www.aanda.org
| Evolutionary Status of 85 Pegasi
We have investigated the evolutionary status of the visual binary 85 Peg(HD 224930) using a high-resolution spectroscopic analysis and theastrometric calibration of the stellar parameters. In spite ofwell-determined stellar parameters from HIPPARCOS astrometry and fromspectroscopy, 85 Peg has for a long time revealed peculiar propertieswhen attempts have been made to combine an evolutionary model withobservation. There has been a chronic problem of the mass ratio of thetwo components when comparing 85 Peg's dynamical properties to thephotometric magnitude difference. Moreover, 85 Peg has been suspected tohave unique characteristics in its chemical composition. In order todetermine accurate spectroscopic abundances, we have obtainedhigh-resolution echelle spectra for 85 Peg. From our elemental analysis,we found that 85 Peg is ?-enhanced with respect to the scaledsolar abundance by a factor of 2. We then produced, within the frameworkof the standard stellar theory, grids of stellar model using the mostrecent observational results. To avoid many-fold degeneracy amongphysical quantities, a statistical minimization test was carried outbetween theoretical model grids. Enforcing consistency between themodeling and statistical constraints, we derived a reliable set ofphysical parameters and confirmed the trinarity of the system. In thecontext of asteroseismology, the theoretical frequency spectrum of 85Peg was calculated. 85 Peg A is expected to have a first order spacing?? ~ 165 ?Hz at the reference frequency ?0 =790 ?Hz.
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Дева |
| Прямое восхождение: | 12h15m10.56s |
| Склонение: | -10°18'44.7" |
| Видимая звёздная величина: | 6.113 |
| Собственное движение RA: | 34.8 |
| Собственное движение Dec: | -1014.8 |
| B-T magnitude: | 6.637 |
| V-T magnitude: | 6.157 |
Каталоги и обозначения:
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