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Improved Laboratory Transition Probabilities for Hf II and Hafnium Abundances in the Sun and 10 Metal-poor Stars
Radiative lifetimes from laser-induced fluorescence measurements,accurate to ~+/-5%, are reported for 41 odd-parity levels of Hf II. Thelifetimes are combined with branching fractions measured using Fouriertransform spectrometry to determine transition probabilities for 150lines of Hf II. Approximately half of these new transition probabilitiesoverlap with recent independent measurements using a similar approach.The two sets of measurements are found to be in good agreement for linesin common. Our new laboratory data are applied to refine the hafniumphotospheric solar abundance and to determine hafnium abundances in 10metal-poor giant stars with enhanced r-process abundances. For the Sunwe derive logɛ(Hf)=0.88+/-0.08 from four lines; the uncertaintyis dominated by the weakness of the lines and their blending by otherspectral features. Within the uncertainties of our analysis, ther-process-rich stars possess constant Hf/La and Hf/Eu abundance ratios,logɛ(Hf/La)=-0.13+/-0.02(σ=0.06) andlogɛ(Hf/Eu)=+0.04+/-0.02 (σ=0.06). The observed averagestellar abundance ratio of Hf/Eu and La/Eu is larger than previousestimates of the solar system r-process-only value, suggesting asomewhat larger contribution from the r-process to the production of Hfand La. The newly determined Hf values could be employed as part of thechronometer pair, Th/Hf, to determine radioactive stellar ages.
| Improved Laboratory Transition Probabilities for Gd II and Application to the Gadolinium Abundances of the Sun and Three r-Process Rich, Metal-poor Stars
Radiative lifetimes, accurate to +/-5%, have been measured for 49even-parity and 14 odd-parity levels of Gd II using laser-inducedfluorescence. The lifetimes are combined with branching fractionsmeasured using Fourier transform spectrometry to determine transitionprobabilities for 611 lines of Gd II. This work is the largest-scalelaboratory study to date of Gd II transition probabilities and the firstusing a high-performance Fourier transform spectrometer. This improveddata set has been used to determine a new solar photospheric Gdabundance, logɛ=1.11+/-0.03. Revised Gd abundances have alsobeen derived for the r-process-rich metal-poor giant stars CS 22892-052,BD +17 3248, and HD 115444. The resulting Gd/Eu abundance ratios are invery good agreement with the solar system r-process ratio. We haveemployed the increasingly accurate stellar abundance determinations,resulting in large part from the more precise laboratory atomic data, topredict directly the solar system r-process elemental abundances for Gd,Sm, Ho, and Nd. Our analysis of the stellar data suggests slightlyhigher recommended values for the r-process contribution and total solarsystem values, consistent with the photospheric determinations, for theelements for Gd, Sm, and Ho.
| Near-Ultraviolet Observations of HD 221170: New Insights into the Nature of r-Process-rich Stars
Employing high-resolution spectra obtained with the near-UV-sensitivedetector on the Keck I HIRES, supplemented by data obtained with theMcDonald Observatory 2d-coudé, we have performed a comprehensivechemical composition analysis of the bright r-process-rich metal-poorred giant star HD 221170. Analysis of 57 individual neutral and ionizedspecies yielded abundances for a total of 46 elements and significantupper limits for an additional five. Model stellar atmosphere parameterswere derived with the aid of ~200 Fe peak transitions. From more than350 transitions of 35 neutron-capture (Z>30) species, abundances for30 neutron-capture elements and upper limits for three others werederived. Utilizing 36 transitions of La, 16 of Eu, and seven of Th, wederive ratios of logε(Th/La)=-0.73 (σ=0.06) andlogε(Th/Eu)=-0.60 (σ=0.05), values in excellent agreementwith those previously derived for other r-process-rich metal-poor starssuch as CS 22892-052, BD +17 3248, and HD 115444. Based on the Th/Euchronometer, the inferred age is 11.7+/-2.8 Gyr. The abundancedistribution of the heavier neutron-capture elements (Z>=56) isfitted well by the predicted scaled solar system r-process abundances,as also seen in other r-process-rich stars. Unlike other r-process-richstars, however, we find that the abundances of the lighterneutron-capture elements (37
| Atmospheres, Chemical Compositions, and Evolutionary Histories of Very Metal-Poor Red Horizontal-Branch Stars in the Galactic Field and in NGC 7078 (M15)
We have conducted spectrum analyses of 24 field metal-poor([Fe/H]<-2) red horizontal-branch (RHB) stars identified in the HKobjective-prism survey and 6 such stars in the globular cluster M15,based on high-quality spectra (R~40,000, S/N~100) obtained with theMagellan Inamori Kyocera Echelle spectrograph at the Clay 6.5 mtelescope at Las Campanas Observatory. The atmospheric parameters of theRHB stars provide interesting bridges between turnoff stars of similartemperature and red giant branch (RGB) stars of similar gravity, andthey permit investigations of abundance trends [X/Fe] versus [Fe/H] in arelatively unexplored region of the temperature-gravity plane. We findthat the Teff, logg, vt, and [Fe/H] valuesdetermined from our spectra are consistent with expectations fromliterature spectroscopic studies of other evolved metal-poor stellarclasses. We show that the RHB stars have abundance distributions thatare consistent with typical halo stars of similar metallicities. Thephotometric and spectroscopic gravities of the M15 stars differ byamounts that grow with declining temperature. We use a regressionderived from these differences to calculate photometric gravities forthe field RHB stars. Then we use the locations of the field RHB starsamong the evolutionary tracks of Cassisi et al. in the logg versuslogTeff plane to estimate their masses and lifetimes as RHBstars. We use these lifetimes to estimate the size of the metal-poor HBpopulation from which they arise. Then, using counts of HB and RGB starsin metal-poor globular clusters, we conclude that the number ofmetal-poor RGB stars at high latitudes (|b|>30deg)brighter than V=15 exceeds those identified in extant objective-prismsurveys by more than an order of magnitude. Finally, we deduce theeffective temperature of the fundamental red edge of the metal-poor RRLyrae instability strip, logTeff(FRE)=3.80+/-0.01, from theinterface between the temperature distributions of metal-poor field RHBstars and the RR Lyrae stars of similar [Fe/H] in five metal-poorglobular clusters.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center, California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation. This paper includes data gatheredwith the 6.5 m Magellan telescopes located at Las Campanas Observatory,Chile.
| The Hamburg/ESO R-process enhanced star survey (HERES). III. HE 0338-3945 and the formation of the r + s stars
We have derived abundances of 33 elements and upper limits for 6additional elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE0338-3945 from high-quality VLT-UVES spectra. The star is heavilyenriched, by about a factor of 100 relative to iron and the Sun, in theheavy s-elements (Ba, La, ...). It is also heavily enriched in Eu, whichis generally considered an r-element, and in other similar elements. Itis less enriched, by about a factor of 10, in the lighter s-elements(Sr, Y and Zr). C is also strongly enhanced and, to a somewhat lesserdegree, N and O. These abundance estimates are subject to severeuncertainties due to NLTE and thermal inhomogeneities which are nottaken into detailed consideration. However, an interesting result, whichis most probably robust in spite of these uncertainties, emerges: theabundances derived for this star are very similar to those of otherstars with an overall enhancement of all elements beyond the iron peak.We have defined criteria for this class of stars, r+s stars, and discussnine different scenarios to explain their origin. None of theseexplanations is found to be entirely convincing. The most plausiblehypotheses involve a binary system in which the primary component goesthrough its giant branch and asymptotic giant branch phases and producesCNO and s-elements which are dumped onto the observed star. Whether ther-element Eu is produced by supernovae before the star was formed(perhaps triggering the formation of a low-mass binary), by a companionas it explodes as a supernova (possibly triggered by mass transfer), orwhether it is possibly produced in a high-neutron-density version of thes-process is still unclear. Several suggestions are made on how toclarify this situation.
| Galaxy merging and number vs. apparent magnitude relation for the universe with a time-decaying cosmological term
Aims. An attempt is made to constrain the values of the cosmologicalparameters together with the galaxy merging factor η on the basis ofa comparison between the observed galaxy number counts vs. theirapparent magnitudes relation (N-m relation) with those theoreticallyconstructed for the universe with a time-decaying cosmological termΛ. Methods. We assume that the galaxy number density evolutioncan be represented sufficiently well by a function of the redshift z ofthe form φ^*(z)∝ (1+z)^η. Three variations of thecosmological term with time τ are considered, (1) Λ∝τ-l, (2) Λ ∝ a-m with a being thescale factor, and (3) Λ ∝ Hn with H the Hubbleparameter. The optimum ranges for the decaying parameters (l, m, and n),the density parameters ΩΛ,0 andΩm,0, as well as T_mg (the timescale for the merger ofa pair of galaxies) and the redshift z_mg for the first onset of galaxymerger are sought based on statistical analysis using likelihoodfunctions given by χ2 evaluations. Results. In the caseof the type I models, for instance, we find thatl=0.75+0.55-0.75, T_mg = 0.3 Gyr, z_mg=3,ΩΛ,0=0.71+0.17-0.09 andΩm,0=0.29+0.09-0.17, from which acosmic age of 17.2+1.3-1.2 Gyr results. Thesemodel parameters lead to η=1.73+0.14-0.07. Asa consistency check, we have also carried out computations of the cosmicmicrowave background radiation (CMBR) spectrum, and have madecomparisons with WMAP measurements. We found that it is necessary tosomewhat modify the parameter values obtained above on account of thehigh sensitivity of η to the value of T_mg. The final model that wasfound to account for both the observed N-m relation and the WMAPmeasurements of the CMBR spectrum is as follows: z_mg=3.0, T_mg=0.2 Gyr,l=0.04, {ΩΛ,0}=0.77, η =2.2269. The age ofthis model universe is 14.6 Gyr, which is still sufficiently high tocope with the "new" cosmic age problem.
| Improved Laboratory Transition Probabilities for Sm II and Application to the Samarium Abundances of the Sun and Three r-Process-rich, Metal-poor Stars
Radiative lifetimes, accurate to +/-5%, have been measured for 212odd-parity levels of Sm II using laser-induced fluorescence. Thelifetimes are combined with branching fractions measured using Fouriertransform spectrometry to determine transition probabilities for morethan 900 lines of Sm II. This work is the largest scale laboratory studyto date of Sm II transition probabilities using modern methods. Thisimproved data set has been used to determine a new solar photospheric Smabundance, logɛ=1.00+/-0.03, from 26 lines. The spectra of threevery metal-poor, neutron-capture-rich stars also have been analyzed,employing between 55 and 72 Sm II lines per star. The abundance ratiosof Sm relative to other rare earth elements in these stars are inagreement and are consistent with ratios expected from rapidneutron-capture nucleosynthesis (the r-process).
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| Chemical compositions of neutron-process elements in low-metallicity stars - tracers of r and r+s nucleosynthesis processes .
Employing spectra obtained with the near-UV sensitive detector on theKeck I HIRES, comprehensive chemical composition analyses have beenperformed on the neutron-capture-element-rich star, HD221170 ([Fe/H] =-2.2; Ivans et al 2006), and the binary blue metal-poor star, BPSCS29497-030 (-2.6; Ivans et al. 2005). The heavy element abundances ofHD221170 are fit exquisitely well by a scaled-solar r-process abundancepattern with no contribution from the s-process. In contrast, theabundance pattern of CS29497-030 seems best fit by an AGB model withs-processing that also includes very significant amounts ofpre-enrichment of r-process material in the protostellar cloud out ofwhich the binary system formed.
| 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.
| Supernova Neutrino Effects on r-Process Nucleosynthesis in Black Hole Formation
Very massive stars with mass >=8 Msolar culminate theirevolution by supernova explosions, which are presumed to be the mostviable candidates for the astrophysical sites of r-processnucleosynthesis. If the models for the supernova r-process are correct,then the results of nucleosynthesis could also put a significantconstraint on the remnants of supernova explosions, i.e., a neutron staror black hole. In the case of very massive core collapse for aprogenitor mass 20-40 Msolar, a remnant stellar black hole isthought to be formed. Intense neutrino flux from the neutronized coreand the neutrinosphere might suddenly cease during the Kelvin-Helmholtzcooling phase because of the black hole formation. It is important andinteresting to explore the observable consequences of such a neutrinoflux truncation. It has recently been argued in the literature that eventhe neutrino mass can be determined from the time delay of the deformedneutrino energy spectrum after the cessation of neutrino ejection(neutrino cutoff effect). Here we study the expected theoreticalresponse of the r-process nucleosynthesis to the neutrino cutoff effectin order to look for another independent signature of this phenomenon.We found a sensitive response of the r-process yield if the neutrinocutoff occurs after the critical time when the expanding materials inthe neutrino-driven wind drop out of nuclear statistical equilibrium(NSE). The r-process nucleosynthesis yields drastically change if thecutoff occurs during the r-process, having maximal effect on the changein abundance of 232Th and 235,238U. There is alarge probability of finding this effect in elemental abundances ofr-process-enhanced metal-deficient halo stars whose chemical compositionis presumed to be affected by Population III supernovae in the earlyGalaxy. Using this result, connected with future detection of the timevariation of the SN neutrino spectrum, we are able to identify when theblack hole formation occurs in the course of SN collapse.
| 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.
| The Discovery and Analysis of Very Metal-Poor Stars in the Galaxy
We discuss the importance of very metal-poor stars to develop anunderstanding of the nature of the first stars that formed in theUniverse and the nucleosynthesis events associated with them, as well asto refine models of galaxy formation, in particular for large spiralgalaxies such as the Milky Way. After briefly reviewing the history ofthe search for very metal-deficient stars in the Galaxy, we summarizeongoing efforts, concentrating on the two large objective-prism surveysthat have led to the discovery of the majority of stars with [Fe/H] < 2.0 known at present: the HK survey of Beers and collaborators and theHamburg/ESO survey of Christlieb and collaborators. We then consider thewealth of information that can be gleaned from high-resolutionspectroscopic study of very metal-poor stars. We close with a list ofopen questions and a discussion of new survey techniques that willexpand the sample of recognized very metal-deficient stars in the Galaxyby several orders of magnitude.
| Hubble Space Telescope Observations of Heavy Elements in Metal-Poor Galactic Halo Stars
We present new abundance determinations of neutron-capture elements Ge,Zr, Os, Ir, and Pt in a sample of 11 metal-poor(-3.1<=[Fe/H]<=-1.6) Galactic halo giant stars, based on HubbleSpace Telescope UV and Keck I optical high-resolution spectroscopy. Thestellar sample is dominated by r-process-rich stars such as thewell-studied CS 22892-052 and BD +17°3248 but also includes ther-process-poor, bright giant HD 122563. Our results demonstrate thatabundances of the third r-process peak elements Os, Ir, and Pt in thesemetal-poor halo stars are very well correlated among themselves and withthe abundances of the canonical r-process element Eu (determined inother studies), thus arguing for a common origin or site for r-processnucleosynthesis of heavier (Z>56) elements. However, the large (andcorrelated) scatters of [Eu, Os, Ir, Pt/Fe] suggest that the heaviestneutron-capture r-process elements are not formed in all supernovae. Incontrast, the Ge abundances of all program stars track their Feabundances, very well. An explosive process on iron peak nuclei (e.g.,the α-rich freezeout in supernovae), rather than neutron capture,appears to have been the dominant synthesis mechanism for this elementat low metallicities: Ge abundances seem completely uncorrelated withEu. The correlation (with very small scatter) of Ge and Fe abundancessuggests that Ge must have been produced rather commonly in stars, evenat early times in the Galaxy, over a wide range of metallicity. The Zrabundances show much the same behavior as Ge with (perhaps) somewhatmore scatter, suggesting some variations in abundance with respect toFe. The Zr abundances also do not vary cleanly with Eu abundances,indicating a synthesis origin different than that of heavierneutron-capture elements. Detailed abundance distributions for CS22892-052 and BD +17°3248, combining the new elementaldeterminations for Os-Pt and recently published Nd and Ho measurements,show excellent agreement with the solar system r-process curve from theelements Ba to Pb. The lighter n-capture elements, including Ge, ingeneral fall below the same solar system r-process curve that matchesthe heavier elements.
| Improved Laboratory Transition Probabilities for Pt I and Application to the Platinum Abundances of BD +17°3248 and the Sun
Radiative lifetimes, accurate to +/-5%, have been measured for 58odd-parity levels of Pt I using laser-induced fluorescence. Thelifetimes were combined with branching fractions measured using gratingand Fourier transform spectrometry to determine transition probabilitiesfor 127 lines of Pt I. The new Pt lifetime measurements were found to bein good agreement with previous but less extensive measurements based onlaser-induced fluorescence. The new branching fraction measurements werefound to be in fair agreement with one earlier study. Absolute atomictransition probabilities from the new measurements were used todetermine the platinum abundance in the metal-poor Galactic halo star BD+17°3248. An attempt to refine the solar photospheric abundance ofplatinum was unsuccessful; the single Pt I line used in an earlierabundance determination was found to be even more severely blended thanexpected from earlier work.
| Thorium-rich halo star HD 221170: Further evidence against the universality of the r-process
We report the abundance determination in the atmosphere of the brighthalo star HD 221170. The spectra were taken with the TerskolObservatory's 2.0-m telescope with a resolution R= 45 000 andsignal-to-noise ratio up to 250 in the wavelength region 3638-10 275Å. The adopted atmospheric parameters correspond to an effectivetemperature Teff = 4475 K, a surface gravity \log g = 1.0, amicroturbulent velocity vmicro = 1.7 km s-1, and amacroturbulent velocity vmacro = 4 km s-1. Theabundances of 43 chemical elements were determined with the method ofspectrum synthesis. The large overabundances (by 1 dex relative to iron)of elements with Z > 38 are shown to follow the same pattern as thesolar r-abundances. The present HD 221170 analysis confirms thenon-universality of the r-process, or more exactly the observation thatthe astrophysical sites hosting the r-process do not always lead to aunique relative abundance distribution for the bulk Ba to Hg elements,the Pb-peak elements, and the actinides.Based on observations obtained at the 2-m telescope of Peak Terskolobservatory, International Center for Astronomical, Medical andEcological Research, Ukraine & Russia.Tables \ref{tab1} and \ref{tab2} are only available in electronic format http://www.edpsciences.org
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| 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.
| Lead abundance in the uranium star CS 31082-001
In a previous paper we were able to measure the abundance of uranium andthorium in the very-metal poor halo giant BPS CS31082-001, but only obtained an upper limit for the abundanceof lead (Pb). We have got from ESO 17 h of additional exposure on thisstar in order to secure a detection of the minimum amount of leadexpected to be present in CS 31082-001, the amountarising from the decay of the original content of Th and U in the star.We report here this successful detection. We find an LTE abundancelog(Pb/H)+12=-0.55 ± 0.15 dex, one dex below the upper limitsgiven by other authors for the similar stars CS22892-052 and BD +17°3248, alsoenhanced in r-process elements. From the observed present abundances ofTh and U in the star, the expected amount of Pb produced by the decay of232Th, and 238U alone, over 12-15 Gyr is-0.73± 0.17 dex. The decay of 235U is more difficultto estimate, but is probably slightly below the contribution of238U, making the contribution of the 3 actinides onlyslightly below, or even equal to, the measured abundance. Thecontribution from the decay of 234U has was not included, forlack of published data. In this sense our determination is a lower limitto the contribution of actinides to lead production. We comment thisresult, and we note that if a NLTE analysis, not yet possible, doublesour observed abundance, the decay of the 3 actinides will stillrepresent 50 per cent of the total lead, a proportion higher than thevalues considered so far in the literature.Based on observations obtained with the Very Large Telescope of theEuropean Southern Observatory at Paranal, Chile.
| The Hamburg/ESO R-process Enhanced Star survey (HERES). I. Project description, and discovery of two stars with strong enhancements of neutron-capture elements
We report on a dedicated effort to identify and study metal-poor starsstrongly enhanced in r-process elements ([r/Fe]>1 dex; hereafter r-IIstars), the Hamburg/ESO R-process Enhanced Star survey (HERES).Moderate-resolution (˜2 Å) follow-up spectroscopy has beenobtained for metal-poor giant candidates selected from the Hamburg/ESOobjective-prism survey (HES) as well as the HK survey to identifysharp-lined stars with [Fe/H]<-2.5 dex. For several hundred confirmedmetal-poor giants brighter than B˜ 16.5 mag (most of them from theHES), ``snapshot'' spectra (R˜ 20 000; S/N ˜ 30 per pixel)are being obtained with VLT/UVES, with the main aim of finding the 2-3%r-II stars expected to be among them. These are studied in detail bymeans of higher resolution and higher S/N spectra. In this paper wedescribe a pilot study based on a set of 35 stars, including 23 from theHK survey, eight from the HES, and four comparison stars. We discoveredtwo new r-II stars, CS 29497-004 ([Eu/Fe]=1.64± 0.22) and CS29491-069 ([Eu/Fe]=1.08± 0.23). A first abundance analysis of CS29497-004 yields that its abundances of Ba to Dy are on average enhancedby 1.5 dex with respect to iron and the Sun and match a scaled solarr-process pattern well, while Th is underabundant relative to thatpattern by 0.3 dex, which we attribute to radioactive decay. That is, CS29497-004 seems not to belong to the class of r-process enhanced starsdisplaying an ``actinide boost'', like CS 31082-001 (Hill et al. 2002),or CS 30306-132 (Honda et al. 2004b). The abundance pattern agrees wellwith predictions of the phenomenological model of Qian & Wasserburg.Based in large part on observations collected at the European SouthernObservatory, Paranal, Chile (proposal number 68.B-0320).}
| Abundances of Extremely Metal-poor Star Candidates
We present chemical abundances for 110 stars identified inobjective-prism surveys as candidates for being very metal-poor. Theabundances are derived from high-S/N, intermediate-resolution spectraobtained with the Keck Observatory Echellette Spectrograph and Imager(ESI). An additional 25 stars with well-determined abundances rangingfrom [Fe/H]=-1.5 to -3.2 were observed and the results used to helpcalibrate our analysis and determine the accuracy of our abundancedeterminations. Abundances for the program stars were measured for Fe,Mg, Ca, Ti, Cr, and Ba with an accuracy of approximately 0.3 dex.Fifty-three of the stars in our sample have [Fe/H]<=-2, 22 have[Fe/H]<=-2.5, and 13 have [Fe/H]<=-2.9. Surprisingly,approximately one-third of the sample is relatively metal-rich, with[Fe/H]>-1.5. In addition to identifying a number of extremelymetal-poor stars, this study also shows that moderate-resolution spectraobtained with the Keck ESI yield relatively accurate abundances forstars as faint as V=14 with modest exposure time (~20 minutes). Thiscapability will prove useful if the so-far elusive stars at [Fe/H]<-4turn out to be mostly fainter than V=15.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| 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.
| A New Contributor to Chemical Evolution in High-Redshift Galaxies
The recent discovery of a new population of stars exhibiting unusualelemental abundance patterns characterized by enhanced Ti-to-Ga elementsand low α- and n-capture elements suggests the contribution of anew class of supernovae, probably a kind of Type Ia supernovaeassociated with close binary evolution. The role of these supernovae inchemical evolution is negligible in normal galaxies that undergomoderate star formation such as our own. Thus, while the frequency ofoccurrence would be too low to detect in low-redshift galaxies, it mayrepresent a prominent population in high-redshift objects such as earlyepoch massive elliptical galaxies and QSOs. The chemical contributor ofthis proposed type of supernova in combination with recognizedsupernovae is shown to be compatible with the recent observationalfeatures in the distant universe, successfully reproducing the Type IIsupernova-like abundance pattern with enhancement of Ga and Ge in thegas of newborn massive galaxies and high iron abundances in QSOs even atredshifts of around 6.
| 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.
| Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. I. Observational Data
We have obtained high-resolution (R~=50,000 or 90,000), high-quality(S/N>~100) spectra of 22 very metal-poor stars ([Fe/H]<~-2.5) withthe High Dispersion Spectrograph fabricated for the 8.2 m SubaruTelescope. The spectra cover the wavelength range from 3500 to 5100Å equivalent widths are measured for isolated lines of numerouselemental species, including the α-elements, the iron-peakelements, and the light and heavy neutron-capture elements. Errors inthe measurements and comparisons with previous studies are discussed.These data will be used to perform detailed abundance analyses in thefollowing papers of this series. Radial velocities are also reported andare compared with previous studies. At least one moderatelyr-process-enhanced metal-poor star, HD 186478, exhibits evidence of asmall-amplitude radial velocity variation, confirming the binary statusnoted previously. During the course of this initial program, we havediscovered a new moderately r-process-enhanced, very metal-poor star, CS30306-132 ([Fe/H]=-2.4 [Eu/Fe]=+0.85), which is discussed in detail inthe companion paper.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. II. The r-Process Elements, Including Thorium
We have obtained high-resolution, high signal-to-noise near-UV-bluespectra of 22 very metal-poor stars ([Fe/H]<-2.5) with the SubaruHigh Dispersion Spectrograph and measured the abundances of elementsfrom C to Th. The metallicity range of the observed stars is-3.2<[Fe/H]<-2.4. As found by previous studies, the star-to-starscatter in the measured abundances of neutron-capture elements in thesestars is very large, much greater than could be assigned toobservational errors, in comparison with the relatively small scatter inthe α- and iron-peak elements. In spite of the large scatter inthe ratios of the neutron-capture elements relative to iron, theabundance patterns of heavy neutron-capture elements (56<=Z<~72)are quite similar within our sample stars. The Ba/Eu ratios in the 11very metal-poor stars in our sample in which both elements have beendetected are nearly equal to that of the solar system r-processcomponent. Moreover, the abundance patterns of the heavy neutron-captureelements (56<=Z<=70) in seven objects with clear enhancements ofthe neutron-capture elements are similar to that of the solar systemr-process component. These results prove that heavy neutron-captureelements in these objects are primarily synthesized by the r-process. Incontrast, the abundance ratios of the light neutron-capture elements(38<=Z<=46) relative to the heavier ones (56<=Z<=70) exhibita large dispersion. Our inspection of the correlation between Sr and Baabundances in very metal-poor stars reveals that the dispersion of theSr abundances clearly decreases with increasing Ba abundance. This trendis naturally explained by hypothesizing the existence of two processes,one that produces Sr without Ba and another that produces Sr and Ba insimilar proportions. This result should provide a strong constraint onthe origin of the light neutron-capture elements at low metallicity. Wehave identified a new highly r-process element enhanced, metal-poorstar, CS 22183-031, a giant with [Fe/H]=-2.93 and [Eu/Fe]=+1.2. We alsoidentified a new, moderately r-process-enhanced, metal-poor star, CS30306-132, a giant with [Fe/H]=-2.42 and [Eu/Fe]=+0.85. The abundanceratio of the radioactive element Th (Z=90) relative to the stablerare-earth elements (e.g., Eu) in very metal-poor stars has been used asa cosmochronometer by a number of previous authors. Thorium is detectedin seven stars in our sample, including four objects for which thedetection of Th has already been reported. New detections of thoriumhave been made for the stars HD 6268, HD 110184, and CS 30306-132. TheTh/Eu abundance ratios [log(Th/Eu)], are distributed over the range-0.10 to -0.59, with typical errors of 0.10 to 0.15 dex. In particular,the ratios in two stars, CS 31082-001 and CS 30306-132, aresignificantly higher than the ratio in the well-studied object CS22892-052 and those of other moderately r-process-enhanced metal-poorstars previously reported. Since these very metal-poor stars arebelieved to be formed in the early Galaxy, this result suggests that theabundance ratios between Th and stable rare-earth elements such as Eu,both of which are presumably produced by r-process nucleosynthesis, mayexhibit real star-to-star scatter, with implications for (1) theastrophysical sites of the r-process, and (2) the use of Th/Eu as acosmochronometer.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| The r-Process in Supernovae: Impact of New Microscopic Mass Formulae
The astrophysical origin of r-process nuclei is a long-standing mystery.Although some astrophysical scenarios show some promise, manyuncertainties involved in both the astrophysical conditions and in thenuclear properties far from the β-stability have inhibited us fromunderstanding the nature of the r-process. The purpose of the presentpaper is to examine the effects of the newly derived microscopicHartree-Fock-Bogoliubov (HFB) mass formulae on r-process nucleosynthesisand analyze to what extent a solar-like r-abundance distribution can beobtained. The r-process calculations with the HFB-2 mass formula areperformed, adopting the parameterized model of the prompt explosion froma collapsing O-Ne-Mg core for the physical conditions, and compared withthe results obtained with the HFB-7 and droplet-type mass formulae.Because of its weak shell effect at the neutron magic numbers in theneutron-rich region, the microscopic mass formulae (HFB-2 and HFB-7)give rise to a spread of the abundance distribution in the vicinity ofthe r-process peaks (A=130 and 195). While this effect resolves thelarge underproduction at A~115 and 140 obtained with droplet-type massformulae, large deviations compared to the solar pattern are found nearthe third r-process peak. It is shown that a solar-like r-processpattern can be obtained if the dynamical timescales of the outgoing masstrajectories are increased by a factor of about 2-3, or if theβ-decay rates are systematically increased by the same factor.
| The s-Process in Metal-Poor Stars: Abundances for 22 Neutron-Capture Elements in CS 31062-050
The CH star CS 31062-050 ([Fe/H]=-2.42) is one of the most useful starsyet discovered for evaluating the s-process in metal-poor stars. It isvery abundant in heavy elements (e.g., [La/Fe]=2.2), and its relativelycool temperature and low gravity mean that there are many lines ofinteresting elements present in the spectrum. We measured the abundancesof 22 elements with Z>=29, including the rarely measured Lu and Pd.We derive an upper limit on the Th abundance as well. The abundances inCS 31062-050 show a similar pattern to many other metal-poor CH stars:high [Pb/Fe] and [Pb/La] ratios, low [Y/La] ratios and high [Eu/La]values compared to the solar system s-process. However, the Th limit,with additional assumptions, is not consistent with the idea that theexcess Eu in CS 31062-050 is contributed by the r-process. In addition,the observed [Eu/Tb] cannot be explained by any ratio of solar-systems-process and r-process abundances. We therefore argue that theabundance pattern in CS 31062-050 is most likely the result of thes-process, and we discuss possible modifications that could explain thenon-solar-system pattern observed.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Improved Atomic Data for Ho II and New Holmium Abundances for the Sun and Three Metal-poor Stars
Improved energy levels and hyperfine structure constants for a selectedset of Ho II levels were measured from spectra recorded using the 1 mFourier transform spectrometer (FTS) at the US National SolarObservatory. Branching fractions for the strong blue-UV lines from theselevels were also measured from the FTS data and combined with earlierradiative lifetimes from time-resolved laser-induced fluorescencemeasurements to determine accurate absolute transition probabilities for22 lines of Ho II. These new laboratory measurements, along with arecently reported partition function for ionized Ho, were used toimprove Ho abundance determinations in the Sun and three metal-poorGalactic halo giant stars. The derived solar photospheric holmiumabundance, log10ɛ(Ho)=+0.51+/-0.10, is consistent with itsmeteoritic value, log10ɛ(Ho)=+0.49+/-0.02. In each of themetal-poor, neutron-capture-rich program stars, the holmium abundancerelative to those of other rare earth elements agrees well with solarsystem rapid-neutron-capture abundance values.
| The Pul-3 catalogue of 58483 stars in the Tycho-2 system
A catalogue of positions and proper motions of 58483 stars (Pul-3) hasbeen constructed at the Pulkovo observatory. The Pul-3 is based on theresults of measurements of photographic plates with galaxies (Deutsch'splan). All plates were taken using the Pulkovo Normal Astrograph (thefirst epoch is in the 1950s and the second epoch is in the 1970s).The Pul-3 catalogue contains stars of mainly 12 to 16.5 mag in 146fields with galaxies in the declination zone from -5o to+85o. The Tycho-2 has been used as a reference catalogue.The mean epoch of the Pul-3 is 1963.25. The internal positional accuracyof the Pul-3 catalogue at the mean epoch of observations is ±80mas. The accuracy of the proper motions is mostly within ±3mas/yr to ±12 mas/yr. Comparisons of the Pul-3 with Tycho-2 andARIHIP have been done at the mean epoch of the Pul-3. The Pul-3 externalpositional accuracy relative to Tycho-2 is ±150 mas.The catalogue 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/418/357
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Гончие Псы |
| Прямое восхождение: | 13h16m42.46s |
| Склонение: | +36°22'52.7" |
| Видимая звёздная величина: | 8.99 |
| Расстояние: | 281.69 парсек |
| Собственное движение RA: | 3.9 |
| Собственное движение Dec: | -60.3 |
| B-T magnitude: | 9.888 |
| V-T magnitude: | 9.065 |
Каталоги и обозначения:
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