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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.
| Where, oh where has the r-process gone?
Electronic Article Available from Elsevier Science.
| 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.
| Neutron-Capture Elements in the Metal-poor Globular Cluster M15
We report on observations of six giants in the globular cluster M15 (NGC7078) using the Subaru Telescope to measure neutron-capture elementalabundances. Our abundance analyses, based on high-quality blue spectra,confirm the star-to-star scatter in the abundances of heavyneutron-capture elements (e.g., Eu), and we found no significants-process contribution to them, as was found in previous studies. Wehave found that, for the first time, there are anticorrelations betweenthe abundance ratios of light to heavy neutron-capture elements ([Y/Eu]and [Zr/Eu]) and the heavy neutron-capture elements (e.g., Eu). Thisindicates that the light neutron-capture elements in these stars cannotbe explained by only a single r-process. Another process thatcontributed significantly to the light neutron-capture elements isrequired in M15. Our results suggest a complicated enrichment historyfor M15 and its progenitor.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Improved oscillator strengths and wavelengths in Hf II, with applications to stellar elemental abundances
Aims. We present new and improved radiative lifetimes for eight levelsin Hf I and 18 levels in Hf II, along with oscillator strengths andwavelengths for 195 transitions in Hf II. With these data we determinethe abundance of hafnium in two chemically peculiar stars: the hot-Amstar HR 3383 and the HgMn star χ Lupi, and discuss the implicationsof the new data to the hafnium abundance for the Sun and the metal-poorgalactic halo stars CS 22892-052 and CS 31082-001. Methods: . Theoscillator strengths are derived by combining radiative lifetimesmeasured with the laser induced fluorescence technique and branchingfractions determined from intensity calibrated Fourier transformspectra. The hafnium abundance in the two sharp-lined peculair stars isdetermined by comparison of spectra obtained from instruments onboardthe Hubble Space Telescope with synthetic spectra, while the abundanceof hafnium in the solar photosphere and the metal-poor halo stars isdiscussed in terms of rescaling previous investigations using the new gfvalues. Results: . The abundance enhancement of hafnium has beendetermined in HR 3383 to be +1.7 dex and that for χ Lupi A is +1.3dex. In the course of the analysis we have also determined an abundanceenhancement for molybdenum in HR 3383 to be +1.2 dex, which is similarto that known for χ Lupi A. The abundances in the metal-poor halostars CS 31082-001 and CS 22892-052 were rescaled to log ɛ(Hf) =-0.75 and -0.82 respectively, with smaller 1σ uncertainty. Thishas the effect of improving the theoretical fits of r-processnucleosynthesis to abundance data for heavy elements. The change of gfvalues also implies that the hafnium abundance in the solar photosphereshould be reduced by up to 0.2 dex, thereby inducing a discrepancy withthe meteoritic hafnium abundance.
| 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.
| Sulphur abundances in metal-poor stars
We investigate the debated "sulphur discrepancy" found among metal-poorstars of the Galactic halo with [Fe/H] < -2. This discrepancy stemsin part from the use of two different sets of sulphur lines, the veryweak triplet at 8694-95 Å and the stronger triplet lines at9212-9238 Å. For three representative cases of metal-poor dwarf,turnoff and subgiant stars, we argue that the abundances from theλλ8694-95 lines have been overestimated which has led to acontinually rising trend of [S/Fe] as metallicity decreases. Given thatthe near-IR region is subject to CCD fringing, these weak lines becomeexcessively difficult to measure accurately in the metallicity regime of[Fe/H] < -2. Based on homogeneously determined spectroscopic stellarparameters, we also present updated [S/Fe] ratios from theλλ9212-9238 lines which suggest a plateau-like behavioursimilar to that seen for other alpha elements.
| The 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.
| IX Ophiuchi: A High-Velocity Star Near a Molecular Cloud
The molecular cloud Barnard 59 is probably an outlier of the UpperSco/ρ Oph complex. B59 contains several T Tauri stars (TTSs), butoutside its northwestern edge are three other Hα-emission objectswhose nature has been unclear: IX, KK, and V359 Oph. This paper is adiscussion of all three and of a nearby Be star (HD 154851), basedlargely on Keck HIRES spectrograms obtained in 2004. KK Oph is a close(1.6") double. The brighter component is an HAeBe star, and the fainteris a K-type TTS. The complex BVR variations of the unresolved pairrequire both components to be variable. V359 Oph is a conventional TTS.Thus, these pre-main-sequence stars continue to be recognizable as suchwell outside the boundary of their parent cloud. IX Oph is quitedifferent. Its absorption spectrum is about type G, with manypeculiarities: all lines are narrow but abnormally weak, with structuresthat depend on ion and excitation level and that vary in detail frommonth to month. It could be a spectroscopic binary of small amplitude.Hα and Hβ are the only prominent emission lines. They arebroad, with variable central reversals. However, the most unusualcharacteristic of IX Oph is the very high (heliocentric) radialvelocity: about -310 km s-1, common to all spectrograms, andvery different from the radial velocity of B59, about -7 kms-1. There is no detectable Li I λ6707 line. There isreason to believe that IX Oph is actually a background object, onlyaligned with B59. Several conceivable interpretations are discussed: (1)It is unlikely that it is a high-velocity ejectee from the Upper Sco orUpper Cen-Lup associations (the lack of detectable λ6707 showsthat it is not the product of a very recent event, and the proper motionpoints in the wrong direction) or that it was born in or ejected fromone of the distant high-velocity CO clouds at this longitude(l=357deg). (2) A stronger possibility is that it is simply ametal-poor high-velocity G- or K-type giant (but such stars are notirregularly variable in light and do not have such strong Balmeremission lines). More likely, (3) IX Oph is a member of thehigh-velocity, low-metallicity SRd class of semiregular variables foundin the field and in some globular clusters. At some phases, those starsshow Hα emission like that found in IX Oph and, in one example,emission lines of neutral metals and double absorption lines as in IXOph.
| The Hamburg/ESO R-process enhanced star survey (HERES). II. Spectroscopic analysis of the survey sample
We present the results of analysis of "snapshot" spectra of 253metal-poor halo stars -3.8≤ [Fe/H] ≤ -1.5 obtained in the HERESsurvey. The snapshot spectra have been obtained with VLT/UVES and havetypically S/N˜ 54 per pixel (ranging from 17 to 308), R˜20000, λ = 3760-4980 Å. This sample represents the major partof the complete HERES sample of 373 stars; however, the CH strongcontent of the sample is not dealt with here. The spectra are analysedusing an automated line profile analysis method based on theSpectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elementalabundances of moderate precision (absolute rms errors of order 0.25 dex,relative rms errors of order 0.15 dex) have been obtained for 22elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr,Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 areusually detectable at the 3σ confidence level for our typicalspectra. The remainder can be detected in the least metal-poor stars ofthe sample, spectra with higher than average S/N, or when the abundanceis enhanced. Among the sample of 253 stars, disregarding four previouslyknown comparison stars, we find 8 r-II stars and 35 r-I stars. The r-IIstars, including the two previously known examples CS 22892-052 and CS31082-001, are centred on a metallicity of [Fe/H] = -2.81, with a verysmall scatter, on the order of 0.16 dex. The r-I stars are found acrosspractically the entire metallicity range of our sample. We also findthree stars with strong enhancements of Eu which are s-process rich. Asignificant number of new very metal-poor stars are confirmed: 49 starswith [Fe/H]<-3 and 181 stars with -3<[Fe/H]<-2. We find onestar with [Fe/H]<-3.5. We find the scatter in the abundance ratios ofMg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to besimilar to the estimated relative errors and thus the cosmic scatter tobe small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu,and perhaps Zr, show scatter at [Fe/H] ⪉ -2.5 significantly largerthan can be explained from the errors in the analysis, implying scatterwhich is cosmic in origin. Significant scatter is observed in abundanceratios between light and heavy neutron-capture elements at lowmetallicity and low levels of r-process enrichment.
| 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.
| Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra
The LTE abundances of sulfur (S) of 21 metal-poor stars and one normalstar were explored in the metallicity range of -3 < [Fe/H] ≤ 0,based on the equivalent widths of the S I (1) 9212, 9237Å and S I(6) 8693, 9894Å lines measured on high-resolution spectra, whichwere observed by the OAO 1.88-m telescope equipped with HIDES. Our mainresults are: (1) The abundances derived from the S I (6) lines areconsistent with those from the S I (1) lines among our sample stars inthe range of [Fe/H] > -2 with an average difference of +0.03 ±0.05 dex, whereas a significant discrepancy is observed in the range of[Fe/H] ≤ -2. (2) The behavior of [S(6)/Fe], versus [Fe/H] of ourhalo sample stars exhibits a nearly flat trend with an average of +0.62± 0.09 dex in the range of -3 < [Fe/H] < -1.25, and shows adistribution around +0.29 dex in -1.25 ≤ [Fe/H] ≤ -0.7. Oursample stars with -1.25 ≤ [Fe/H] ≤ -0.5 follow an increasingtrend with decreasing [Fe/H]. The behavior of [S(1)/Fe] of our samplestars also shows essentially the same trend as [S(6)/Fe], though it isquantitatively different. (3) The S behavior in the range of -3 <[Fe/H] ≤ 0 inferred from the abundances of multiplets 6 and 1 arequalitatively consistent with each other, and may be represented by acombination of a nearly flat trend and a linearly increasing trend withdecreasing [Fe/H]. A transition of the trend is likely to occur at[Fe/H] -1.5 dex.
| 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
| The first stars: what we know and do not know.
Not Available
| 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.
| 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).}
| He I λ10830 Absorption in Metal-Poor Red Giants: Probing Fast Chromospheric Outflows
We present the results of a search for near-IR chromospheric He Iabsorption in 10 metal-poor red giants using high-resolution spectraobtained with NIRSPEC on Keck II. Four of these red giants are in thefield, and six are in the globular cluster M13. Two of the field starsand one of the cluster stars show evidence for He I absorption that isconsistent with chromospheric outflow velocities ranging from 30 to 140km s-1. If identified with a wind, the velocity found in theM13 red giant IV-15 (30 km s-1) is comparable to the escapevelocity of the cluster core, suggesting that such winds may beeffective in clearing out the interstellar medium of globular clusters.Absorption is confined to stars with Teff>~4600 K andMV fainter than -1.5. Since previous studies have foundHα and Ca II K2 emission line profiles indicative ofwinds in metal-poor giants brighter than this limit, the mechanismresponsible for He I absorption in metal-poor red giants apparentlyoperates less effectively in the cooler and brighter stars. On the otherhand, the He I line extends the detection of mass outflows further downthe giant branch of metal-poor stars than the Hα observations.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.
| 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.
| Atmospheric Chemical Composition of the Halo Star HD 221170 from a Synthetic-Spectrum Analysis
The atmospheric abundances of 30 chemical elements in the halo star HD221170 are analyzed by fitting synthetic spectra to observed spectra (i)with a resolution of 60 000 and signal-to-noise ratios of about 200taken with the 1.93-m telescope of the Observatoire de Haute Provenceand (ii) with a resolution of 35 000 and signal-to-noise ratios of morethan 100 taken with the 2-m telescope of the Terskol Peak Observatory.The derived parameters of the stellar atmosphere are T eff=4475 K, logg=1.0, [Fe/H]=-2.03, V micro=1.7 km/s, and V macro=4 km/s. Theparameters T eff, log g, [Fe/H], and V micro can be determined byanalyzing the variations of the rms error of the mean iron abundancederived using different model atmospheres. The chemical composition ofthe star’s atmosphere is analyzed. The abundances of a total of 35elements in HD 221170 have been derived in this paper and in previousstudies. Overall, the abundances of elements lighter than praseodymiumare consistent with the elemental abundances in the atmospheres of starswith similar metal deficits. Copper and manganese are underabundant by-2.9 and -2.6 dex, respectively, relative to the Sun (when the analysisincludes the effects of hyperfine structure). Heavy r-process elements(starting from praseodymium) are overabundant compared to iron-groupelements. This can be explained by an enrichment in r-process elementsof the material from which the star was formed.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Oxygen Abundances in Metal-poor Stars
We present oxygen abundances derived from both the permitted andforbidden oxygen lines for 55 subgiants and giants with [Fe/H] valuesbetween -2.7 and solar with the goal of understanding the discrepancy inthe derived abundances. A first attempt, using Teff valuesfrom photometric calibrations and surface gravities from luminositiesobtained agreement between the indicators for turn-off stars, but thedisagreement was large for evolved stars. We find that the difference inthe oxygen abundances derived from the permitted and forbidden lines ismost strongly affected by Teff, and we derive a newTeff scale based on forcing the two sets of lines to give thesame oxygen abundances. These new parameters, however, do not agree withother observables, such as theoretical isochrones or Balmer-line profilebased Teff determinations. Our analysis finds thatone-dimensional, LTE analyses (with published non-LTE corrections forthe permitted lines) cannot fully resolve the disagreement in the twoindicators without adopting a temperature scale that is incompatiblewith other temperature indicators. We also find no evidence ofcircumstellar emission in the forbidden lines, removing such emission asa possible cause for the discrepancy.
| Non-LTE Analysis of the Sodium Abundance of Metal-Poor Stars in the Galactic Disk and Halo
We performed an extensive non-LTE analysis of the neutral sodium linesof Na I 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halostars of types F-K covering a wide metallicity range (-4 <≈[Fe/H] <≈ +0.4), using our own data as well as data collectedfrom the literature. For comparatively metal-rich disk stars (-1<≈ [Fe/H] <≈ +0.4) where the weaker 6154/6161 linesare the best abundance indicators, we confirmed [Na/Fe] 0 with an"upturn" (i.e., a shallow/broad dip around -0.5 <≈ [Fe/H]<≈ 0) as already reported in previous studies. For themetal-deficient halo stars, where the much stronger 5890/5896 or8183/8195 lines subject to considerable (negative) non-LTE correctionsamounting to 0.5 dex have to be used, our analysis suggests mildly"subsolar" [Na/Fe] values down to -0.4 (with a somewhat largescatter of ± 0.2 dex) on the average at the typical halometallicity of [Fe/H] -2, followed by a rise again to a near-solarratio of [Na/Fe] 0 at the very metal-poor regime [Fe/H] -3to -4. These results are discussed in comparison with the previousobservational studies along with the theoretical predictions from theavailable chemical evolution models.
| Sodium Abundances in Stellar Atmospheres with Differing Metallicities
The non-LTE sodium abundances of 100 stars with metallicities-3<[Fe/H]<0.3 are determined using high-dispersion spectra withhigh signal-to-noise ratios. The sodium abundances [Na/Fe] obtained areclose to the solar abundance and display a smaller scatter than valuespublished previously. Giants (logg<3.8) with [Fe/H]<-1 do notdisplay overabundances of sodium, and their sodium abundances do notshow an anticorrelation with the oxygen abundance, in contrast toglobular-cluster giants. They likewise do not show sodium-abundancevariations with motion along the giant branch. No appreciable decreasein the sodium abundance was detected for dwarfs (logg>3.8) withmetallicities -2<[Fe/H]<-1. The observed relation between [Na/Fe]and [Fe/H] is in satisfactory agreement with the theoreticalcomputations of Samland, which take into account the metallicitydependence of the sodium yield and a number of other factors affectingthe distribution of elements in the Galaxy during the course of itsevolution.
| Mapping the Galactic Halo. VI. Spectroscopic Measures of Luminosity and Metallicity
We present our calibration of spectroscopic measures of luminosity andmetallicity for halo giant candidates and give metallicities anddistances for our first sample of spectroscopically confirmed giants.These giants have distances ranging from 15 to 83 kpc. As surveys reachfarther into the Galaxy's halo with K giant samples, identification ofgiants becomes more difficult. This is because the numbers of foregroundhalo K dwarfs rise for V magnitudes of 19-20, typical for halo giants at~100 kpc. Our photometric survey uses the strength of the Mg b/H featurenear 5170 Å to weed K dwarfs out of the disk and thick disk, butwe need spectroscopic measures of the strength of the Ca II K, Ca Iλ4227, and Mg b/H features to distinguish between the verymetal-poor dwarfs and halo giants. Using a full error analysis of ourspectroscopic measures, we show why a signal-to-noise ratio of ~15pixel-1 at Ca I λ4227 and ~10 at Ca II K is needed forreliable luminosity discrimination. We use the Ca II K and Mg b featuresto measure metallicity in our halo giants, with typical errors (randomplus systematic) of 0.3 dex for [Fe/H] values from -0.8 to -3.0.
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are only available in electronic form, at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| Spectroscopic Binaries, Velocity Jitter, and Rotation in Field Metal-poor Red Giant and Red Horizontal-Branch Stars
We summarize 2007 radial velocity measurements of 91 metal-poor fieldred giants. Excluding binary systems with orbital solutions, ourcoverage averages 13.7 yr per star, with a maximum of 18.0 yr. We reportfour significant findings. (1) Sixteen stars are found to bespectroscopic binaries, and we present orbital solutions for 14 of them.The spectroscopic binary frequency of the metal-poor red giants, with[Fe/H]<=-1.4, for periods less than 6000 days, is 16%+/-4%, which isnot significantly different from that of comparable-metallicity fielddwarfs, 17%+/-2%. The two CH stars in our program, BD -1°2582 and HD135148, are both spectroscopic binaries. (2) Velocity jitter is presentamong about 40% of the giants with MV<=-1.4. The twobest-observed cases, HD 3008 and BD +22°2411, showpseudoperiodicities of 172 and 186 days, longer than any knownlong-period variable in metal-poor globular clusters. Photometricvariability seen in HD 3008 and three other stars showing velocityjitter hints that starspots are the cause. However, the phasing of thevelocity data with the photometry data from Hipparcos is not consistentwith a simple starspot model for HD 3008. We argue against orbitalmotion effects and radial pulsation, so rotational modulation remainsthe best explanation. The implied rotational velocities for HD 3008 andBD +22°2411, both with MV<=-1.4 and R~50Rsolar, exceed 12 km s-1. (3) Including HD 3008and BD +22°2411, we have found signs of significant excess linebroadening in eight of the 17 red giants with MV<=-1.4,which we interpret as rotation. In three cases, BD +30°2034, CD-37°14010, and HD 218732, the rotation is probably induced by tidallocking between axial rotation and the observed orbital motion with astellar companion. But this cannot explain the other five stars in oursample that display signs of significant rotation. This high frequencyof elevated rotational velocities does not appear to be caused bystellar mass transfer or mergers: there are too few main-sequencebinaries with short enough periods. We also note that the lack of anynoticeable increase in mean rotation at the magnitude level of the redgiant branch luminosity function ``bump'' argues against the rapidrotation's being caused by the transport of internal angular momentum tothe surface. Capture of a planetary-mass companion as a red giantexpands in radius could explain the high rotational velocities. (4) Wealso find significant rotation in at least six of the roughly 15 (40%)red horizontal-branch stars in our survey. It is likely that theenhanced rotation seen among a significant fraction of both blue and redhorizontal-branch stars arose when these stars were luminous red giants.Rapid rotation alone therefore appears insufficient cause to populatethe blue side of the horizontal branch. While the largest projectedrotational velocities seen among field blue and red horizontal-branchstars are consistent with their different sizes, neither are consistentwith the large values we find for the largest red giants. This suggeststhat some form of angular momentum loss (and possibly mass loss) hasbeen at work. Also puzzling is the apparent absence of rotation seen infield RR Lyrae variables. Angular momentum transfer and conservation inevolved metal-poor field stars thus pose many interesting questions forthe evolution of low-mass stars.
| Classification of Metal-Deficient Dwarfs in the Vilnius Photometric System
Methods used for the quantitative classification of metal-deficientstars in the Vilnius photometric system are reviewed. We present a newcalibration of absolute magnitudes for dwarfs and subdwarfs, based onHipparcos parallaxes. The new classification scheme is applied to asample of Population II visual binaries.
| Abundances of Cu and Zn in metal-poor stars: Clues for Galaxy evolution
We present new observations of copper and zinc abundances in 90metal-poor stars, belonging to the metallicity range -3<[Fe/H]<-0.5. The present study is based on high resolutionspectroscopic measurements collected at the Haute Provence Observatoire(R= 42 000, S/N>100). The trend of Cu and Zn abundances as a functionof the metallicity [Fe/H] is discussed and compared to that of otherheavy elements beyond iron. We also estimate spatial velocities andgalactic orbital parameters for our target stars in order to disentanglethe population of disk stars from that of halo stars using kinematiccriteria. In the absence of a firm a priori knowledge of thenucleosynthesis mechanisms controlling Cu and Zn production, and of therelative stellar sites, we derive constraints on these last from thetrend of the observed ratios [Cu/Fe] and [Zn/Fe] throughout the historyof the Galaxy, as well as from a few well established properties ofbasic nucleosynthesis processes in stars. We thus confirm that theproduction of Cu and Zn requires a number of different sources (neutroncaptures in massive stars, s-processing in low and intermediate massstars, explosive nucleosynthesis in various supernova types). We alsoattempt a ranking of the relative roles played by different productionmechanisms, and verify these hints through a simple estimate of thegalactic enrichment in Cu and Zn. In agreement with suggestionspresented earlier, we find evidence that type Ia Supernovae must play arelevant role, especially for the production of Cu. Based on the spectracollected with the 1.93-m telescope of Haute Provence Observatory.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | פגסוס |
| התרוממות ימנית: | 23h29m28.81s |
| סירוב: | +30°25'57.9" |
| גודל גלוי: | 7.689 |
| מרחק: | 434.783 פארסק |
| תנועה נכונה: | -16.3 |
| תנועה נכונה: | -54.4 |
| B-T magnitude: | 8.99 |
| V-T magnitude: | 7.797 |
קטלוגים וכינוים:
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INHABITED SKY
