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The Ubiquity of the Rapid Neutron-capture Process
To better characterize the abundance patterns produced by the r-process,we have derived new abundances or upper limits for the heavy elementszinc (Zn, Z= 30), yttrium (Y, Z= 39), lanthanum (La, Z= 57), europium(Eu, Z= 63), and lead (Pb, Z= 82). Our sample of 161 metal-poor starsincludes new measurements from 88 high-resolution and highsignal-to-noise spectra obtained with the Tull Spectrograph on the 2.7 mSmith Telescope at the McDonald Observatory, and other abundances areadopted from the literature. We use models of the s-process inasymptotic giant branch stars to characterize the high Pb/Eu ratiosproduced in the s-process at low metallicity, and our new observationsthen allow us to identify a sample of stars with no detectable s-processmaterial. In these stars, we find no significant increase in the Pb/Euratios with increasing metallicity. This suggests that s-processmaterial was not widely dispersed until the overall Galactic metallicitygrew considerably, perhaps even as high as [Fe/H] =-1.4, in contrastwith earlier studies that suggested a much lower mean metallicity. Weidentify a dispersion of at least 0.5 dex in [La/Eu] in metal-poor starswith [Eu/Fe] <+0.6 attributable to the r-process, suggesting thatthere is no unique "pure" r-process elemental ratio among pairs of rareearth elements. We confirm earlier detections of an anti-correlationbetween Y/Eu and Eu/Fe bookended by stars strongly enriched in ther-process (e.g., CS 22892-052) and those with deficiencies of the heavyelements (e.g., HD 122563). We can reproduce the range of Y/Eu ratiosusing simulations of high-entropy neutrino winds of core-collapsesupernovae that include charged-particle and neutron-capture componentsof r-process nucleosynthesis. The heavy element abundance patterns inmost metal-poor stars do not resemble that of CS 22892-052, but thepresence of heavy elements such as Ba in nearly all metal-poor starswithout s-process enrichment suggests that the r-process is a commonphenomenon.This paper includes data taken at The McDonald Observatory of TheUniversity of Texas at Austin.
| The Chemical Compositions of Non-variable Red and Blue Field Horizontal Branch Stars
We present a new detailed abundance study of field red horizontal branch(RHB) and blue horizontal branch (BHB) non-variable stars. Highresolution and high signal-to-noise ratio echelle spectra of 11 RHB and12 BHB were obtained with the McDonald 2.7 m telescope, and the RHBsample was augmented by reanalysis of spectra of 25 stars from a recentsurvey. We derived stellar atmospheric parameters based on spectroscopicconstraints and computed relative abundance ratios for 24 species of 19elements. The species include Si II and Ca II, which have not beenpreviously studied in RHB and BHB (T eff < 9000 K) stars.The abundance ratios are generally consistent with those ofsimilar-metallicity field stars in different evolutionary stages. Weestimated the masses of the RHB and BHB stars by comparing their Teff-log g positions with HB model evolutionary tracks.The mass distribution suggests that our program stars possess masses of~0.5 M sun. Finally, we compared the temperaturedistributions of field RHB and BHB stars with field RR Lyraes in themetallicity range -0.8gsim [Fe/H] gsim-2.5. This yieldedeffective temperature estimates of 5900 K and 7400 K for the red andblue edges of the RR Lyrae instability strip.
| The Hamburg/ESO R-process enhanced star survey (HERES). V. Detailed abundance analysis of the r-process enhanced star HE 2327-5642
Aims: We present a detailed abundance analysis of a stronglyr-process enhanced giant star discovered in the HERES project, HE2327-5642, for which [Fe/H] = -2.78, [r/Fe] = +0.99. Methods: Wedetermined the stellar parameters and element abundances by analyzingthe high-quality VLT/UVES spectra. The surface gravity was calculatedfrom the non-local thermodynamic equilibrium (NLTE) ionization balancebetween ion{Fe}{i} and ion{Fe}{ii}, and ion{Ca}{i} and ion{Ca}{ii}. Results: Accurate abundances for a total of 40 elements and for 23neutron-capture elements beyond Sr and up to Th were determined in HE2327-5642. For every chemical species, the dispersion in the single linemeasurements around the mean does not exceed 0.11 dex. The heavy elementabundance pattern of HE 2327-5642 is in excellent agreement with thosepreviously derived for other strongly r-process enhanced stars, such asCS 22892-052, CS 31082-001, and HE 1219-0312. Elements in the range fromBa to Hf match the scaled Solar r-process pattern very well. No firmconclusion can be drawn about the relationship between the fisrtneutron-capture peak elements, Sr to Pd, in HE 2327-5642 and the Solarr-process, due to the uncertainty in the Solar r-process. A cleardistinction in Sr/Eu abundance ratios was found between the halo starsof different europium enhancement. The strongly r-process enhanced starscontain a low Sr/Eu abundance ratio at [Sr/Eu] = -0.92 ± 0.13,while the stars with 0 < [Eu/Fe] < 1 and [Eu/Fe] < 0 have 0.36dex and 0.93 dex higher Sr/Eu values, respectively. Radioactive datingfor HE 2327-5642 with the observed thorium and rare-earth elementabundance pairs results in an average age of 13.3 Gyr, when based on thehigh-entropy wind calculations, and 5.9 Gyr, when using the Solarr-residuals. We propose that HE 2327-5642 is a radial-velocity variablebased on our high-resolution spectra covering 4.3 years.Based on observations collected at the European Southern Observatory,Paranal, Chile (Proposal numbers 170.D-0010, and 280.D-5011).Table 8 isonly available in electronic form at http://www.aanda.org
| HD209621: abundances of neutron-capture elements
High-resolution spectra obtained from the Subaru Telescope HighDispersion Spectrograph have been used to update the stellar atmosphericparameters and metallicity of the star HD209621. We have derived ametallicity of [Fe/H] = -1.93 for this star, and have found a largeenhancement of carbon and of heavy elements, with respect to iron.Updates on the elemental abundances of four s-process elements (Y, Ce,Pr, Nd) along with the first estimates of abundances for a number ofother heavy elements (Sr, Zr, Ba, La, Sm, Eu, Er, Pb) are reported. Thestellar atmospheric parameters, the effective temperature,Teff, and the surface gravity, logg (4500K, 2.0), aredetermined from local thermodynamic equilibrium analysis using modelatmospheres. Estimated [Ba/Eu] = +0.35, places the star in the group ofCEMP-(r+s) stars; however, the s-elements abundance pattern seen inHD209621 is characteristic of CH stars; notably, the second-peaks-process elements are more enhanced than the first-peak s-processelements. HD209621 is also found to show a large enhancement of thethird-peak s-process element lead (Pb) with [Pb/Fe] = +1.88. Therelative contributions of the two neutron-capture processes, r and s, tothe observed abundances are examined using a parametric model-basedanalysis, which hints that the neutron-capture elements in HD209621primarily originate in s-process.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan, and at HCT, IAO, Hanle,India.E-mail: aruna@iiap.res.in (AG); aoki.wako@nao.ac.jp (WA)
| New Abundance Determinations of Cadmium, Lutetium, and Osmium in the r-process Enriched Star BD +17 3248
We report the detection of Cd I (Z = 48), Lu II (Z = 71), and Os II (Z =76) in the metal-poor star BD +17 3248. These abundances are derivedfrom an ultraviolet spectrum obtained with the Space Telescope ImagingSpectrograph on the Hubble Space Telescope. This is the first detectionof these neutron-capture species in a metal-poor star enriched by the rprocess. We supplement these measurements with new abundances of Mo I,Ru I, and Rh I derived from an optical spectrum obtained with the HighResolution Echelle Spectrograph on Keck. Combined with previousabundance derivations, 32 neutron-capture elements have been detected inBD +17 3248, the most complete neutron-capture abundance pattern in anymetal-poor star to date. The light neutron-capture elements (38 <= Z<= 48) show a more pronounced even-odd effect than expected fromcurrent solar system r-process abundance predictions. The age for BD +173248 derived from the Th II/Os II chronometer is in better agreementwith the age derived from other chronometers than the age derived fromTh II/Os I. New Hf II abundance derivations from transitions in theultraviolet are lower than those derived from transitions in theoptical, and the lower Hf abundance is in better agreement with thescaled solar system r-process distribution.
| Stellar archaeology: Exploring the Universe with metal-poor stars
The abundance patterns of the most metal-poor stars in the Galactic haloand small dwarf galaxies provide us with a wealth of information aboutthe early Universe. In particular, these old survivors allow us to studythe nature of the first stars and supernovae, the relevantnucleosynthesis processes responsible for the formation and evolution ofthe elements, early star- and galaxy formation processes, as well as theassembly process of the stellar halo from dwarf galaxies a long timeago. This review presents the current state of the field of ``stellararchaeology'' - the diverse use of metal-poor stars to explore thehigh-redshift Universe and its constituents. In particular, theconditions for early star formation are discussed, how these ultimatelyled to a chemical evolution, and what the role of the most iron-poorstars is for learning about Population III supernovae yields. Rapidneutron-capture signatures found in metal-poor stars can be used toobtain stellar ages, but also to constrain this complex nucleosynthesisprocess with observational measurements. Moreover, chemical abundancesof extremely metal-poor stars in different types of dwarf galaxies canbe used to infer details on the formation scenario of the halo and therole of dwarf galaxies as Galactic building blocks. I conclude with anoutlook as to where this field may be heading within the next decade. Atable of ˜ 1000 metal-poor stars and their abundances ascollected from the literature is provided in electronic format.
| Empirically Derived Integrated Stellar Yields of Fe-Peak Elements
We present here the initial results of a new study of massive staryields of Fe-peak elements. We have compiled from the literature adatabase of carefully determined solar neighborhood stellar abundancesof seven iron-peak elements, Ti, V, Cr, Mn, Fe, Co, and Ni, and thenplotted [X/Fe] versus [Fe/H] to study the trends as functions ofmetallicity. Chemical evolution models were then employed to force a fitto the observed trends by adjusting the input massive starmetallicity-sensitive yields of Kobayashi et al. Our results suggestthat yields of Ti, V, and Co are generally larger as well asanticorrelated with metallicity, in contrast to the Kobayashi et al.predictions. We also find the yields of Cr and Mn to be generallysmaller and directly correlated with metallicity compared to thetheoretical results. Our results for Ni are consistent with theory,although our model suggests that all Ni yields should be scaled upslightly. The outcome of this exercise is the computation of a set ofintegrated yields, i.e., stellar yields weighted by a slightly flattenedtime-independent Salpeter initial mass function and integrated overstellar mass, for each of the above elements at several metallicitypoints spanned by the broad range of observations. These results aredesigned to be used as empirical constraints on future iron-peak yieldpredictions by stellar evolution modelers. Special attention is paid tothe interesting behavior of [Cr/Co] with metallicity—these twoelements have opposite slopes—as well as the indirect correlationof [Ti/Fe] with [Fe/H]. These particular trends, as well as thoseexhibited by the inferred integrated yields of all iron-peak elementswith metallicity, are discussed in terms of both supernovanucleosynthesis and atomic physics.
| Transition probabilities of astrophysical interest in the niobium ions Nb+ and Nb2+
Aims: We attempt to derive accurate transition probabilities forastrophysically interesting spectral lines of ion{Nb}{ii} andion{Nb}{iii} and determine the niobium abundance in the Sun andmetal-poor stars rich in neutron-capture elements. Methods: Weused the time-resolved laser-induced fluorescence technique to measureradiative lifetimes in ion{Nb}{ii}. Branching fractions were measuredfrom spectra recorded using Fourier transform spectroscopy. Theradiative lifetimes and the branching fractions were combined yieldingtransition probabilities. In addition, we calculated lifetimes andtransition probablities in ion{Nb}{ii} and ion{Nb}{iii} using arelativistic Hartree-Fock method that includes core polarization.Abundances of the sun and five metal-poor stars were derived usingsynthetic spectra calculated with the MOOG code, including hyperfinebroadening of the lines. Results: We present laboratorymeasurements of 17 radiative lifetimes in ion{Nb}{ii}. By combiningthese lifetimes with branching fractions for lines depopulating thelevels, we derive the transition probabilities of 107 ion{Nb}{ii} linesfrom 4d35p configuration in the wavelength region 2240-4700Å. For the first time, we present theoretical transitionprobabilities of 76 Nb III transitions with wavelengths in the range1430-3140 Å. The derived solar photospheric niobium abundance log?_? = 1.44 ± 0.06 is in agreement with themeteoritic value. The stellar Nb/Eu abundance ratio determined for fivemetal-poor stars confirms that the r-process is a dominant productionmethod for the n-capture elements in these stars.
| A holistic approach to carbon-enhanced metal-poor stars
Context. Carbon-enhanced metal-poor (CEMP) stars are known to haveproperties that reflect the nucleosynthesis of the first low- andintermediate-mass stars, because most have been polluted by anow-extinct AGB star. Aims: By considering abundances in thevarious CEMP subclasses separately, we try to derive parameters (such asmetallicity, mass, temperature, and neutron source) characterising AGBnucleosynthesis from the specific signatures imprinted on theabundances, and separate them from the impact of thermohaline mixing,first dredge-up, and dilution associated with the mass transfer from thecompanion. Methods: To place CEMP stars in a broader context, wecollect abundances for about 180 stars of various metallicities (fromsolar to [Fe/H] =-4), luminosity classes (dwarfs and giants), andabundance patterns (e.g. C-rich and poor, Ba-rich and poor), from bothour own sample and the literature. Results: We first show thatthere are CEMP stars that share the properties of CEMP-s stars andCEMP-no stars (which we refer to as CEMP-low-s stars). We also show thatthere is a strong correlation between Ba and C abundances in the s-onlyCEMP stars. This represents a strong detection of the operation of the13C neutron source in low-mass AGB stars. For the CEMP-rsstars (seemingly enriched with elements from both the s- andr-processes), the correlation of the N abundances with abundances ofheavy elements from the 2nd and 3rd s-process peaks bears instead thesignature of the 22Ne neutron source. Since CEMP-rs starsalso exhibit O and Mg enhancements, we conclude that extremely hotconditions prevailed during the thermal pulses of the contaminating AGBstars. We also note that abundances are not affected by the evolution ofthe CEMP-rs star itself (especially by the first dredge-up). Thisimplies that mixing must have occurred while the star was on the mainsequence, and that a large amount of matter must have been accreted soas to trigger thermohaline mixing. Finally, we argue that most CEMP-nostars (with neutron-capture element abundances comparable to non-CEMPstars) are likely the extremely metal-poor counterparts of CEMPneutron-capture-rich stars. We also show that the C enhancement inCEMP-no stars declines with metallicity at extremely low metallicity([Fe/H] < -3.2). This trend is not predicted by any of the currentAGB models.Tables 1-4 are only available in electronic form at http://www.aanda.org
| Non-LTE abundances of Mg and K in extremely metal-poor stars and the evolution of [O/Mg], [Na/Mg], [Al/Mg], and [K/Mg] in the Milky Way
Aims: LTE abundances of light elements in extremely metal-poor(EMP) stars have been previously derived from high quality spectra. Newderivations, free from the NLTE effects, will better constrain themodels of the Galactic chemical evolution and the yields of the veryfirst supernovae. Methods: The NLTE profiles of the magnesium andpotassium lines have been computed in a sample of 53 extremelymetal-poor stars with a modified version of the program MULTI andadjusted to the observed lines in order to derive the abundances ofthese elements. Results: The NLTE corrections for magnesium andpotassium are in good agreement with the works found in the literature.The abundances are slightly changed, reaching a better precision: thescatter around the mean of the abundance ratios has decreased. Magnesiummay be used with confidence as reference element. Together withpreviously determined NLTE abundances of sodium and aluminum, the newratios are displayed, for comparison, along the theoretical trendsproposed by some models of the chemical evolution of the Galaxy, usingdifferent models of supernovae.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory (Large Programme “First Stars”, ID165.N-0276; P.I.: R. Cayrel.
| Accuracy of spectroscopy-based radioactive dating of stars
Context. Combined spectroscopic abundance analyses of stable andradioactive elements can be applied for deriving stellar ages. Theachievable precision depends on factors related to spectroscopy,nucleosynthesis, and chemical evolution. Aims: We quantify theuncertainties arising from the spectroscopic analysis, and compare theseto the other error sources. Methods: We derive formulae for theage uncertainties arising from the spectroscopic abundance analysis, andapply them to spectroscopic and nucleosynthetic data compiled from theliterature for the Sun and metal-poor stars. Results: We obtainedready-to-use analytic formulae of the age uncertainty for the cases ofstable+unstable and unstable+unstable chronometer pairs, and discuss theoptimal relation between to-be-measured age and mean lifetime of aradioactive species. Application to the literature data indicates that,for a single star, the achievable spectroscopic accuracy is limited toabout ±20% for the foreseeable future. At present, theoreticaluncertainties in nucleosynthesis and chemical evolution models form theprecision bottleneck. For stellar clusters, isochrone fitting provides ahigher accuracy than radioactive dating, but radioactive dating becomescompetitive when applied to many cluster members simultaneously,reducing the statistical errors by a factor ?{N}. Conclusions: Spectroscopy-based radioactive stellar dating would benefitfrom improvements in the theoretical understanding of nucleosynthesisand chemical evolution. Its application to clusters can provide strongconstraints for nucleosynthetic models.
| Fast Winds and Mass Loss from Metal-Poor Field Giants
Echelle spectra of the infrared He I λ10830 line were obtainedwith NIRSPEC on the Keck 2 telescope for 41 metal-deficient field giantstars including those on the red giant branch (RGB), asymptotic giantbranch (AGB), and red horizontal branch (RHB). The presence of this He Iline is ubiquitous in stars with T effgsim 4500 K andMV fainter than -1.5, and reveals the dynamics of theatmosphere. The line strength increases with effective temperature for Teffgsim 5300 K in RHB stars. In AGB and RGB stars, the linestrength increases with luminosity. Fast outflows (gsim 60 kms-1) are detected from the majority of the stars andabout 40% of the outflows have sufficient speed as to allow escape ofmaterial from the star as well as from a globular cluster. Outflowspeeds and line strengths do not depend on metallicity for our sample([Fe/H]= -0.7 to -3.0), suggesting the driving mechanism forthese winds derives from magnetic and/or hydrodynamic processes. Gasoutflows are present in every luminous giant, but are not detected inall stars of lower luminosity indicating possible variability. Mass lossrates ranging from ~3 × 10-10 to ~6 ×10-8 M sun yr-1 estimatedfrom the Sobolev approximation for line formation represent values withevolutionary significance for red giants and RHB stars. We estimate that0.2 M sun will be lost on the RGB, and the torque of thiswind can account for observations of slowly rotating RHB stars in thefield. About 0.1-0.2 M sun will be lost on the RHB itself.This first empirical determination of mass loss on the RHB maycontribute to the appearance of extended horizontal branches in globularclusters. The spectra appear to resolve the problem of missingintracluster material in globular clusters. Opportunities exist for"wind smothering" of dwarf stars by winds from the evolved population,possibly leading to surface pollution in regions of high stellardensity.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.
| An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| The Hamburg/ESO R-process enhanced star survey (HERES). IV. Detailed abundance analysis and age dating of the strongly r-process enhanced stars CS 29491-069 and HE 1219-0312
We report on a detailed abundance analysis of two strongly r-processenhanced, very metal-poor stars newly discovered in the HERES project,CS 29491-069 ([Fe/H]=-2.51, [r/Fe]=+1.1) andHE 1219-0312 ([Fe/H]=-2.96, [r/Fe]=+1.5). Theanalysis is based on high-quality VLT/UVES spectra and MARCS modelatmospheres. We detect lines of 15 heavy elements in the spectrum ofCS 29491-069, and 18 in HE1219-0312; in both cases including the Th II 4019 Åline. The heavy-element abundance patterns of these two stars are mostlywell-matched to scaled solar residual abundances not formed by thes-process. We also compare the observed pattern with recent high-entropywind (HEW) calculations, which assume core-collapse supernovae ofmassive stars as the astrophysical environment for the r-process, andfind good agreement for most lanthanides. The abundance ratios of thelighter elements strontium, yttrium, and zirconium, which are presumablynot formed by the main r-process, are reproduced well by the model.Radioactive dating for CS 29491-069 with the observedthorium and rare-earth element abundance pairs results in an average ageof 9.5 Gyr, when based on solar r-process residuals, and 17.6 Gyr, whenusing HEW model predictions. Chronometry seems to fail in the case ofHE 1219-0312, resulting in a negative age due to itshigh thorium abundance. HE 1219-0312 could thereforeexhibit an overabundance of the heaviest elements, which is sometimescalled an “actinide boost”.Based on observations collected at the European Southern Observatory,Paranal, Chile (Proposal Number 170.D-0010). Table 8 is only availablein electronic form at http://www.aanda.org
| First stars XII. Abundances in extremely metal-poor turnoff stars, and comparison with the giants
Context: The detailed chemical abundances of extremely metal-poor (EMP)stars are key guides to understanding the early chemical evolution ofthe Galaxy. Most existing data, however, treat giant stars that may haveexperienced internal mixing later. Aims: We aim to compare theresults for giants with new, accurate abundances for all observableelements in 18 EMP turnoff stars. Methods: VLT/UVES spectra at R~ 45 000 and S/N ~ 130 per pixel (?? 330-1000 nm) areanalysed with OSMARCS model atmospheres and the TURBOSPECTRUM code toderive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, andBa. Results: For Ca, Ni, Sr, and Ba, we find excellentconsistency with our earlier sample of EMP giants, at all metallicities.However, our abundances of C, Sc, Ti, Cr, Mn and Co are ~0.2 dex largerthan in giants of similar metallicity. Mg and Si abundances are ~0.2 dexlower (the giant [Mg/Fe] values are slightly revised), while Zn is again~0.4 dex higher than in giants of similar [Fe/H] (6 stars only). Conclusions: For C, the dwarf/giant discrepancy could possibly have anastrophysical cause, but for the other elements it must arise fromshortcomings in the analysis. Approximate computations of granulation(3D) effects yield smaller corrections for giants than for dwarfs, butsuggest that this is an unlikely explanation, except perhaps for C, Cr,and Mn. NLTE computations for Na and Al provide consistent abundancesbetween dwarfs and giants, unlike the LTE results, and would be highlydesirable for the other discrepant elements as well. Meanwhile, werecommend using the giant abundances as reference data for Galacticchemical evolution models.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory, Chile (Large Programme “First Stars”,ID 165.N-0276; P.I.: R. Cayrel, and Programme 078.B-0238; P.I.: M.Spite). Appendices A-C are only available in electronic form athttp://www.aanda.org Table 7 is only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/519
| Potassium Abundances in Red Giants of Mildly to Very Metal-Poor Globular Clusters
A non-LTE analysis of K I resonance lines at 7664.91 and 7698.97 A wascarried out for 15 red giants belonging to three globular clusters ofdifferent metallicity (M 4, M 13, and M 15) along with two referenceearly-K giants (rho Boo and alpha Boo), in order to check whether the Kabundances are uniform within a cluster and to investigate the behaviorof [K/Fe] ratio at the relevant metallicity range of -2.5 <[Fe/H]< -1. We confirmed that [K/H] (as well as [Fe/H]) is almosthomogeneous within each cluster to a precision of < ~0.1 dex, thoughdubiously large deviations are exceptionally seen for two peculiar starsshowing signs of considerably increased turbulence in the upperatmosphere. The resulting [K/Fe] ratios are mildly supersolar by a fewtenths of dex for three clusters, tending to gradually increase from~+0.1-0.2 at [Fe/H] ~-1 to ~+0.3 at [Fe/H] ~ -2.5. This result connectsreasonably well with the [K/Fe] trend of disk stars (-1 < [Fe/H]) andthat of extremely metal-poor stars (-4 <[Fe/H] < -2.5). That is,[K/Fe] appears to continue a gradual increase from [Fe/H]~0 toward alower metallicity regime down to [Fe/H]~-3, where a broad maximum of[K/Fe]~+0.3-0.4 is attained, possibly followed by a slight downturn at[Fe/H]<~-3.
| The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy
We examine the Pb and Th abundances in 27 metal-poor stars(-3.1< [Fe/H] <-1.4) whose very heavy metal (Z >56) enrichment was produced only by the rapid (r-) nucleosynthesisprocess. New abundances are derived from Hubble Space Telescope/SpaceTelescope Imaging Spectrograph, Keck/High Resolution EchelleSpectrograph, and Very Large Telescope/UV-Visual Echelle Spectrographspectra and combined with other measurements from the literature to forma more complete picture of nucleosynthesis of the heaviest elementsproduced in the r-process. In all cases, the abundance ratios among therare earth elements and the third r-process peak elements considered(La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solarsystem r-process abundance distribution. We compare the stellarobservations with r-process calculations within the classical"waiting-point" approximation. In these computations a superposition of15 weighted neutron-density components in the range 23 <=lognn <= 30 is fit to the r-process abundance peaks tosuccessfully reproduce both the stable solar system isotopicdistribution and the stable heavy element abundance pattern between Baand U in low-metallicity stars. Under these astrophysical conditions,which are typical of the "main" r-process, we find very good agreementbetween the stellar Pb r-process abundances and those predicted by ourmodel. For stars with anomalously high Th/Eu ratios (the so-calledactinide boost), our observations demonstrate that any nucleosyntheticdeviations from the main r-process affect—at most—only theelements beyond the third r-process peak, namely Pb, Th, and U. Ourtheoretical calculations also indicate that possible r-process abundance"losses" by nuclear fission are negligible for isotopes along ther-process path between Pb and the long-lived radioactive isotopes of Thand U.
| New Rare Earth Element Abundance Distributions for the Sun and Five r-Process-Rich Very Metal-Poor Stars
We have derived new abundances of the rare earth elements Pr, Dy, Tm,Yb, and Lu for the solar photosphere and for five very metal-poor,neutron-capture r-process-rich giant stars. The photospheric values forall five elements are in good agreement with meteoritic abundances. Forthe low-metallicity sample, these abundances have been combined with newCe abundances from a companion paper, and reconsideration of a few otherelements in individual stars, to produce internally consistent Ba, rareearth, and Hf (56 <= Z <= 72) element distributions. These havebeen used in a critical comparison between stellar and solar r-processabundance mixes.
| Improved Laboratory Transition Probabilities for Ce II, Application to the Cerium Abundances of the Sun and Five r-Process-Rich, Metal-Poor Stars, and Rare Earth Lab Data Summary
Recent radiative lifetime measurements accurate to ±5% usinglaser-induced fluorescence (LIF) on 43 even-parity and 15 odd-paritylevels of Ce II have been combined with new branching fractions measuredusing a Fourier transform spectrometer (FTS) to determine transitionprobabilities for 921 lines of Ce II. This improved laboratory data sethas been used to determine a new solar photospheric Ce abundance, log? = 1.61 ± 0.01 (? = 0.06 from 45 lines), a valuein excellent agreement with the recommended meteoritic abundance, log? = 1.61 ± 0.02. Revised Ce abundances have also beenderived for the r-process-rich metal-poor giant stars BD+17°3248, CS22892-052, CS 31082-001, HD 115444, and HD 221170. Between 26 and 40lines were used for determining the Ce abundance in these five stars,yielding a small statistical uncertainty of ±0.01 dex similar tothe solar result. The relative abundances in the metal-poor stars of Ceand Eu, a nearly pure r-process element in the Sun, matchesr-process-only model predictions for solar system material. Thisconsistent match with small scatter over a wide range of stellarmetallicities lends support to these predictions of elemental fractions.A companion paper includes an interpretation of these new precisionabundance results for Ce as well as new abundance results andinterpretation for Pr, Dy, and Tm.
| Taking Another Look: Light n-Capture Element Abundances in Metal-Poor Halo Stars
Elements are produced in stars through a variety of processes; some arewell known, others are still the object of active research. The elementszirconium (Zr) and yttrium (Y) reside in the mass range where there isuncertainty about the production mechanism at early time. The rapidn-capture elements.
| Evolution of the barium abundance in the early Galaxy from a NLTE analysis of the Ba lines in a homogeneous sample of EMP stars
Context: Barium is a key element in constraining the evolution of the(not well understood) r-process in the first galactic stars andcurrently the Ba abundances in these very metal-poor stars were mostlymeasured under the Local Thermodynamical Equilibrium (LTE) assumption,which may lead in general to an underestimation of Ba. Aims: We presenthere determinations of the barium abundance taking into account thenon-LTE (NLTE) effects in a sample of extremely metal-poor stars (EMPstars): 6 turnoff stars and 35 giants. Methods: The NLTE profiles ofthe three unblended Ba II lines (4554 Å, 5853 Å, 6496Å) have been computed. The computations were made with a modifiedversion of the MULTI code, applied to an atomic model of the Ba atomwith 31 levels of Ba I, 101 levels of Ba II, and compared to theobservations. Results: The ratios of the NLTE abundances of bariumrelative to Fe are slightly shifted towards the solar ratio. In the plotof [Ba/Fe] versus [Fe/H], the slope of the regression line is slightlyreduced as is the scatter. In the interval -3.3 <[Fe/H] < -2.6,[Ba/Fe] decreases with a slope of about 1.4 and a scatter close to 0.44.For [Fe/H] <-3.3 the number of stars is not sufficient to decidewhether [Ba/Fe] keeps decreasing (and then CD-38:245 should beconsidered as a peculiar “barium-rich star”) or if a plateauis reached as soon as [Ba/Fe] ≈ -1. In both cases the scatter remainsquite large, larger than what can be accounted for by the measurementand determination errors, suggesting the influence of a complex processof Ba production, and/or inefficient mixing in the early Galaxy.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory (Large Programme “First Stars”, ID165.N-0276; P.I.: R. Cayrel.
| Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature withdetailed chemical abundance information (spanning –4.2lsim [Fe/H]lsim+0.3), and we compute their space velocities and Galactic orbitalparameters. We identify members of the inner and outer stellar halopopulations in our sample based only on their kinematic properties andthen compare the abundance ratios of these populations as a function of[Fe/H]. In the metallicity range where the two populations overlap(–2.5lsim [Fe/H] lsim–1.5), the mean [Mg/Fe] of the outerhalo is lower than the inner halo by –0.1 dex. For [Ni/Fe] and[Ba/Fe], the star-to-star abundance scatter of the inner halo isconsistently smaller than in the outer halo. The [Na/Fe], [Y/Fe],[Ca/Fe], and [Ti/Fe] ratios of both populations show similar means andlevels of scatter. Our inner halo population is chemically homogeneous,suggesting that a significant fraction of the Milky Way stellar halooriginated from a well-mixed interstellar medium. In contrast, our outerhalo population is chemically diverse, suggesting that anothersignificant fraction of the Milky Way stellar halo formed in remoteregions where chemical enrichment was dominated by local supernovaevents. We find no abundance trends with maximum radial distance fromthe Galactic center or maximum vertical distance from the Galactic disk.We also find no common kinematic signature for groups of metal-poorstars with peculiar abundance patters, such as the α-poor stars orstars showing unique neutron-capture enrichment patterns. Several starsand dwarf spheroidal systems with unique abundance patterns spend themajority of their time in the distant regions of the Milky Way stellarhalo, suggesting that the true outer halo of the Galaxy may have littleresemblance to the local stellar halo.
| Abundances of Sr, Y, and Zr in Metal-Poor Stars and Implications for Chemical Evolution in the Early Galaxy
We have attributed the elements from Sr through Ag in stars of lowmetallicities ([Fe/H]<~-1.5) to charged-particle reactions (CPRs) inneutrino-driven winds, which are associated with neutron star formationin low-mass and normal supernovae (SNe) from progenitors of ~8-11Msolar and ~12-25 Msolar, respectively. Using thisrule and attributing all Fe production to normal SNe, we previouslydeveloped a phenomenological two-component model, which predicts that[Sr/Fe]>=-0.32 for all metal-poor stars. This is in direct conflictwith the high-resolution data now available, which show that there is agreat shortfall of Sr relative to Fe in many stars with [Fe/H]<~-3.The same conflict also exists for the CPR elements Y and Zr. We showthat the data require a stellar source leaving behind black holes andthat hypernovae (HNe) from progenitors of ~25-50 Msolar arethe most plausible candidates. If we expand our previous model toinclude three components (low-mass and normal SNe and HNe), we find thatessentially all of the data are very well described by the new model.The HN yield pattern for the low-A elements from Na through Zn(including Fe) is inferred from the stars deficient in Sr, Y, and Zr. Weestimate that HNe contributed ~24% of the bulk solar Fe inventory whilenormal SNe contributed only ~9% (not the usually assumed ~33%). Thisimplies a greatly reduced role of normal SNe in the chemical evolutionof the low-A elements.
| Nucleosynthesis modes in the high-entropy-wind of type II supernovae
The exact conditions for the supernova high-entropy wind (HEW) as one ofthe favored sites for the rapid neutron-capture (r-) process stillcannot be reproduced selfconsistently in present hydrodynamicsimulations. Therefore, we have performed large-scale networkcalculations within a parameterized HEW model to constrain the necessaryconditions for a full r-process, and to compare our results with recentastronomical observations. A superposition of entropy trajectories withmodel-inherent weightings results in an excellent reproduction of theoverall solar-system isotopic abundances (Nr,?) of the“main” r-process elements beyond Sn. For the lighterr-elements, our model supports earlier qualitative ideas about amultiplicity of nucleosynthesis processes in the Fe-group region. In thehigh-entropy-wind scenario, these suggestions are quantified, and theorigin of the “missing” abundances to Nr,?is determined to be a rapid primary charged-particle (?-) process,thus excluding a classical “weak” neutron-capture component.This explains the recent halo-star observations of a non-correlation ofCu Ge and Sr Zr with metallicity [Fe/H] and r-process enrichment [Eu/H].Moreover, for the first time a partial correlation with the“main” r-process is identified for Ru and Pd.
| Neutron-Capture Elements in the Early Galaxy
The content of neutron-capture (trans-iron-peak) elements in thelow-metallicity Galactic halo varies widely from star to star. Thedifferences are both in bulk amount of the neutron-capture elements withrespect to lighter ones and in element-to-element ratios amongthemselves. Several well-defined abundance distributions have emergedthat reveal characteristic rapid and slow neutron-capturenucleosynthesis patterns. In this review we summarize these observedmetal-poor star's abundances, contrasting them with the Solar-systemvalues, comparing them to theoretical predictions, using them to assessthe types of stars responsible for their specific anomalies, andspeculating on the timing and nature of early Galactic nucleosynthesis.
| Improved Laboratory Transition Probabilities for Er II and Application to the Erbium Abundances of the Sun and Five r-Process-rich, Metal-poor Stars
Recent radiative lifetime measurements accurate to +/-5% (Stockett etal. 2007, J. Phys. B 40, 4529) using laser-induced fluorescence (LIF) on7 even-parity and 63 odd-parity levels of Er II have been combined withnew branching fractions measured using a Fourier transform spectrometer(FTS) to determine transition probabilities for 418 lines of Er II. Thiswork moves Er II onto the growing list of rare-earth spectra withextensive and accurate modern transition probability measurements usingLIF plus FTS data. This improved laboratory data set has been used todetermine a new solar photospheric Er abundance,log?=0.96+/-0.03 (?=0.06 from 8 lines), a value inexcellent agreement with the recommended meteoritic abundance,log?=0.95+/-0.03. Revised Er abundances have also been derivedfor the r-process-rich metal-poor giant stars CS 22892-052, BD +17 3248,HD 221170, HD 115444, and CS 31082-001. For these five stars the averageEr/Eu abundance ratio, =0.42, is in very good agreement with thesolar-system r-process ratio. This study has further strengthened thefinding that r-process nucleosynthesis in the early Galaxy, whichenriched these metal-poor stars, yielded a very similar pattern to ther-process, which enriched later stars including the Sun.
| Detailed Abundances for 28 Metal-poor Stars: Stellar Relics in the Milky Way
We present the results of an abundance analysis for a sample of starswith -4<[Fe/H]<-2. The data were obtained with the HIRESspectrograph at Keck Observatory. The set includes 28 stars, witheffective temperature ranging from 4800 to 6600 K. For 13 stars with[Fe/H]<-2.6, including nine with [Fe/H]<-3.0 and one with[Fe/H]=-4.0, these are the first reported detailed abundances. For themost metal-poor star in our sample, CS 30336-049, we measure anabundance pattern that is very similar to stars in the range[Fe/H]~-3.5, including a normal C+N abundance. We also find that it hasvery low but measurable Sr and Ba, indicating some neutron-captureactivity even at this low of a metallicity. We explore this issuefurther by examining other very neutron capture-deficient stars and findthat, at the lowest levels, [Ba/Sr] exhibits the ratio of the mainr-process. We also report on a new r-process-enhanced star, CS31078-018. This star has [Fe/H]=-2.85, [Eu/Fe]=1.23, and [Ba/Eu]=-0.51.CS 31078-018 exhibits an ``actinide boost,'' i.e., much higher [Th/Eu]than expected and at a similar level to CS 31082-001. Our spectra allowus to further constrain the abundance scatter at low metallicities,which we then use to fit to the zero-metallicity Type II supernovayields of Heger & Woosley (2008). We find that supernovae withprogenitor masses between 10 and 20 Msolar provide the bestmatches to our abundances.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.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.
| NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
Aims. Aluminium is a key element to constrain the models of the chemicalenrichment and the yields of the first supernovae. But obtaining preciseAl abundances in extremely metal-poor (EMP) stars requires that thenon-LTE effects be carefully taken into account. Methods: The NLTEprofiles of the blue resonance aluminium lines have been computed in asample of 53 extremely metal-poor stars with a modified version of theprogram MULTI applied to an atomic model of the Al atom with 78 levelsof Al I and 13 levels of Al II, and compared to the observations. Results: With these new determinations, all the stars of the sample showa ratio Al/Fe close to the solar value: [Al/Fe] = -0.06±0.10 witha very small scatter. These results are compared to the models of thechemical evolution of the halo using different models of SN II and arecompatible with recent computations. The sodium-rich giants are notfound to be also aluminium-rich and thus, as expected, the convection inthese giants only brings to the surface the products of the Ne-Na cycle.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory, Chile (Large Programme “First Stars”,ID 165.N-0276(A); P.I.: R. Cayrel).
| Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars
We report the results for rotational velocities, Vrot sin i,and macroturbulence dispersions, ζRT, for 12 metal-poorfield red giant branch (RGB) stars and 7 metal-poor field red horizontalbranch (RHB) stars. The results are based on Fourier transform analysesof absorption line profiles from high-resolution (R ≈ 120,000),high-S/N (≈215 per pixel; ≈345 per resolution element) spectraobtained with the Gecko spectrograph at the Canada-France-HawaiiTelescope (CFHT). The stars were selected from the authors' previousstudies of 20 RHB and 116 RGB stars, based primarily onlarger-than-average line-broadening values. We find thatζRT values for the metal-poor RGB stars are very similarto those for metal-rich disk giants studied earlier by Gray and hiscollaborators. Six of the RGB stars have small rotational values, lessthan 2.0 km s-1, while five show significantrotation/enhanced line broadening, over 3 km s-1. We confirmthe rapid rotation rate for RHB star HD 195636, found earlier byPreston. This star's rotation is comparable to that of the fastest knownrotating blue horizontal branch (BHB) stars, when allowance is made fordifferences in radii and moments of inertia. The other six RHB starshave somewhat lower rotation but show a trend to higher values at highertemperatures (lower radii). Comparing our results with those for BHBstars from Kinman et al., we find that the fraction of rapidly rotatingRHB stars is somewhat lower than is found among BHB stars. The number ofrapidly rotating RHB stars is also smaller than we would have expectedfrom the observed rotation of the RGB stars. We devise two empiricalmethods to translate our earlier line-broadening results intoVrot sin i for all the RGB and RHB stars they studied.Binning the RGB stars by luminosity, we find that most metal-poor fieldRGB stars show no detectable sign, on average, of rotation, which is notsurprising given the stars' large radii. However, the most luminousstars, with MV <= -1.5, do show net rotation, with meanvalues of 2-4 km s-1, depending on the algorithm employed,and also show signs of radial velocity jitter and mass loss. This"rotation" may in fact prove to be due to other line-broadening effects,such as shock waves or pulsation.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique de France, and the University of Hawaii.
| Line Broadening in Field Metal-Poor Red Giant and Red Horizontal Branch Stars
We report 349 radial velocities for 45 metal-poor field red giant branch(RGB) and red horizontal branch (RHB) stars, with time coverage rangingfrom 1 to 21 years. We have identified one new spectroscopic binary, HD4306, and one possible such system, HD 184711. We also provide 57 radialvelocities for 11 of the 91 stars reported in our previous work. All butone of the 11 stars had been found to have variable radial velocities.New velocities for the long-period spectroscopic binaries BD-1 2582 andHD 108317 have extended the time coverage to 21.7 and 12.5 years,respectively, but in neither case have we yet completed a full orbitalperiod. As was found in the previous study, radial velocity "jitter" ispresent in many of the most luminous stars. Excluding stars showingspectroscopic binary orbital motion, all 7 of the red giants withestimated MV values more luminous than -2.0 display jitter,as well as 3 of the 14 stars with -2.0 < MV <= -1.4. Wehave also measured the line broadening in all the new spectra, usingsynthetic spectra as templates. Comparison with results fromhigh-resolution and higher signal-to-noise (S/N) spectra employed byother workers shows good agreement down to line-broadening levels of 3km s-1, well below our instrumental resolution of 8.5 kms-1. As the previous work demonstrated, the majority of themost luminous red giants show significant line broadening, as do many ofthe red horizontal branch stars, and we briefly discuss possible causes.The line broadening appears related to velocity jitter, in that bothappear primarily among the highest luminosity red giants.
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