Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Rubidium and Lead Abundances in Giant Stars of the Globular Clusters M13 and NGC 6752
We present measurements of the neutron-capture elements Rb and Pb infive giant stars of the globular cluster NGC 6752 and Pb measurements infour giants of the globular cluster M13. The abundances were derived bycomparing synthetic spectra with high-resolution, high signal-to-noiseratio spectra obtained using HDS on the Subaru telescope and MIKE on theMagellan telescope. The program stars span the range of the O-Alabundance variation. In NGC 6752, the mean abundances are[Rb/Fe]=-0.17+/-0.06 (σ=0.14), [Rb/Zr]=-0.12+/-0.06(σ=0.13), and [Pb/Fe]=-0.17+/-0.04 (σ=0.08). In M13 the meanabundance is [Pb/Fe]=-0.28+/-0.03 (σ=0.06). Within the measurementuncertainties, we find no evidence for star-to-star variation for eitherRb or Pb within these clusters. None of the abundance ratios [Rb/Fe],[Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 mayhave slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to thesmall sample of field stars at the same metallicity. For M13 and NGC6752 the Pb abundances are in accord with predictions from a Galacticchemical evolution model. If metal-poor intermediate-mass asymptoticgiant branch stars did produce the globular cluster abundance anomalies,then such stars do not synthesize significant quantities of Rb or Pb.Alternatively, if such stars do synthesize large amounts of Rb or Pb,then they are not responsible for the abundance anomalies seen inglobular clusters.Based in part on data collected at the Subaru Telescope, which isoperated by the National Astronomical Observatory of Japan, and onobservations made with the Magellan Clay Telescope at Las CampanasObservatory.
| Rubidium and lead abundances in globular clusters .
We present a brief and biased summary of key star-to-star abundancevariations recently observed in globular clusters and some possibleexplanations for these variations. Measurements of the neutron-captureelements rubidium (Rb) and lead (Pb) in the globular clusters M 13 andNGC 6752 are then presented along with preliminary measurements in M 4and M 5. The abundance ratios [Rb/Zr] and [Pb/Fe] are used to test theglobular cluster AGB pollution scenario and to gain insight into AGBnucleosynthesis.
| Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman
We develop and test a method for the estimation of metallicities([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhancedmetal-poor (CEMP) stars based on the application of artificial neuralnetworks, regressions, and synthesis models to medium-resolution (1-2Å) spectra and J-K colors. We calibrate this method by comparisonwith metallicities and carbon abundance determinations for 118 starswith available high-resolution analyses reported in the recentliterature. The neural network and regression approaches make use of apreviously defined set of line-strength indices quantifying the strengthof the Ca II K line and the CH G band, in conjunction with J-K colorsfrom the Two Micron All Sky Survey Point Source Catalog. The use ofnear-IR colors, as opposed to broadband B-V colors, is required becauseof the potentially large affect of strong molecular carbon bands onbluer color indices. We also explore the practicality of obtainingestimates of carbon abundances for metal-poor stars from the spectralinformation alone, i.e., without the additional information provided byphotometry, as many future samples of CEMP stars may lack such data. Wefind that although photometric information is required for theestimation of [Fe/H], it provides little improvement in our derivedestimates of [C/Fe], and hence, estimates of carbon-to-iron ratios basedsolely on line indices appear sufficiently accurate for most purposes.Although we find that the spectral synthesis approach yields the mostaccurate estimates of [C/Fe], in particular for the stars with thestrongest molecular bands, it is only marginally better than is obtainedfrom the line index approaches. Using these methods we are able toreproduce the previously measured [Fe/H] and [C/Fe] determinations withan accuracy of ~0.25 dex for stars in the metallicity interval-5.5<=[Fe/H]<=-1.0 and with 0.2<=(J-K)0<=0.8. Athigher metallicity, the Ca II K line begins to saturate, especially forthe cool stars in our program, and hence, this approach is not useful insome cases. As a first application, we estimate the abundances of [Fe/H]and [C/Fe] for the 56 stars identified as possibly carbon-rich, relativeto stars of similar metal abundance, in the sample of ``strong G-band''stars discussed by Beers, Preston, and Shectman.
| Improved Laboratory Transition Probabilities for Pt I and Application to the Platinum Abundances of BD +17°3248 and the Sun
Radiative lifetimes, accurate to +/-5%, have been measured for 58odd-parity levels of Pt I using laser-induced fluorescence. Thelifetimes were combined with branching fractions measured using gratingand Fourier transform spectrometry to determine transition probabilitiesfor 127 lines of Pt I. The new Pt lifetime measurements were found to bein good agreement with previous but less extensive measurements based onlaser-induced fluorescence. The new branching fraction measurements werefound to be in fair agreement with one earlier study. Absolute atomictransition probabilities from the new measurements were used todetermine the platinum abundance in the metal-poor Galactic halo star BD+17°3248. An attempt to refine the solar photospheric abundance ofplatinum was unsuccessful; the single Pt I line used in an earlierabundance determination was found to be even more severely blended thanexpected from earlier work.
| 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.
| The r-Process in Supernovae: Impact of New Microscopic Mass Formulae
The astrophysical origin of r-process nuclei is a long-standing mystery.Although some astrophysical scenarios show some promise, manyuncertainties involved in both the astrophysical conditions and in thenuclear properties far from the β-stability have inhibited us fromunderstanding the nature of the r-process. The purpose of the presentpaper is to examine the effects of the newly derived microscopicHartree-Fock-Bogoliubov (HFB) mass formulae on r-process nucleosynthesisand analyze to what extent a solar-like r-abundance distribution can beobtained. The r-process calculations with the HFB-2 mass formula areperformed, adopting the parameterized model of the prompt explosion froma collapsing O-Ne-Mg core for the physical conditions, and compared withthe results obtained with the HFB-7 and droplet-type mass formulae.Because of its weak shell effect at the neutron magic numbers in theneutron-rich region, the microscopic mass formulae (HFB-2 and HFB-7)give rise to a spread of the abundance distribution in the vicinity ofthe r-process peaks (A=130 and 195). While this effect resolves thelarge underproduction at A~115 and 140 obtained with droplet-type massformulae, large deviations compared to the solar pattern are found nearthe third r-process peak. It is shown that a solar-like r-processpattern can be obtained if the dynamical timescales of the outgoing masstrajectories are increased by a factor of about 2-3, or if theβ-decay rates are systematically increased by the same factor.
| First stars IV. CS 29497-030: Evidence for operation of the s-process at very low metallicity
We present an abundance analysis of the very metal-poor, carbon-enhancedstar CS 29497-030. Our results indicate that this unusually hot turnoffstar (Teff = 6650 K, log g = 3.5) has a metallicity [Fe/H] =-2.8, and exhibits large overabundances of carbon ([C/Fe] = +2.38),nitrogen ([N/Fe] = +1.88), and oxygen ([O/Fe] = +1.67). This star alsoexhibits a large enhancement in its neutron-capture elements; thepattern follows that expected to arise from the s-process. Inparticular, the Pb abundance is found to be very high with respect toiron ([Pb/Fe] = +3.5), and also with respect to the second peaks-process elements (e.g., Ba, La, Ce, Nd), which fits into the newlyintroduced classification of lead (Pb) stars. The known spectroscopicbinary status of this star, along with the observed s-process abundancepattern, suggest that it has accreted matter from a companion, whichformerly was an Asymptotic Giant-Branch (AGB) star. In a preliminaryanalysis, we have also identified broad absorption lines of metallicspecies that suggest a large axial rotational velocity for this star,which may be the result of spin-up associated with the accretion ofmaterial from its previous AGB companion. In addition, this star isclearly depleted in the light element Li. When considered along with itsrather high inferred temperature, these observations are consistent withthe expected properties of a very low metallicity halo blue straggler.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (program ID 165.N-0276(A)).Table \ref{tab6} is only available in electronic form athttp://www.edpsciences.org
| r-Process abundance universality and actinide cosmochronology
We review recent observational and theoretical results concerning thepresence of actinide nuclei on the surfaces of old halo stars and theiruse as an age determinant. We present model calculations which show thatthe observed universality of abundances for 56=75 and possibly Z<56 aswell. This introduces an uncertainty into the use of the Th/Euchronometer as a means to estimate the ages of the metal deficientstars. We do find, however, that the U/Th ratio is a robust chronometer.This is because the initial production ratio of U to Th is almostindependent of the astrophysical nucleosynthesis environment. Thelargest remaining uncertainties in the U/Th initial production ratio aredue to the input nuclear physics models.
| Comparing Deep Mixing in Globular Cluster and Halo Field Giants: Carbon Abundance Data from the Literature
The behavior of carbon abundance as a function of luminosity is used tocompare the rates of deep mixing within red giants of four globularclusters and the Galactic halo field population. Measurements of [C/Fe]for the clusters M92, NGC 6397, M3, and M13 have been compiled from theliterature, together with the Gratton et al. data for halo field stars.Plots of [C/Fe] versus absolute visual magnitude show that forMV<+1.6 the rate of decline of carbon abundance withincreasing luminosity on the red giant branch isd[C/Fe]/dMV~0.22+/-0.03 among the field stars, as well as inM92, NGC 6397, and M3. Among giants fainter than MV=+1.6 thevariation of [C/Fe] with absolute magnitude is much less. The dataindicate that the rate at which deep mixing introduces carbon-depletedmaterial into the convective envelopes of field halo stars during theupper red giant branch phase of evolution is similar to that of manyglobular cluster giants. The notable exception appears to be M13, inwhich stars exhibit deep mixing at a greater rate; this may account forthe high incidence of very low oxygen abundances among the most luminousgiants in M13 in comparison to M3.
| Improved oscillator strengths and wavelengths for Os I and Ir I, and new results on early r-process nucleosynthesis
The radioactive decay of 238U and 232Th hasrecently been used to determine ages for some of the oldest stars in theUniverse. This has highlighted the need for accurate observationalconstraints on production models for the heaviest r-process elementswhich might serve as stable references, notably osmium and iridium. Inorder to provide a firmer basis for the observed abundances, we haveperformed laser-induced fluorescence measurements and Fourier TransformSpectroscopy to determine new radiative lifetimes and branchingfractions for selected levels in Os I and Ir I. From these data, wedetermine new absolute oscillator strengths and improved wavelengths for18 Os I and 4 Ir I lines. A reanalysis of VLT spectra of CS 31082-001and new results for other stars with Os and Ir detections show that (i):the lines in the UV and lambda 4260 Å yield reliable Osabundances, while those at lambda lambda 4135, 4420 Å areheavily affected by blending; (ii): the Os and Ir abundances areidentical in all the stars; (iii): the heavy-element abundances in verymetal-poor stars conform closely to the scaled solar r-process patternthroughout the range 56 <= Z <= 77; and (iv): neither Os or Ir norany lighter species are suitable as reference elements for theradioactive decay of Th and U.Based in part on observations obtained with the Very Large Telescope ofthe European Southern Observatory at Paranal, Chile.
| Thorium and Uranium Chronometers Applied to CS 31082-001
We use the classical r-process model to explore the implications of therecently reported first observation of U in the extremely metal-poor,r-process element-enriched halo star CS 31082-001 for U and Thcosmochronometry. Using updated nuclear physics input and performing anew, conservative, analysis of the remaining uncertainties in theclassical r-process model, we confirm that U (together with Th)abundance observations in metal-poor stars are a promising tool fordating r-process events in the early Galaxy, independent of assumptionson Galactic chemical evolution. We show that nuclear physicsuncertainties limit the present accuracy of estimated U/Th ages to about2 Gyr. Critical nuclear data that are required to lower this uncertaintyinclude β-delayed fission branchings and reliable predictions ofthe onset of deformation in the vicinity of the N=184 shell closurearound 244Tl, as both directly affect predicted U/Th ratiosin r-process models. In this paper we apply, for the first time, the newHFBCS-1 mass model within the framework of the classical r-processmodel. We find that the predicted U and Th abundances are incompatiblewith the solar U and Th abundances and trace this back to a differentprediction of the onset of deformation around 244Tl. In thecase of CS 31082-001, we find it likely that the zero-age U and Thabundances were enhanced by about a factor of 2.5 compared to both (1) atheoretical extrapolation from the observed stable elements using theclassical r-process model and (2) the zero-age abundances of Th and U inother r-process-enhanced, metal-poor halo stars. Although presently adhoc, this ``actinide boost'' assumption solves the apparent problem ofthe relative age difference compared with other metal-poor halo starsand, at the same time, the problem of the inconsistency of ages based onU/(stable nucleus), Th/(stable nucleus) and U/Th ratios. There clearlyexist differences, among some r-process-enhanced, metal-poor stars, inthe level of the elemental abundances of actinides beyond the thirdr-process peak. Whether CS 31082-001 is a relatively rare case orcommonplace awaits the identification of larger numbers ofr-process-enhanced, metal-poor stars in which both U and Th can bemeasured. Using the U/Th ratio, we obtain a best age estimate for ther-process elements in CS 31082-001 of 15.5+/-3.2 Gyr. Futureobservations of Pb and Bi and a better determination of the r-processcontribution to solar Pb are needed to put the age estimates for thisand other stars on a more solid basis. For our most likely scenario, weprovide predictions of the expected upper and lower limits on theabundances of the elements Pb and Bi in CS 31082-001.
| A Statistical Model for Predicting the Average Abundance Patterns of Heavier Elements in Metal-Poor Stars
We have collected nearly all the available observed data of the elementsfrom Ba to Dy in halo and disk stars in the metallicity range-4.0<[Fe/H]<0.5. Based on the observed data of Ba and Eu, weevaluated the least-squares regressions of [Ba/Fe] on [Fe/H], and [Eu/H]on [Ba/H]. Assuming that the heavy elements (heavier than Ba) areproduced by a combination of the main components of s- and r-processesin metal-poor stars, and choosing Ba and Eu as respective representativeelements of the main s- and the main r-processes, a statistical modelfor predicting the Galactic chemical evolution of the heavy elements ispresented. With this model, we calculate the mean abundance trends ofthe heavy elements La, Ce, Pr, Nd, Sm, and Dy with the metallicity. Wecompare our results with the observed data at various metallicities,showing that the predicted trends are in good agreement with theobserved trends, at least for the metallicity range [Fe/H]>= -2.5.Finally, we discuss our results and deduce some important informationabout the Galactic chemical evolution.
| The Chemical Composition and Age of the Metal-poor Halo Star BD +17°3248
We have combined new high-resolution spectra obtained with the HubbleSpace Telescope (HST) and ground-based facilities to make acomprehensive new abundance analysis of the metal-poor, halo star BD+17°3248. We have detected the third r-process peak elements osmium,platinum, and (for the first time in a metal-poor star) gold, elementswhose abundances can only be reliably determined using HST. Ourobservations illustrate a pattern seen in other similar halo stars withthe abundances of the heavier neutron capture elements, including thethird r-process peak elements, consistent with a scaled solar systemr-process distribution. The abundances of the lighter neutron captureelements, including germanium and silver, fall below that same scaledsolar r-process curve, a result similar to that seen in theultra-metal-poor star CS 22892-052. A single site with two regimes orsets of conditions, or perhaps two different sites for the lighter andheavier neutron capture elements, might explain the abundance patternseen in this star. In addition, we have derived a reliable abundance forthe radioactive element thorium. We tentatively identify U II at 3859Å in the spectrum of BD +17°3248, which makes this the seconddetection of uranium in a very metal-poor halo star. Our combinedobservations cover the widest range in proton number (from germanium touranium) thus far of neutron capture elements in metal-poor Galactichalo stars. Employing the thorium and uranium abundances in comparisonwith each other and with several stable elements, we determine anaverage cosmochronological age for BD +17°3248 of 13.8+/-4 Gyr,consistent with that found for other similar metal-poor halo stars.Based on observations made at three facilities: (1) the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute(STScI), which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555 (2) theKeck I Telescope of the W. M. Keck Observatory, which is operated by theCalifornia Association for Research in Astronomy (CARA, Inc.) on behalfof the University of California and the California Institute ofTechnology; and (3) the H. J. Smith Telescope of McDonald Observatory,which is operated by the University of Texas at Austin.
| SN 1987A Revisited as a Major Production Site for r-Process Elements
The origin of nucleosynthesis products of rapid neutron capturereactions (the r-process) is a long-standing astrophysical problem.Recent analyses of elemental abundances for extremely metal-poor starsshed light on the elemental abundances of individual supernovae.Comparison of the abundance distributions of some extremely metal-poorstars with those of the best-observed supernova SN 1987A clearlyindicates that the overabundances of barium and strontium found in SN1987A that have been ascribed to the slow neutron capture process mustbe results of r-process nucleosynthesis. The mass of freshly synthesizedbarium in SN 1987A is estimated to be 6×10-6Msolar based on the observed surface abundance and detailedhydrodynamical models for this supernova. These new findings lead to theconclusion that 20 Msolar stars, one of which is theprogenitor star of SN 1987A, are the predominant production sites forr-process elements in the Galaxy and the r-process element donors fornotable neutron capture-rich giant stars, CS 22892-052 and CS 31082-001.
| A Model for Abundances in Metal-poor Stars
A model is presented that seeks to explain quantitatively the stellarabundances of r-process elements and other elements associated with ther-process sites. It is argued that the abundances of all these elementsin stars with -3<~[Fe/H]<-1 can be explained by the contributionsof three sources. The sources are the first generations of very massive(>~100 Msolar) stars that are formed from big bang debrisand are distinct from Type II supernovae (SNe II) and two types of SNeII, the H and L events, which can occur only at [Fe/H]>~-3. The Hevents are of high frequency and produce dominantly heavy (A>130)r-elements but no Fe (presumably leaving behind black holes). The Levents are of low frequency and produce Fe and dominantly light(A<~130) r-elements (essentially none above Ba). By using theobserved abundances in two ultra-metal-poor stars and the solarr-abundances, the initial or prompt inventory of elements produced bythe first generations of very massive stars and the yields of H and Levents can be determined. The abundances of a large number of elementsin a star can then be calculated from the model by using only theobserved Eu and Fe abundances. To match the model results and theobservational data for stars with -3<[Fe/H]<-1 requires that thesolar r-abundances for Sr, Y, Zr, and Ba must be significantly increasedfrom the standard values. No such changes appear to be required for allother elements. If the changes in the solar r-abundances for Sr, Y, Zr,and Ba are not permitted, the model fails at -3<[Fe/H]<-1 butstill works at [Fe/H]~-3 for these four elements. By using the correctedsolar r-abundances for these elements, good agreement is obtainedbetween the model results and data over the range -3<[Fe/H]<-1. Noevidence of s-process contributions is found in this region, but all theobservational data in this region now show regular increases of Ba/Euabove the standard solar r-process value. Whether the solar r-componentsof Sr, Y, Zr, and Ba used here to obtain a fit to the stellar data canbe reconciled with those obtained from solar abundances by subtractingthe s-components calculated from models is not clear.
| Discovery of three lead-rich stars
About half of the stable nuclei heavier than iron are believed to besynthesized during the late stages of evolution of stars with masses inthe range 0.8-8 solar masses. These elements are then expelled into theinterstellar medium through stellar winds after being `dredged up'towards the surface of the stars. These processes occur when the star isin the `asymptotic giant branch' (AGB) phase of its life. Nuclei (mainlyiron) deep inside the star slowly capture neutrons and progressivelybuild up heavier elements (the `s-process'). For AGB stars that formedearly in the history of the Galaxy, and that therefore have very lowabundances of elements heavier than helium (`metals'), models predictthat the s-process will accumulate synthesized material with atomicweights in the Pb-Bi region. Such stars will therefore have largeoverabundances of lead relative to other heavy elements. Here we reportthe discovery of large amounts of lead in three metal-poor stars(HD187861, HD196944 and HD224959). Our analysis shows that these starsare more enriched in lead than in any other element heavier than iron.The excellent agreement between the observed and predicted abundancesreinforces our current understanding of the detailed operation of thes-process deep in the interiors of AGB stars.
| Lead: Asymptotic Giant Branch Production and Galactic Chemical Evolution
The enrichment of Pb in the Galaxy is followed in the framework of adetailed model of Galactic chemical evolution that already provedadequate to reproduce the chemical enrichment of O and of the elementsfrom Ba to Eu. The stellar yields are computed through nucleosynthesiscalculations in the asymptotic giant branch (AGB) phase of low- andintermediate-mass stars covering a wide range of metallicities. Thephysical parameters of the stellar structure were derived from fullstellar evolutionary models computed previously. We show that low-massAGB stars are the main producers of Pb in the Galaxy, with a complexdependence on metallicity and a maximum efficiency at [Fe/H]~-1. Ourcalculations succeed in reproducing the abundances of Pb isotopes in thesolar system: the role attributed by the classical analysis of thes-process to the strong component, in order to explain more than 50% ofsolar 208Pb, is actually played by the high production of Pbin low-mass and low-metallicity AGB stars. We then follow the Galacticchemical evolution of Pb isotopes and give our expectations on thes-process contribution to each of them at the epoch of the solar systemformation. Finally, we present new spectroscopic estimates of Pbabundance on a sample of field stars and compare them, together with afew other determinations available, with the predicted trend of [Pb/Fe]in the Galaxy.
| Neutron-Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal-poor Giants
New abundances for neutron-capture (n-capture) elements in a largesample of metal-poor giants from the Bond survey are presented. Thespectra were acquired with the KPNO 4 m echelle and coudé feedspectrographs, and have been analyzed using LTE fine-analysis techniqueswith both line analysis and spectral synthesis. Abundances of eightn-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, and Dy) in 43 stars havebeen derived from blue (λλ4070-4710, R~20,000, S/Nratio~100-200) echelle spectra and red (λλ6100-6180,R~22,000, S/N ratio~100-200) coudé spectra, and the abundance ofBa only has been derived from the red spectra for an additional 27stars. Overall, the abundances show clear evidence for a largestar-to-star dispersion in the heavy element-to-iron ratios. Thiscondition must have arisen from individual nucleosynthetic events inrapidly evolving halo progenitors that injected newly manufacturedn-capture elements into an inhomogeneous early Galactic halointerstellar medium. The new data also confirm that at metallicities[Fe/H]<~-2.4, the abundance pattern of the heavy (Z>=56) n-captureelements in most giants is well-matched to a scaled solar systemr-process nucleosynthesis pattern. The onset of the main r-process canbe seen at [Fe/H]~-2.9 this onset is consistent with the suggestion thatlow mass Type II supernovae are responsible for the r-process.Contributions from the s-process can first be seen in some stars withmetallicities as low as [Fe/H]~-2.75 and are present in most stars withmetallicities [Fe/H]>-2.3. The appearance of s-process contributionsas metallicity increases presumably reflects the longer stellarevolutionary timescale of the (low-mass) s-process nucleosynthesissites. The lighter n-capture elements (Sr-Y-Zr) are enhanced relative tothe heavier r-process element abundances. Their production cannot beattributed solely to any combination of the solar system r- and mains-processes, but requires a mixture of material from the r-process andfrom an additional n-capture process that can operate at early Galactictime. This additional process could be the weak s-process in massive(~25 Msolar) stars, or perhaps a second r-process site, i.e.,different from the site that produces the heavier (Z>=56) n-captureelements.
| Galactic [O/Fe] and [C/Fe] Ratios: The Influence of New Stellar Parameters
We consider the effects of recent NLTE gravities and Fe abundances onstellar [O/Fe] and [C/Fe] ratios. The NLTE parameters greatly reduce oreliminate the well-known discrepancy between CH- and C I-based Cabundances in metal-poor stars and previously seen trends ofatomic-based [C/Fe] and [O/Fe] with Teff. With the NLTEparameters, the metal-poor molecular-based [C/Fe] ratio maintains itsincrease with declining [Fe/H] this may also be demonstrated by therevised atomic-based ratios. [O/Fe] values derived from OH and O Ifeatures are considerably reduced and typically show improved agreementbut are 0.1-0.2 dex larger than those exhibited by the Lick-Texassyndicate's recent [O I]-based giant determinations. The revised [O/Fe]ratios still show an increase down to at least [Fe/H]~-2 we suggest thatrecent field giant data show an increase with similar slope. Evenadopting uniform NLTE parameters, study-to-study abundance differencescan be significant; moreover, different NLTE studies yield differinggravities and Fe abundances even after taking Teffdifferences into account. Comparison of metal-poor giant gravities andcluster abundances with isochrones, trigonometric gravities, andnear-turnoff cluster abundances yields conflicting indications aboutwhether the evolved gravities might be underestimated as suggested formetal-poor dwarfs. Regardless, we argue that even extreme gravityrevisions do not affect the [O/Fe]-[Fe/H] relation derived from theextant results. Combining what we believe the most reliable giant anddwarf data considered here, we find[O/Fe]=-0.184(+/-0.022)×[Fe/H]+0.019 with an rms scatter of only0.13 dex; there is no indication of a break or slope change atintermediate [Fe/H]. The gentle slope is in very reasonable agreementwith some chemical evolution models employing yields with small mass andmetallicity dependences. Finally, two notes are made concerning Naabundance-spatial position and element-to-element correlations in M13giants.
| Detection of Lead in the Carbon-rich, Very Metal-poor Star LP 625-44: A Strong Constraint on S-Process Nucleosynthesis at Low Metallicity
We report the detection of the Pb I λ4057.8 line in the verymetal-poor ([Fe/H]=-2.7), carbon-rich star, LP 625-44. We determine theabundance of Pb ([Pb/Fe]=2.65) and 15 other neutron-capture elements.The abundance pattern between Ba and Pb agrees well with a scaled solarsystem s-process component, while the lighter elements (Sr-Zr) are lessabundant than Ba. The enhancement of s-process elements is interpretedas a result of mass transfer in a binary system from a previousasymptotic giant branch (AGB) companion, an interpretation stronglysupported by radial velocity variations of this system. The detection ofPb makes it possible, for the first time, to compare model predictionsof s-process nucleosynthesis in AGB stars with observations of elementsbetween Sr and Pb. The Pb abundance is significantly lower than theprediction of recent models (e.g., Gallino et al.), which succeeded inexplaining the metallicity dependence of the abundance ratios of lights-elements (Sr-Zr) to heavy ones (Ba-Dy) found in previously observeds-process-enhanced stars. This suggests that one should either (1)reconsider the underlying assumptions concerning the 13C-richs-processing site (13C pocket) in the present models or (2)investigate alternative sites of s-process nucleosynthesis in verymetal-poor AGB stars.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Abundances of light elements in metal-poor stars. III. Data analysis and results
We present the results of the analysis of an extensive set of new andliterature high quality data concerning Fe, C, N, O, Na, and Mg. Thisanalysis exploited the T_eff scale determined in Gratton et al. (1996a),and the non-LTE abundance corrections computed in Gratton et al.(1999a). Results obtained with various abundance indices are discussedand compared. Detailed comparison with models of galactic chemicalevolution will be presented in future papers of this series. Our non-LTEanalysis yields the same O abundances from both permitted and forbiddenlines for stars with T_eff >4600 K, in agreement with King (1993),but not with other studies using a lower T_eff -scale for subdwarfs.However, we obtain slightly smaller O abundances for the most luminousmetal-poor field stars than for fainter stars of similar metallicities,an effect attributed to inadequacies of the adopted model atmospheres(Kurucz 1992, with overshooting) for cool stars. We find a nearlyconstant O overundance in metal-poor stars ([Fe/H]<-0.8), at a meanvalue of 0.46+/- 0.02 dex (sigma =0.12, 32 stars), with only a gentleslope with [Fe/H] ( ~ -0.1); this result is different from the steeperslope recently obtained using OH band in the near UV. If only bonafideunmixed stars are considered, C abundances scale with Fe ones (i.e.[C/Fe]~ 0) down to [Fe/H] ~ -2.5. Due to our adoption of a differentT_eff scale, we do not confirm the slight C excess in the most metalpoor disk dwarfs (-0.8<[Fe/H]<-0.4) found in previousinvestigations. Na abundances scale as Fe ones in the high metallicityregime, while metal-poor stars present a Na underabundance. None of thefield stars analyzed belong to the group of O-poor and Na-rich starsobserved in globular clusters. Na is deficient with respect to Mg inhalo and thick disk stars; within these populations, Na deficiency maybe a slow function of [Mg/H]. Solar [Na/Mg] ratios are obtained for thindisk stars. Tables~ 2 to 9 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strabg.fr/Abstract.html
| Revised Strömgren metallicity calibration for red giants
A new calibration of the Strömgren (b-y),m_1 diagram in terms ofiron abundance of red giants is presented. This calibration is based ona homogeneous sample of giants in the globular clusters omega Centauri,M 22, and M 55 as well as field giants from the list of Anthony-Twarog& Twarog (\cite{anth98}). Towards high metallicities, the newcalibration is connected to a previous calibration by Grebel &Richtler (\cite{greb92}), which was unsatisfactory for iron abudanceslower than -1.0 dex. The revised calibration is valid for CN-weak/normalred giants in the abundance range of -2.0 <[Fe/H]< 0.0 dex, and acolor range of 0.5 < (b-y) < 1.1 mag. As shown for red giants inomega Centauri, CN-weak stars with Strömgren metallicities higherthan -1.0 dex cannot be distinguished in the (b-y),m_1 diagram fromstars with lower iron abundances but higher CN band strengths. Based ondata collected at the European Southern Observatory, La Silla, Chile
| Mixing along the red giant branch in metal-poor field stars
We have determined Li, C, N, O, Na, and Fe abundances, and12C/13C isotopic ratios for a sample of 62 fieldmetal-poor stars in the metallicity range -2<=[Fe/H]<= -1. Starswere selected in order to have accurate luminosity estimates from theliterature, so that evolutionary phases could be clearly determined foreach star. We further enlarged this dataset by adding 43 more starshaving accurate abundances for some of these elements and similarly welldefined luminosities from the literature. This large sample was used toshow that (small mass) lower-RGB stars (i.e. stars brighter than thefirst dredge-up luminosity and fainter than that of the RGB bump) haveabundances of light elements in agreement with predictions fromclassical evolutionary models: only marginal changes occur for CNOelements, while dilution within the convective envelope causes thesurface Li abundance to decrease by a factor of ~ 20. A second, distinctmixing episode occurs in most (perhaps all) small mass metal-poor starsjust after the RGB bump, when the molecular weight barrier left by themaximum inward penetration of the convective shell is canceled by theoutward expansion of the H-burning shell, in agreement with recenttheoretical predictions. In field stars, this second mixing episode onlyreaches regions of incomplete CNO burning: it causes a depletion of thesurface 12C abundance by about a factor of 2.5, and acorresponding increase in the N abundance by about a factor of 4. The12C/13C is lowered to about 6 to 10 (close to butdistinctly higher than the equilibrium value of 3.5), while practicallyall remaining Li is burnt. However an O-Na anti-correlation such astypically observed amongst globular cluster stars, is not present infield stars. None of the 29 field stars more evolved than the RGB bump(including 8 RHB stars) shows any sign of an O depletion or Naenhancement. This means that the second mixing episode is not deepenough to reach regions were ON-burning occurs in field stars. Based inpart on observations made at the ESO La Silla ObservatoryTables 1, 2, 3, 5 and 6 are available in electronic form only at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Prompt Iron Enrichment, Two r-Process Components, and Abundances in Very Metal-Poor Stars
We present a model to explain the wide range of abundances for heavyr-process elements (mass number A>130) at low [Fe/H]. This modelrequires rapid star formation and/or an initial population ofsupermassive stars in the earliest condensed clots of matter in order toprovide a prompt or initial Fe inventory. Subsequent Fe and r-processenrichment was provided by two types of supernovae: one producing heavyr-elements with no Fe on a rather short timescale and the otherproducing light r-elements (A<=130) with Fe on a much longertimescale.
| Stellar abundances in the early Galaxy and two r-process components.
Electronic Article Available from Elsevier Science.
| The abundances of neutron-capture elements in metal-poor stars.
Not Available
| R-Process Abundances and Chronometers in Metal-poor Stars
Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations,employing internally consistent and physically realistic nuclear physicsinput (quasi-particle random-phase approximation [QRPA] beta-decayproperties and the recent extended Thomas-Fermi with Strutinsky integraland quenching (ETFSI-Q) nuclear mass model), have been performed. Thesetheoretical computations assume the classical waiting-pointapproximation of (n,gamma)⇄(gamma,n) equilibrium. Thecalculations reproduce the solar isotopic r-abundances in detail,including the heaviest stable Pb and Bi isotopes. These calculations arethen compared with ground-based and Hubble Space Telescope observationsof neutron-capture elements in the metal-poor halo stars CS 22892-052,HD 115444, HD 122563, and HD 126238. The elemental abundances in allfour metal-poor stars are consistent with the solar r-process elementaldistribution for the elements Z>=56. These results strongly suggest,at least for those elements, that the relative elemental r-processabundances have not changed over the history of the Galaxy. Thisindicates also that it is unlikely that the solar r-process abundancesresulted from a random superposition of varying abundance patterns fromdifferent r-process nucleosynthesis sites. This further suggests thatthere is one r-process site in the Galaxy, at least for elementsZ>=56. Employing the observed stellar abundances of stable elements,in conjunction with the solar r-process abundances to constrain thecalculations, we present predictions for the zero decay-age abundancesof the radioactive elements Th and U. We compare these predictions(obtained with the mass model ETFSI-Q, which reproduces solarr-abundances best) with newly derived observational values in three verymetal-poor halo stars: HD 115444, CS 22892-052, and HD 122563. Withinthe observational errors the ratio of [Th/Eu] is the same in both CS22892-052 and HD 115444. Comparing with the theoretical ratio suggestsan average age of these two very metal-poor stars to be 15.6+/-4.6 Gyr,consistent with earlier radioactive age estimates and recent globularand cosmological age estimates. Our upper limit on the uranium abundancein HD 115444 also implies a similar age. Such radioactive agedeterminations of very low metallicity stars avoid uncertainties inGalactic chemical evolution models. They still include uncertainties dueto the involved nuclear physics far from beta-stability. However, wegive an extensive overview of the possible variations expected and cometo the conclusion that this aspect alone should not exceed limits of 3Gyr. Therefore this method shows promise as an independent datingtechnique for the Galaxy.
| Abundance Ratios in Extreme Metal-Poor Stars
In extremely metal-poor stars ([Fe/H]≤ ‑ 2.5) the neutroncapture elementsare characterized by a 300-fold dispersion in M/Feratios which decreases with increasing metallicity, the median M/Feratio increases with increasing [Fe/H], but the averageM/Fe number ratiois approximately constant. These observations are consistent withahighly dispersed intrinsic yield of neutron-capture elements insupernova (SN) events,and a progression to increasing metallicity bystochastic chemical evolution.The abundance trends indicate that thesynthesis of elements heavier thanbarium was dominated by the r-process.The Sr/Ba ratio shows a dispersionwhich suggests a stochastic source ofSr in excess of the r-process value;possibly due to the alpha-richfreeze out.The iron-peak elements Cr, Mn, and Co show non-solarabundance ratios forextreme metal-poor stars, and no measurableintrinsicdispersion relative to iron. We discuss chemical evolution models whichexplain these observations.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Lobo |
| Ascensión Recta: | 14h25m29.96s |
| Declinación: | -43°38'36.9" |
| Magnitud Aparente: | 7.689 |
| Distancia: | 262.467 parsecs |
| Movimiento Propio en Ascensión Recta: | 27 |
| Movimiento Propio en Declinación: | -103.8 |
| B-T magnitude: | 8.665 |
| V-T magnitude: | 7.77 |
Catálogos y designaciones:
|
