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Investigation of the single neutron exposure model for the s-process: the primary nature of the neutron source
The primary nature of the 13C neutron source is verysignificant for the studies of the s-process nucleosynthesis. In thispaper we present an attempt to fit the element abundances observed in 16s-rich stars using parametric model of the single neutron exposure. Thecalculated results indicate that almost all s-elements were made in asingle neutron exposure for nine sample stars. Although a large spreadof neutron exposure is obtained, the maximum value of the neutronexposure will reach about 7.0 mbarn-1, which is close to thetheoretical predictions by the asymptotic giant branch (AGB) model. Thecalculated result is a significant evidence for the primary nature ofthe neutron source. Combining the result obtained in this work and theneutron exposure-initial mass relations, a large spread of neutronexposure can be explained by the different initial stellar mass andtheir time evolution. The possibility that the rotationally inducedmixing process can lead to a spread of the neutron exposure in AGB starsis also existent.
| Carbon-enhanced Metal-poor Stars. I. Chemical Compositions of 26 Stars
The chemical compositions of 26 metal-poor stars that exhibit strong CHand/or C2 molecular bands are determined. Twenty-two stars inour sample satisfy our definition for carbon-enhanced metal-poor (CEMP)stars based on the carbon abundance ratio ([C/Fe]) and the evolutionarystatus. In addition, we measure Na abundances for nine knowncarbon-enhanced stars. Combining our new sample with the results ofprevious work, we investigate the abundance and evolutionary status of atotal of 64 CEMP stars. The following results are obtained: (1) All butone of the 37 stars with [Fe/H]>=-2.6 exhibit large excesses ofbarium ([Ba/Fe]>+0.5), while the other 27 stars with lowermetallicity exhibit a large scatter in their barium abundance ratios(-1.2<[Ba/Fe]<+3.3). (2) A correlation between the carbon andbarium abundance ratios ([C/Fe] and [Ba/Fe]) is found in Ba-enhancedobjects (comprising 54 stars), suggesting that the origin of theobserved carbon excess in Ba-enhanced stars is nucleosynthesis in AGBstars, where the main s-process occurs. (3) The majority of theBa-enhanced stars have -1.0<[C/H]<0.0, and a clear cutoff existsat [C/H]~0, which we take as the limit of carbon enrichment bymetal-poor AGB stars. The [C/H] values of Ba-normal stars are relativelylow, with a wide distribution. (4) The difference in the distributionsof evolutionary status between Ba-enhanced and Ba-normal CEMP starssuggested by our previous work is not statistically confirmed by thepresent, enlarged sample. (5) Excesses of Na are found in stars withextremely large enhancements of C, N, and Ba, suggesting efficientproduction of this element by AGB nucleosynthesis. The implications ofthese results on the origins of carbon in CEMP stars are discussed.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| The origin of the lead-rich stars in the Galactic halo: investigation of model parameters for the s-process
Several stars at the low-metallicity extreme of the Galactic halo showlarge spreads of lead and associated `heavy' s-process elements([Pb/hs]). Theoretically, an s-process pattern should be obtained froman AGB star with a fixed metallicity and initial mass. For the thirddredge-up and the s-process model, several important properties dependprimarily on the core mass of AGB stars. Zijlstra reported that theinitial-to-final mass relation steepens at low metallicity, due to lowmass-loss efficiency. This might affect the model parameters of the AGBstars, e.g. the overlap factor and the neutron irradiation time, inparticular at low metallicity. The calculated results do indeed showthat the overlap factor and the neutron irradiation time aresignificantly small at low metallicities, especially for3.0Msolar AGB stars. The scatter of [Pb/hs] found in lowmetallicities can therefore be explained naturally when varying theinitial mass of the low-mass AGB stars.
| Neutron-Capture Elements in the s- and r-Process-rich Stars: Constraints on Neutron-Capture Nucleosynthesis Processes
The chemical abundances of the very metal-poor double-enhanced stars areexcellent information for setting new constraints on models ofneutron-capture processes at low metallicity. These stars are known as s+ r stars, since they show enhancements of both s-process and r-processelements. The observed abundance ratios for the double-enhanced starscan be explained by those of stars that were polluted by an AGB star andsubsequently accreted very significant amounts of r-process material outof an AIC (accretion-induced collapse) or Type 1.5 supernova. In thispaper we present for the first time an attempt to fit the elementalabundances observed in the s- and r-rich, very metal-poor stars using aparametric model and suggest a new concept of component coefficients todescribe the contributions of the individual neutron-capture processesto double-enhanced stars. We find that the abundance ratios of thesestars are best fitted by enrichments of s- and r-process material. Theoverlap factor in the AGB stars where the observed s-process elementswere produced lies between 0.1 and 0.81. Taking into account thedependence of the initial-final mass relations on metallicity, this widerange of values could possibly be explained by a wide range of core-massvalues of AGB stars at low metallicity. The component coefficient of ther-process is strongly correlated with the component coefficient of thes-process for the double-enhanced stars. This is significant evidencethat the r-process material in double-enhanced stars comes from an AICor Type 1.5 supernova.
| The Hamburg/ESO R-process enhanced star survey (HERES). III. HE 0338-3945 and the formation of the r + s stars
We have derived abundances of 33 elements and upper limits for 6additional elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE0338-3945 from high-quality VLT-UVES spectra. The star is heavilyenriched, by about a factor of 100 relative to iron and the Sun, in theheavy s-elements (Ba, La, ...). It is also heavily enriched in Eu, whichis generally considered an r-element, and in other similar elements. Itis less enriched, by about a factor of 10, in the lighter s-elements(Sr, Y and Zr). C is also strongly enhanced and, to a somewhat lesserdegree, N and O. These abundance estimates are subject to severeuncertainties due to NLTE and thermal inhomogeneities which are nottaken into detailed consideration. However, an interesting result, whichis most probably robust in spite of these uncertainties, emerges: theabundances derived for this star are very similar to those of otherstars with an overall enhancement of all elements beyond the iron peak.We have defined criteria for this class of stars, r+s stars, and discussnine different scenarios to explain their origin. None of theseexplanations is found to be entirely convincing. The most plausiblehypotheses involve a binary system in which the primary component goesthrough its giant branch and asymptotic giant branch phases and producesCNO and s-elements which are dumped onto the observed star. Whether ther-element Eu is produced by supernovae before the star was formed(perhaps triggering the formation of a low-mass binary), by a companionas it explodes as a supernova (possibly triggered by mass transfer), orwhether it is possibly produced in a high-neutron-density version of thes-process is still unclear. Several suggestions are made on how toclarify this situation.
| s-Process in low metallicity Pb stars.
We consider a sample of very metal-poor, C-rich, s-rich and lead-richstars observed at high-resolution spectroscopy, and some recentspectroscopic data of C+s-rich stars obtained at moderate resolution.The spectroscopic data of these stars are interpreted with AGBtheoretical models of different 13C-pocket efficiencies,initial mass and initial r-enrichment. When lead is not measured we giveour theoretical prediction. The observed stars are not on the AGB phase,but are main sequence or giant stars. They acquired the C and senrichments by mass transfer in a close binary system from the moremassive companion while on the AGB (now a white dwarf). A considerablefraction of the stars show both high s and r enrichments. To explain thes+r enriched stars we assume a parental cloud already enriched inr-elements. The measurement of Nb is an indicator of an extrinsic AGB ina binary system. The intrinsic indicator [hs/ls] constrains the initialmass, while [Pb/hs] and [Pb/ls] are a measure of the s-processefficiency. The apparent discrepancies of C and N abundances may bereconciled by assuming a strong cool bottom process occurring during theAGB. An important primary production of light elements, from Ne to Si,increasing with the star mass, is predicted for AGB models at very lowmetallicity, induced by n capture on primary 22Ne and itsprogenies.
| The Origins of Two Classes of Carbon-enhanced, Metal-poor Stars
We have compiled composition, luminosity, and binarity information forcarbon-enhanced, metal-poor (CEMP) stars reported by recent studies. Wedivided the CEMP star sample into two classes having high and lowabundances, respectively, of the s-process elements and consider theabundances of several isotopes, in particular, 12C,13C, and 14N, as well as the likely evolutionarystages of each star. Despite the fact that objects in both groups wereselected from the same surveys (primarily the HK survey), without apriori knowledge of their s-process element abundances, we identify thefollowing remarkable differences between the two classes: s-element-richCEMP (CEMP-s) stars occupy a wide range of evolutionary states, but donot have a strongly evolved 13C/14N ratio, whereass-element-normal CEMP stars (CEMP-no) are found only high up thefirst-ascent giant branch and possess 13C/14Nratios approaching the CN cycle equilibrium value. We argue that theseobservational constraints can be accommodated by the followingscenarios. CEMP-s stars acquire their distinctive surface compositionsduring their lifetimes when mass is transferred from an AGB companionthat has recently synthesized 12C and s-process elements.Such mass-accreting stars can be enriched at almost any stage of theirevolution and hence are found throughout the H-R diagram. Dilution oftransferred surface material as the accretor ascends the giant branchand its surface convective zone deepens may reduce the number of suchstars, whose surfaces remain C-rich at high luminosities. Many, but notnecessarily all, such stars should currently be in binary systems.Li-preserving CEMP-s stars may require a different explanation. Incontrast, a CEMP-no star is proposed to have formed from gas that wasenriched in 12C from the triple-α process in a previousgeneration of stars, some of which has been converted to 13Cand 14N during the present star's giant branch evolution. Thebinary fraction of such stars should be the same as that ofnon-carbon-enhanced, metal-poor stars.
| 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.
| A detailed spectroscopy of the carbon-rich star BD +57°2161
An LTE abundance analysis based on high-resolution spectra is presentedfor the carbon-rich star BD +57°2161, whose evolutionary status isunknown. With [C/Fe] =+0.4 dex and a mean s-process overabundance of[s/Fe] ≃ +1.5 dex the peculiar atmospheric composition of BD+57°2161 is confirmed. The 12C/13C abundanceratio was found to be about 10. The mild iron deficiency, [Fe/H] = -0.2,supports the idea that BD +57°2161 could be an old-disk-populationobject. Radial-velocity measurements confirm the binary nature of thestar. Therefore the peculiar chemical composition could be due to themass transfer from the secondary - AGB star in the past. Orbitalparameters are estimated for another star of this group, BD +75°348.
| Chemical abundances in 43 metal-poor stars
We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe,Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood,most of them subgiants or turn-off-point stars with iron abundances[Fe/H] ranging from -0.4 to -3.0. About half of this sample has not beenspectroscopically analysed in detail before. Effective temperatures wereestimated from uvby photometry, and surface gravities primarily fromHipparcos parallaxes. The analysis is differential relative to the Sun,and was carried out with plane-parallel MARCS models. Various sources oferror are discussed and found to contribute a total error of about0.1-0.2 dex for most elements, while relative abundances, such as[Ca/Fe], are most probably more accurate. For the oxygen abundances,determined in an NLTE analysis of the 7774 Å triplet lines, theerrors may be somewhat larger. We made a detailed comparison withsimilar studies and traced the reasons for the, in most cases,relatively small differences. Among the results we find that [O/Fe]possibly increases beyond [Fe/H] = -1.0, though considerably less sothan in results obtained by others from abundances based on OH lines. Wedid not trace any tendency toward strong overionization of iron, andfind the excesses, relative to Fe and the Sun, of the α elementsMg, Si, and Ca to be smaller than those of O. We discuss someindications that also the abundances of different α elementsrelative to Fe vary and the possibility that some of the scatter aroundthe trends in abundances relative to iron may be real. This may supportthe idea that the formation of Halo stars occurred in smaller systemswith different star formation rates. We verify the finding by Gratton etal. (2003b, A&A, 406, 131) that stars that do not participate in therotation of the galactic disk show a lower mean and larger spread in [α/Fe] than stars participating in the general rotation. The latterstars also seem to show some correlation between [ α/Fe] androtation speed. We trace some stars with peculiar abundances, amongthese two Ba stars, HD 17072 and HD196944, the second already known to be rich in s elements.Finally we advocate that a spectroscopic study of a larger sample ofhalo stars with well-defined selection criteria is very important, inorder to add to the very considerable efforts that various groups havealready made.
| The Binary Frequency Among Carbon-enhanced, s-Process-rich, Metal-poor Stars
We discuss radial velocities for a sample of carbon-enhanced,s-process-rich, very metal-poor (CEMP-s) stars, analyzed withhigh-resolution spectroscopy obtained over multiple epochs. We find that~68% of the stars in the sample show evidence of radial velocityvariations. The expected detection fraction for these stars, adoptingthe measured binary fraction in the field (~60%) and assuming that theyshare the same period and eccentricity distribution, is ~22%. Even ifone assumes that the true binary fraction of these stars is 100%, theexpected detection percentage is ~36%. These values indicate that thebinary fraction among CEMP-s stars is higher than the field binaryfraction, suggesting that all of these objects are in double (ormultiple) systems. The fact that the observed frequency of velocityvariation exceeds the expected detection fraction in the case of anassumed binary fraction of 100% is likely due to a more restricteddistribution of orbital periods for these objects, as compared to normalfield binaries. Our results indicate that CEMP-s stars are themetal-poor analogs of classical CH stars.Based in part on observations collected at the European SouthernObservatory, Paranal, Chile (ESO Programme 167.D-0173).INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5,I-35122 Padova, Italy.
| New analysis of the two carbon-rich stars CS 22948-27 and CS 29497-34: Binarity and neutron capture elements
We have carried out a new determination of abundances in the verymetal-poor CH/CN strong stars CS 22948-27 and CS 29497-34, usinghigh-resolution spectra obtained with the HARPS spectrograph at the 3.6m telescope of ESO, La Silla, that covers the range λλ4000-6900 Å at a resolution of R = 100 000. Both stars are foundto be long period binaries. It is confirmed that the abundance patternsshow an enhancement of the α-elements (like Mg, Ca), of the protoncapture elements (like Na and Al) and a strong enrichment in ``r'' and``s'' process elements, where the s-enrichment is probably due to a masstransfer episode from a companion in its AGB phase. The possible originsof the abundance pattern and especially of the strong enhancement ofboth ``s'' and ``r'' elements are discussed.Observations collected at the European Southern Observatory (ESO), ESOProgramme 72.A-0244 (PI Petitjean), and including data obtained from theESO/ST-ECF Science Archive Facility.
| The first stars: what we know and do not know.
Not Available
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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
| On the binarity of carbon-enhanced, metal-poor stars
We report on a programme to monitor the radial velocities of a sample ofcandidate and confirmed carbon-enhanced, metal-poor (CEMP) stars. Weobserved 45 targets using the Echelle Spectrographs of three 4-m classtelescopes. Radial velocities for these objects were calculated bycross-correlation of their spectra with the spectrum of HD 140283, andhave errors < 1 km s-1. Sixteen of our programme's targetshave reported carbon excess, and nine of these objects also exhibits-process enhancements (CEMP-s). We combine these stars' radialvelocities with other literature studies in search of binarity. Thesearch reveals that four of our CEMP-s stars ( 44%) are in binarysystems. Using the analysis of Lucatello et al. (2004), we find that allthe CEMP-s stars in our sample are binaries. This conclusion impliesthat CEMP-s stars may be the very metal-poor relatives of CH and Ba IIstars, which are believed to have acquired their peculiar abundancepatterns by mass transfer from a thermally-pulsing AGB companion.
| Binary Blue Metal-poor Stars: Evidence for Asymptotic Giant Branch Mass Transfer
We present new abundance analyses of six blue metal-poor (BMP) starswith very low iron abundances ([Fe/H]<-2), based on newhigh-resolution echelle spectra. Three are spectroscopic binaries, andthree have constant radial velocities. The chemical compositions ofthese two groups are very different, as the binary BMP stars have largeenhancements of carbon and neutron-capture elements that are products ofs-process nucleosynthesis. One star, CS 29497-030, has an extremeenhancement of lead, [Pb/Fe]=+3.7, the largest abundance in any star yetdiscovered. It probably also has an oxygen overabundance compared to theother BMP stars of our sample. The binary BMP stars must have attainedtheir status via mass transfer during the asymptotic giant branch (AGB)evolutions of their companion stars, which are now unseen and mostlikely are compact objects. We have not found any examples of AGB masstransfer among BMP binaries with [Fe/H]>-2.
| More lead stars
The standard model for the operation of the s-process in asymptoticgiant branch (AGB) stars predicts that low-metallicity ([Fe/H] <~ -1)AGB stars should exhibit large overabundances of Pb and Bi as comparedto other s-elements. The discovery of the first three such ``leadstars'' (defined as stars enriched in s-elements with [Pb/hs] >~ 1,hs being any of Ba, La or Ce) among CH stars has been reported in aprevious paper (Van Eck et al. \cite{VanEck-01}). Five more CH stars(with [Fe/H] ranging from -1.5 to -2.5) are studied in the presentpaper, and two of them appear to be enriched in lead (with [Pb/Ce] =~0.7). The Pb I line at lambda4057 .812 Å is detected and clearlyresolved thanks to high-resolution spectra (R = lambda /Delta lambda =135ts000 ). The abundances for these two stars (HD 198269 and HD 201626)are consistent with the predictions for the s-process operating inlow-metallicity AGB stars as a consequence of the ``partial mixing'' ofprotons below the convective hydrogen envelope. Another two stars (HD189711 and V Ari) add to a growing number of low-metallicity stars (alsoincluding LP 625-44 and LP 706-7, as reported by Aoki et al.\cite{Aoki2001}) which do not conform to these predictions. Variationson the canonical proton-mixing scenario for the operation of thes-process in low-metallicity stars, that could account for thesediscrepant stars, are briefly discussed.Based on observations carried out at the European Southern Observatory(La Silla, Chile; Program 65.L-0354) and at the Observatoire de HauteProvence (operated by CNRS, France).
| Abundance Analysis of HE 2148-1247, A Star with Extremely Enhanced Neutron Capture Elements
Abundances for 27 elements in the very metal-poor dwarf star HE2148-1247 are presented, including many of the neutron capture elements.We establish that HE 2148-1247 is a very highly s-process-enhanced starwith anomalously high Eu as well, Eu/H ~ half-solar, demonstrating thelarge addition of heavy nuclei at [Fe/H] = -2.3 dex. Ba and La areenhanced by a somewhat larger factor and reach the solar abundance,while Pb significantly exceeds it, thus demonstrating the addition ofsubstantial s-process material. Ba/Eu is 10 times the solar r-processratio but much less than that of the s-process, indicating a substantialr-process addition as well. C and N are also very highly enhanced. Wehave found that HE 2148-1247 is a radial velocity variable; it isprobably a small-amplitude long-period binary. The C, N, and thes-process element enhancements were thus presumably produced throughmass transfer from a former asymptotic giant branch (AGB) binarycompanion. The large enhancement of heavy r-nuclides also requires anadditional source as this is far above any inventory in the interstellarmedium at such low [Fe/H]. We consider that the s-process material wasadded by mass transfer of a more massive companion during its thermallypulsating AGB phase and ending up as a white dwarf. We furtherhypothesize that accretion onto the white dwarf from the envelope of thestar caused accretion-induced collapse of the white dwarf, forming aneutron star, which then produced heavy r-nuclides and againcontaminated its companion. This mechanism in a binary system can thusenhance the envelope of the lower mass star in s- and r-process materialsequentially. Through analysis of the neutron capture element abundancestaken from the literature for a large sample of very metal-poor stars,we demonstrate, as exemplified by HE 2148-1247, that mass transfer in asuitable binary can be very efficient in enhancing the heavy elements ina star; it appears to be capable of enhancing the s-process elements invery metal-poor stars to near the solar abundance but not substantiallyabove it. The yield of Pb relative to Ba appears to vary among verymetal-poor stars.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the California Institute of Technology, theUniversity of California, and the National Aeronautics and SpaceAdministration.
| Stellar Archaeology: A Keck Pilot Program on Extremely Metal-Poor Stars From the Hamburg/ESO Survey. III. The Lead (Pb) Star HE 0024-2523
We present a detailed abundance analysis, including spectral syntheses,of a very metal-poor ([Fe/H]~=-2.7) peculiar main-sequence star (HE0024-2523) detected during the course of the Keck Pilot Program. Radialvelocities of this star were obtained during four different observingruns over a time span of 1.1 yr and demonstrate that it is clearly ashort-period spectroscopic binary. An orbital solution was obtained, andorbital parameters were determined with high precision. The rotationalvelocity was also measured (vrotsini=9.7+/-1.5 kms-1) rotation appears likely to be synchronous with theorbit. The abundance analysis and spectral syntheses indicate that theobject is a CH star characterized by extreme s-process enrichment,likely due to mass accretion from an evolved companion that has nowprobably become a white dwarf. The lead (Pb) abundance of (HE 0024-2523)is very high, the same as that of the recently discovered lead-rich,metal-poor star CS 29526-110, [Pb/Fe]=+3.3. The abundance ratio of theheavy to light s-elements, as characterized by Pb and Ba, [Pb/Ba]=+1.9,is the highest yet found for any metal-poor star and is about 0.7 dexhigher than that of CS 29526-110. On the basis of the measured isotopicratio of carbon (C12/C13~6) we argue that the massdonor must have had an original mass of at least ~3 Msolar.The unusually short period of this CH star suggests that it underwent apast common-envelope phase with its evolved companion. Our results arecompared with the latest available models for asymptotic giant branchyields and s-process nucleosynthesis. We also discuss the possibleconnection between HE 0024-2523, the lithium depletion of halo stars,and halo blue straggler formation.Based in part on observations obtained at the W. M. Keck Observatory,which is operated jointly by the California Institute of Technology, theUniversity of California, and NASA.
| Subaru/HDS Study of the Extremely Metal-Poor Star CS 29498-043: Abundance Analysis Details and Comparison with Other Carbon-Rich Objects
We report on the details of an abundance study using the SubaruTelescope High Dispersion Spectrograph for the extremely metal-poor([Fe/H] = 3.75) star CS 29498-043, whose overabundances of C, N, Mg,and Si were reported in our previous work. The effect of the largeoverabundance of Mg found in this object on the stellar atmosphere isincluded in the abundance analysis. We also performed a similar analysisfor the other eight carbon-rich, metal-poor (-2.7 < [Fe/H] < -1.9)stars with excesses of s-process elements, and for the carbon-richobject CS 22957-027 which shows no excess of neutron-capture elements.Neutron-capture elements in CS 29498-043, as well as CS 22949-037 whichis also known to have large excesses of C, N, Mg, and Si, do not showany overabundance. This fact clearly distinguishes these two stars fromother carbon-rich objects with excesses of s-process elements. Theabundances of Mg, Al, and Si in CS 29498-043 and CS 22949-037 aresignificantly higher than those in other carbon-rich objects andnon-carbon-rich stars with similar metallicities. The abundance natureof the se two stars suggests the existence of a class of extremelymetal-poor stars with large excesses of C, N, Mg, Al, and Si, whichpresumably originate from supernovae in which relatively little materialescaped from the iron core.
| A Subaru/High Dispersion Spectrograph Study of Lead (Pb) Abundances in Eight s-Process Element-rich, Metal-poor Stars
We report the abundances of neutron-capture elements in eightcarbon-rich, metal-poor (-2.7<=[Fe/H]<=-1.9) stars observed withthe Subaru Telescope High Dispersion Spectrograph. The derived abundancepatterns indicate that the neutron-capture elements in these objectsprimarily originated from s-process nucleosynthesis, although the[Ba/Eu] abundance ratios in some objects are lower than that of thesolar system s-process component. The present analysis has yielded thePb abundances for seven objects as well as an upper limit for one objectfrom use of the Pb I λ4057 and λ3683 lines. The values of[Pb/Ba] in these objects cover a wide range, between -0.3 and 1.2.Theoretical studies of s-process nucleosynthesis at low metallicity arerequired to explain this large dispersion of the [Pb/Ba] values.Variations in radial velocity have been found for two of the eightobjects, suggesting that, at least in these instances, the observedexcess of s-process elements is due to the transfer of material across abinary system including an AGB star. Comparisons with predictions of AGBnucleosynthesis models are discussed. Based on data collected at theSubaru Telescope, which is operated by the National AstronomicalObservatory of Japan.
| Chemical Composition of Carbon-Rich, Very Metal-Poor Subgiant LP 625-44 Observed with the Subaru/HDS
We have obtained high-resolution (R ~ 90000) spectra of the carbon- ands-process-element-rich, very metal-poor ([Fe/H] = -2.7) subgiant LP625-44, as well as those of HD 140283 (a metal-poor subgiant with normalabundance ratio) for a comparison, with the High Dispersion Spectrograph(HDS) for the Subaru Telescope for detailed abundance study. The excessof oxygen in LP 625-44 seems to be remarkable (perhaps by nearly afactor 10) compared with that of HD 140283 derived from the O I tripletaround 7770Å, though the oxygen abundance derived from these linesis uncertain. The Na enhancement in LP 625-44 is by about a factor 50,suggesting hydrogen burning in the 22Ne-rich layer in anasymptotic giant branch star which produced the abundance pattern ofthis object. In our new spectrum of LP 625-44, the Pb Iλ3683Å line has been detected, confirming the Pb abundance[log ∈(Pb) ~ 1.9] derived from the Pb I λ4057Å line byprevious work. The abundance ratio of s-process elements at the secondpeak (e.g., La, Ce, and Nd) to that at the third peak (Pb) in LP 625-44is significantly higher (by a factor 5) than that in other threes-process element-rich objects recently studied by van Eck et al. Recenttheoretical studies have modeled s-process nucleosynthesis in theradiative layer of asymptotic giant branch stars in the inter-pulsephase. The above results mean that these processes produced a largescatter in the abundance ratios, or different processes (e.g., s-processnucleosynthesis during thermal pulses) contributed to heavy elements inthe early Galaxy.
| Discovery of lead stars with the ESO 3.6-m telescope and CES
Not Available
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
| 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.
| Nucleosynthesis and Mixing on the Asymptotic Giant Branch. III. Predicted and Observed s-Process Abundances
We present the results of s-process nucleosynthesis calculations forasymptotic giant branch (AGB) stars of different metallicities anddifferent initial stellar masses (1.5 and 3 Msolar), and wepresent comparisons of them with observational constraints fromhigh-resolution spectroscopy of evolved stars over a wide metallicityrange. The computations were based on previously published stellarevolutionary models that account for the third dredge-up phenomenonoccurring late on the AGB. Neutron production is driven by the13C(α,n)16O reaction during the interpulseperiods in a tiny layer in radiative equilibrium at the top of the He-and C-rich shell. The neutron source 13C is manufacturedlocally by proton captures on the abundant 12C; a few protonsare assumed to penetrate from the convective envelope into the radiativelayer at any third dredge-up episode, when a chemical discontinuity isestablished between the convective envelope and the He- and C-richzones. A weaker neutron release is also guaranteed by the marginalactivation of the reaction 22Ne(α,n)25Mgduring the convective thermal pulses. Owing to the lack of a consistentmodel for 13C formation, the abundance of 13Cburnt per cycle is allowed to vary as a free parameter over a wideinterval (a factor of 50). The s-enriched material is subsequently mixedwith the envelope by the third dredge-up, and the envelope compositionis computed after each thermal pulse. We follow the changes in thephotospheric abundance of the Ba-peak elements (heavy s [hs]) and thatof the Zr-peak ones (light s [ls]), whose logarithmic ratio [hs/ls] hasoften been adopted as an indicator of the s-process efficiency (e.g., ofthe neutron exposure). Our model predictions for this parameter show acomplex trend versus metallicity. Especially noteworthy is theprediction that the flow along the s-path at low metallicities drainsthe Zr and Ba peaks and builds an excess at the doubly magic208Pb, which is at the termination of the s-path. We thendiscuss the effects on the models of variations in the crucialparameters of the 13C pocket, finding that they are notcritical for interpreting the results. The theoretical predictions arecompared with published abundances of s-elements for AGB giants ofclasses MS, S, SC, post-AGB supergiants, and for various classes ofbinary stars, which supposedly derive their composition by mass transferfrom an AGB companion. This is done for objects belonging both to theGalactic disk and to the halo. The observations in general confirm thecomplex dependence of neutron captures on metallicity. They suggest thata moderate spread exists in the abundance of 13C that isburnt in different stars. Although additional observations are needed,it seems that a good understanding has been achieved of s-processoperation in AGB stars. Finally, the detailed abundance distributionincluding the light elements (CNO) of a few s-enriched stars atdifferent metallicities are examined and satisfactorily reproduced bymodel envelope compositions.
| The Abundance Pattern of Two Barium Stars in the Galactic Halo: HD 104340 and HD 206983
We present the abundance pattern of two barium stars in the Galactichalo, HD 104340 and HD 206983, based on high-resolution optical spectra.We also determined the spectroscopic stellar atmospheric parameters,temperature, and microturbulent velocity, as well as stellar surfacegravity from a solution of excitation and ionization equilibria of Fe Iand Fe II lines under the assumption of local thermodynamic equilibrium.The abundance analysis reveals HD 104340 to be a metal-poor K giant with[Fe/H]=-1.72 and HD 206983 also a metal-poor K giant with [Fe/H]=-1.43.From a set of Fe I lines, the radial velocity is found to be 263.3+/-0.6km s-1 and -319.2+/-4.4 km s-1 for HD 104340 andHD 206983, respectively. Their high velocity, low metallicity, and highgalactic latitude imply that both stars are members of a Galactic halopopulation. From our study and by using information from the literaturewe believe that HD 206983 is another member of a group known asmetal-deficient barium stars. We compare the abundance pattern with theabundances of a halo population. We found that the abundances of theiron group, α-elements, manganese, copper, and zinc, as well assodium and magnesium, of HD 104340 and HD 206983 follow the abundancepattern of a halo population. The heavy element abundance pattern ofboth stars shows enhancement by a factor of 4-8 with respect to themetal-poor stars with the same metallicity as that analyzed by us. Wealso discuss the abundances of the s-process elements and compare ourresults with other objects that display the same degree of enrichmentdue to neutron capture reactions, binary systems, and AGB stars, througha diagram of metallicity versus neutron exposure given by the [hs/ls]index. Based on the observations made with the 1.52 m telescope at theEuropean Southern Observatory (La Silla, Chile) under agreement withObservatório Nacional (Brazil).
| Estimation of Stellar Metal Abundance. II. A Recalibration of the Ca II K Technique, and the Autocorrelation Function Method
We have recalibrated a method for the estimation of stellar metalabundance, parameterized as [Fe/H], based on medium-resolution (1-2Å) optical spectra (the majority of which cover the wavelengthrange 3700-4500 Å). The equivalent width of the Ca II K line (3933Å) as a function of [Fe/H] and broadband B-V color, as predictedfrom spectrum synthesis and model atmosphere calculations, is comparedwith observations of 551 stars with high-resolution abundances availablefrom the literature (a sevenfold increase in the number of calibrationstars that were previously available). A second method, based on theFourier autocorrelation function technique first described by Ratnatunga& Freeman, is used to provide an independent estimate of [Fe/H], ascalibrated by comparison with 405 standard-star abundances.Metallicities based on a combination of the two techniques for dwarfsand giants in the color range 0.30<=(B-V)_0<=1.2 exhibit anexternal 1 sigma scatter of approximately 0.10-0.20 dex over theabundance range -4.0<=[Fe/H]<=0.5. Particular attention has beengiven to the determination of abundance estimates at the metal-rich endof the calibration, where our previous attempt suffered from aconsiderable zero-point offset. Radial velocities, accurate toapproximately 10 km s^-1, are reported for all 551 calibration stars.
| The chemical composition of HD 196944: a carbon and s-process rich, very metal-poor star
An LTE abundance analysis of the post-AGB candidate star HD 196944 ispresented based on high resolution, high S/N spectra. The radialvelocity of HD 196944 is found to be -174 km s(-1) indicating that itbelongs to the halo population. The spectroscopic analysis provides theatmospheric parameters T_eff = 5250 K, log g = 1.7 (cgs), xi_t = 1.9 kms(-1) corresponding to those for G2-5 (bright) giants. A low ironabundance, [Fe/H] = -2.45, is derived confirming the old, low massnature of the star. With [C/Fe] = +1.4 and a mean s-processoverabundance of [s/Fe] = +1.1 the peculiar atmospheric composition ofHD 196944 is confirmed. Possible evolutionary stages of HD 196944, thatcan explain its atmospheric parameters and composition, are discussed.Based on observations carried out at the European Southern Observatory,La Silla, Chile
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