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Evolution of the barium abundance in the early Galaxy from a NLTE analysis of the Ba lines in a homogeneous sample of EMP stars
Context: Barium is a key element in constraining the evolution of the(not well understood) r-process in the first galactic stars andcurrently the Ba abundances in these very metal-poor stars were mostlymeasured under the Local Thermodynamical Equilibrium (LTE) assumption,which may lead in general to an underestimation of Ba. Aims: We presenthere determinations of the barium abundance taking into account thenon-LTE (NLTE) effects in a sample of extremely metal-poor stars (EMPstars): 6 turnoff stars and 35 giants. Methods: The NLTE profiles ofthe three unblended Ba II lines (4554 Å, 5853 Å, 6496Å) have been computed. The computations were made with a modifiedversion of the MULTI code, applied to an atomic model of the Ba atomwith 31 levels of Ba I, 101 levels of Ba II, and compared to theobservations. Results: The ratios of the NLTE abundances of bariumrelative to Fe are slightly shifted towards the solar ratio. In the plotof [Ba/Fe] versus [Fe/H], the slope of the regression line is slightlyreduced as is the scatter. In the interval -3.3 <[Fe/H] < -2.6,[Ba/Fe] decreases with a slope of about 1.4 and a scatter close to 0.44.For [Fe/H] <-3.3 the number of stars is not sufficient to decidewhether [Ba/Fe] keeps decreasing (and then CD-38:245 should beconsidered as a peculiar “barium-rich star”) or if a plateauis reached as soon as [Ba/Fe] ≈ -1. In both cases the scatter remainsquite large, larger than what can be accounted for by the measurementand determination errors, suggesting the influence of a complex processof Ba production, and/or inefficient mixing in the early Galaxy.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory (Large Programme “First Stars”, ID165.N-0276; P.I.: R. Cayrel.
| Chemical Inhomogeneities in the Milky Way Stellar Halo
We have compiled a sample of 699 stars from the recent literature withdetailed chemical abundance information (spanning –4.2lsim [Fe/H]lsim+0.3), and we compute their space velocities and Galactic orbitalparameters. We identify members of the inner and outer stellar halopopulations in our sample based only on their kinematic properties andthen compare the abundance ratios of these populations as a function of[Fe/H]. In the metallicity range where the two populations overlap(–2.5lsim [Fe/H] lsim–1.5), the mean [Mg/Fe] of the outerhalo is lower than the inner halo by –0.1 dex. For [Ni/Fe] and[Ba/Fe], the star-to-star abundance scatter of the inner halo isconsistently smaller than in the outer halo. The [Na/Fe], [Y/Fe],[Ca/Fe], and [Ti/Fe] ratios of both populations show similar means andlevels of scatter. Our inner halo population is chemically homogeneous,suggesting that a significant fraction of the Milky Way stellar halooriginated from a well-mixed interstellar medium. In contrast, our outerhalo population is chemically diverse, suggesting that anothersignificant fraction of the Milky Way stellar halo formed in remoteregions where chemical enrichment was dominated by local supernovaevents. We find no abundance trends with maximum radial distance fromthe Galactic center or maximum vertical distance from the Galactic disk.We also find no common kinematic signature for groups of metal-poorstars with peculiar abundance patters, such as the α-poor stars orstars showing unique neutron-capture enrichment patterns. Several starsand dwarf spheroidal systems with unique abundance patterns spend themajority of their time in the distant regions of the Milky Way stellarhalo, suggesting that the true outer halo of the Galaxy may have littleresemblance to the local stellar halo.
| The Abundance Spread in the Boötes I Dwarf Spheroidal Galaxy
We present medium-resolution spectra of 16 radial velocity red-giantmembers of the low-luminosity Boötes I dwarf spheroidal (dSph)galaxy that have sufficient S/N for abundance determination, based onthe strength of the Ca II K line. Assuming [Ca/Fe] ~ 0.3, the abundancerange in the sample is Δ[Fe/H] ~ 1.7 dex, with one star having[Fe/H] = -3.4. The dispersion is σ([Fe/H]) = 0.45 +/- 0.08-similarto those of the Galaxy's more luminous dSph systems and ωCentauri. This suggests that the large mass (>~107Msolar) normally assumed to foster self-enrichment and theproduction of chemical abundance spreads was provided by the nonbaryonicmaterial in Boötes I.
| Abundances of Sr, Y, and Zr in Metal-Poor Stars and Implications for Chemical Evolution in the Early Galaxy
We have attributed the elements from Sr through Ag in stars of lowmetallicities ([Fe/H]<~-1.5) to charged-particle reactions (CPRs) inneutrino-driven winds, which are associated with neutron star formationin low-mass and normal supernovae (SNe) from progenitors of ~8-11Msolar and ~12-25 Msolar, respectively. Using thisrule and attributing all Fe production to normal SNe, we previouslydeveloped a phenomenological two-component model, which predicts that[Sr/Fe]>=-0.32 for all metal-poor stars. This is in direct conflictwith the high-resolution data now available, which show that there is agreat shortfall of Sr relative to Fe in many stars with [Fe/H]<~-3.The same conflict also exists for the CPR elements Y and Zr. We showthat the data require a stellar source leaving behind black holes andthat hypernovae (HNe) from progenitors of ~25-50 Msolar arethe most plausible candidates. If we expand our previous model toinclude three components (low-mass and normal SNe and HNe), we find thatessentially all of the data are very well described by the new model.The HN yield pattern for the low-A elements from Na through Zn(including Fe) is inferred from the stars deficient in Sr, Y, and Zr. Weestimate that HNe contributed ~24% of the bulk solar Fe inventory whilenormal SNe contributed only ~9% (not the usually assumed ~33%). Thisimplies a greatly reduced role of normal SNe in the chemical evolutionof the low-A elements.
| NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
Aims. Aluminium is a key element to constrain the models of the chemicalenrichment and the yields of the first supernovae. But obtaining preciseAl abundances in extremely metal-poor (EMP) stars requires that thenon-LTE effects be carefully taken into account. Methods: The NLTEprofiles of the blue resonance aluminium lines have been computed in asample of 53 extremely metal-poor stars with a modified version of theprogram MULTI applied to an atomic model of the Al atom with 78 levelsof Al I and 13 levels of Al II, and compared to the observations. Results: With these new determinations, all the stars of the sample showa ratio Al/Fe close to the solar value: [Al/Fe] = -0.06±0.10 witha very small scatter. These results are compared to the models of thechemical evolution of the halo using different models of SN II and arecompatible with recent computations. The sodium-rich giants are notfound to be also aluminium-rich and thus, as expected, the convection inthese giants only brings to the surface the products of the Ne-Na cycle.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory, Chile (Large Programme “First Stars”,ID 165.N-0276(A); P.I.: R. Cayrel).
| Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants
We present the parameters of 891 stars, mostly clump giants, includingatmospheric parameters, distances, absolute magnitudes, spatialvelocities, galactic orbits and ages. One part of this sample consistsof local giants, within 100 pc, with atmospheric parameters eitherestimated from our spectroscopic observations at high resolution andhigh signal-to-noise ratio, or retrieved from the literature. The otherpart of the sample includes 523 distant stars, spanning distances up to1 kpc in the direction of the North Galactic Pole, for which we haveestimated atmospheric parameters from high resolution but lowsignal-to-noise Echelle spectra. This new sample is kinematicallyunbiased, with well-defined boundaries in magnitude and colours. Werevisit the basic properties of the Galactic thin disk as traced byclump giants. We find the metallicity distribution to be different fromthat of dwarfs, with fewer metal-rich stars. We find evidence for avertical metallicity gradient of -0.31 dex kpc-1 and for atransition at ~4-5 Gyr in both the metallicity and velocities. Theage-metallicity relation (AMR), which exhibits a very low dispersion,increases smoothly from 10 to 4 Gyr, with a steeper increase for youngerstars. The age-velocity relation (AVR) is characterized by thesaturation of the V and W dispersions at 5 Gyr, and continuous heatingin U.
| First stars. VIII. Enrichment of the neutron-capture elements in the early Galaxy
Context: Extremely metal-poor (EMP) stars in the halo of the Galaxy aresensitive probes of the production of the first heavy elements and theefficiency of mixing in the early interstellar medium. The heaviestmeasurable elements in such stars are our main guides to understandingthe nature and astrophysical site(s) of early neutron-capturenucleosynthesis. Aims: Our aim is to measure accurate, homogeneousneutron-capture element abundances for the sample of 32 EMP giant starsstudied earlier in this series, including 22 stars with [Fe/H]< -3.0. Methods: Based on high-resolution, high S/N spectra from the ESOVLT/UVES, 1D, LTE model atmospheres, and synthetic spectrum fits, wedetermine abundances or upper limits for the 16 elements Sr, Y, Zr, Ba,La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb in all stars. Results: As found earlier, [Sr/Fe], [Y/Fe], [Zr/Fe] and [Ba/Fe] arebelow Solar in the EMP stars, with very large scatter. However, we finda tight anti-correlation of [Sr/Ba], [Y/Ba], and [Zr/Ba] with [Ba/H] for-4.5 <[Ba/H] < -2.5, also when subtracting the contribution of themain r-process as measured by [Ba/H]. Spectra of even higher S/N ratioare needed to confirm and extend these results below [Fe/H] ≃ -3.5.The huge, well-characterised scatter of the [n-capture/Fe] ratios in ourEMP stars is in stark contrast to the negligible dispersion in the [α/Fe] and [Fe-peak/Fe] ratios for the same stars found in Paper V. Conclusions: These results demonstrate that a second(“weak” or LEPP) r-process dominates the production of thelighter neutron-capture elements for [Ba/H] < -2.5. The combinationof very consistent [ α/Fe] and erratic [n-capture/Fe] ratiosindicates that inhomogeneous models for the early evolution of the haloare needed. Our accurate data provide strong constraints on futuremodels of the production and mixing of the heavy elements in the earlyGalaxy.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (program ID 165.N-0276(A); P.I: R. Cayrel).
| Halo Star Streams in the Solar Neighborhood
We have assembled a sample of halo stars in the solar neighborhood tolook for halo substructure in velocity and angular momentum space. Oursample (231 stars) includes red giants, RR Lyrae variable stars, and redhorizontal branch stars within 2.5 kpc of the Sun with [Fe/H] less than-1.0. It was chosen to include stars with accurate distances, spacevelocities, and metallicities, as well as well-quantified errors. Withour data set, we confirm the existence of the streams found by Helmi andcoworkers, which we refer to as the H99 streams. These streams have adouble-peaked velocity distribution in the z-direction (out of theGalactic plane). We use the results of modeling of the H99 streams byHelmi and collaborators to test how one might use vz velocityinformation and radial velocity information to detect kinematicsubstructure in the halo. We find that detecting the H99 streams withradial velocities alone would require a large sample (e.g.,approximately 150 stars within 2 kpc of the Sun and within 20° ofthe Galactic poles). In addition, we use the velocity distribution ofthe H99 streams to estimate their age. From our model of the progenitorof the H99 streams, we determine that it was accreted between 6 and 9Gyr ago. The H99 streams have [α/Fe] abundances similar to otherhalo stars in the solar neighborhood, suggesting that the gas thatformed these stars were enriched mostly by Type II supernovae. We havealso discovered in angular momentum space two other possiblesubstructures, which we refer to as the retrograde and progradeoutliers. The retrograde outliers are likely to be halo substructure,but the prograde outliers are most likely part of the smooth halo. Theretrograde outliers have significant structure in the vφdirection and show a range of [α/Fe], with two having low[α/Fe] for their [Fe/H]. The fraction of substructure stars in oursample is between 5% and 7%. The methods presented in this paper can beused to exploit the kinematic information present in future largedatabases like RAVE, SDSS-II/SEGUE, and Gaia.
| NLTE determination of the sodium abundance in a homogeneous sample of extremely metal-poor stars
Context: Abundance ratios in extremely metal-poor (EMP) stars are a goodindication of the chemical composition of the gas in the earliest phasesof the Galaxy evolution. It had been found from an LTE analysis that atlow metallicity, and in contrast with most of the other elements, thescatter of [Na/Fe] versus [Fe/H] was surprisingly large and that, ingiants, [Na/Fe] decreased with metallicity. Aims: Since it iswell-known that the formation of sodium lines is very sensitive tonon-LTE effects, to firmly establish the behaviour of the sodiumabundance in the early Galaxy, we have used high quality observations ofa sample of EMP stars obtained with UVES at the VLT, and we have takeninto account the non-LTE line formation of sodium. Methods: Theprofiles of the two resonant sodium D lines (only these sodium lines aredetectable in the spectra of EMP stars) have been computed in a sampleof 54 EMP giants and turn-off stars (33 of them with [Fe/H]<-3.0)with a modified version of the code MULTI, and compared to the observedspectra. Results: With these new determinations in the range {-4<[Fe/H]< -2.5}, both [Na/Fe] and [Na/Mg] are almost constant witha low scatter. In the turn-off stars and "unmixed" giants (located inthe low RGB): [Na/Fe] = -0.21 ± 0.13 or [Na/Mg] = -0.45 ±0.16. These values are in good agreement with the recent determinationsof [Na/Fe] and [Na/Mg] in nearby metal-poor stars. Moreover we confirmthat all the sodium-rich stars are "mixed" stars (i.e., giants locatedafter the bump, which have undergone an extra mixing). None of theturn-off stars is sodium-rich. As a consequence it is probable that thesodium enhancement observed in some mixed giants is the result of a deepmixing.
| First stars IX - Mixing in extremely metal-poor giants. Variation of the 12C/13C, [Na/Mg] and [Al/Mg] ratios
Context: .Extremely metal-poor (EMP) stars preserve a fossil record ofthe composition of the ISM when the Galaxy formed. It is crucial,however, to verify whether internal mixing has modified their surfacecomposition, especially in the giants where most elements can bestudied. Aims: .We aim to understand the CNO abundance variationsfound in some, but not all EMP field giants analysed earlier. Mixingbeyond the first dredge-up of standard models is required, and itsorigin needs clarification. Methods: .The 12C/^{13C}ratio is the most robust diagnostic of deep mixing, because it isinsensitive to the adopted stellar parameters and should be uniformlyhigh in near-primordial gas. We have measured 12C and ^{13C} abundances in 35 EMP giants (including 22 with {[Fe/H] <-3.0}) from high-quality VLT/UVES spectra analysed with LTE modelatmospheres. Correlations with other abundance data are used to studythe depth of mixing. Results: .The 12C/^{13C} ratio isfound to correlate with [C/Fe] (and Li/H), and clearly anti-correlatewith [N/Fe], as expected if the surface abundances are modified by CNOprocessed material from the interior. Evidence for such deep mixing isobserved in giants above {log L/Lȯ = 2.6}, brighter thanin less metal-poor stars, but matching the bump in the luminosityfunction in both cases. Three of the mixed stars are also Na- andAl-rich, another signature of deep mixing, but signatures of the ONcycle are not clearly seen in these stars. Conclusions: .Extramixing processes clearly occur in luminous RGB stars. They cannot beexplained by standard convection, nor in a simple way by rotatingmodels. The Na- and Al-rich giants could be AGB stars themselves, but aninhomogeneous early ISM or pollution from a binary companion remainpossible alternatives.
| Early Cosmic Chemical Evolution: Relating the Origin of a Diffuse Intergalactic Medium and the First Long-Lived Stars
Nucleosynthetic signatures in common are found between the gasresponsible for the high-redshift Lyα forest and a subsample ofextremely metal-poor stars. A simple mass-loss model of chemicalevolution with physically motivated parameters provides a consistentpicture in which the gas is identified with that lost bysupernova-driven winds during the first generation of star formation.Substantial mass loss occurs, which can account for a diffuse IGM withup to 80% of the total baryon content and a peak [C-O/H] abundance of~-2.9. This mass-loss component differs from one produced later, duringgalaxy formation and evolution, which contributes to a circumgalacticmedium (CGM). The CGM has earlier been shown to have a mass of ~10% ofall baryons and peak [Fe/H]~-1.
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| First stars VI - Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements
We have investigated the poorly-understood origin of nitrogen in theearly Galaxy by determining N abundances from the NH band at 336 nm in35 extremely metal-poor halo giants, with carbon and oxygen abundancesfrom Cayrel et al. (\cite{CDS04}, A&A, 416, 1117), usinghigh-quality ESO VLT/UVES spectra (30 of our 35 stars are in the range-4.1 < [Fe/H] < -2.7 and 22 stars have [Fe/H] < -3.0). Nabundances derived both from the NH band and from the CN band at 389 nmfor 10 stars correlate well, but show a systematic difference of 0.4dex, which we attribute to uncertainties in the physical parameters ofthe NH band (line positions, gf values, dissociation energy, etc.).Because any dredge-up of CNO processed material to the surface maycomplicate the interpretation of CNO abundances in giants, we have alsomeasured the surface abundance of lithium in our stars as a diagnosticof such mixing. Our sample shows a clear dichotomy between two groups ofstars. The first group shows evidence of C to N conversion through CNcycling and strong Li dilution, a signature of mixing; these stars aregenerally more evolved and located on the upper Red Giant Branch (RGB)or Horizontal Branch (HB). The second group has [N/Fe] < 0.5, showsno evidence for C to N conversion, and Li is only moderately diluted;these stars belong to the lower RGB and we conclude that their C and Nabundances are very close to those of the gas from which they formed inthe early Galaxy, they are called ``unmixed stars''. The [O/Fe] and[(C+N)/Fe] ratios are the same in the two groups, confirming that thedifferences between them are caused by dredge-up of CN-processedmaterial in the first group, with negligible contributions from the O-Ncycle. The ``unmixed'' stars reflect the abundances in the early Galaxy:the [C/Fe] ratio is constant (about +0.2 dex) and the [C/Mg] ratio isclose to solar at low metallicity, favouring a high C production bymassive zero-metal supernovae. The [N/Fe] and [N/Mg] ratios scatterwidely. Their mean values in each metallicity bin decrease withincreasing metallicity, but this trend could be a statistical effect.The larger values of these ratios define a flat upper plateau ([N/Mg] =0.0, [N/Fe] = +0.1), which could reflect higher values within a widerange of yields of zero-metal SNe II. Alternatively, by analogy with theDLAs, the lower abundances ([N/Mg] = -1.1, [N/Fe] = -0.7) could reflectgenerally low yields from the first SNe II, the other stars being Nenhanced by winds of massive Asymptotic Giant Branch (AGB) stars. Sinceall the stars show clear [α/Fe] enhancements, they were formedbefore any significant enrichment of the Galactic gas by SNe Ia, andtheir composition should reflect the yields of the first SNe II.However, if massive AGB stars or AGB supernovae evolved more rapidlythan SNe Ia and contaminated the ISM, our stars would also reflect theyields of these AGB stars. At present it cannot be decided whetherprimary N is produced primarily in SNe II or in massive AGB stars, or inboth. The stellar N abundances and [N/O] ratios are compatible withthose found in Damped Lyman-α (DLA) systems. They extend thewell-known DLA ``plateau'' at [N/O] ≈ -0.8 to lower metallicities,albeit with more scatter; no star is found below the putative ``low[N/α] plateau'' at [N/O] ≈ -1.55 in DLAs.Based on observations obtained with the ESO VLT under ESO programme ID165.N-0276(A). This work has made use of the SIMBAD database.
| 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.
| Cu and Zn in the early Galaxy
We present Cu and Zn abundances for 38 FGK stars, mostly dwarfs,spanning a metallicity range between solar and [Fe/H] = -3. Theabundances were obtained using Kurucz's local thermal equilibrium (LTE)model atmospheres and the near-UV lines of Cu I 3273.95 Å and Zn I3302.58 Å observed at high spectral resolution. The trend of[Cu/Fe] versus [Fe/H] is almost solar for [Fe/H] > -1 and thendecreases to a plateau <[Cu/Fe]> = -0.98 at [Fe/H] < -2.5,whereas the [Zn/Fe] trend is essentially solar for [Fe/H] > -2 andthen slightly increases at lower metallicities to an average value of<[Zn/Fe]> = +0.18. We compare our results with previous work onthese elements, and briefly discuss them in terms of nucleosynthesisprocesses. Predictions of halo chemical evolution fairly reproduce thetrends, especially the [Cu/Fe] plateau at very low metallicities, but toa lesser extent the higher [Zn/Fe] ratios at low metallicities,indicating possibly missing yields.
| First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy
In the framework of the ESO Large Programme ``First Stars'', veryhigh-quality spectra of some 70 very metal-poor dwarfs and giants wereobtained with the ESO VLT and UVES spectrograph. These stars are likelyto have descended from the first generation(s) of stars formed after theBig Bang, and their detailed composition provides constraints on issuessuch as the nature of the first supernovae, the efficiency of mixingprocesses in the early Galaxy, the formation and evolution of the haloof the Galaxy, and the possible sources of reionization of the Universe.This paper presents the abundance analysis of an homogeneous sample of35 giants selected from the HK survey of Beers et al. (\cite{BPS92},\cite{Be99}), emphasizing stars of extremely low metallicity: 30 of our35 stars are in the range -4.1 <[Fe/H]< -2.7, and 22 stars have[Fe/H] < -3.0. Our new VLT/UVES spectra, at a resolving power ofR45 000 and with signal-to-noise ratios of 100-200 per pixel overthe wavelength range 330-1000 nm, are greatly superior to those of theclassic studies of McWilliam et al. (\cite{MPS95}) and Ryan et al.(\cite{RNB96}).The immediate objective of the work is to determine precise,comprehensive, and homogeneous element abundances for this large sampleof the most metal-poor giants presently known. In the analysis wecombine the spectral line modeling code ``Turbospectrum'' with OSMARCSmodel atmospheres, which treat continuum scattering correctly and thusallow proper interpretation of the blue regions of the spectra, wherescattering becomes important relative to continuous absorption (λ< 400 nm). We obtain detailed information on the trends of elementalabundance ratios and the star-to-star scatter around those trends,enabling us to separate the relative contributions of cosmic scatter andobservational/analysis errors.Abundances of 17 elements from C to Zn have been measured in all stars,including K and Zn, which have not previously been detected in starswith [Fe/H] < -3.0. Among the key results, we discuss the oxygenabundance (from the forbidden [OI] line), the different and sometimescomplex trends of the abundance ratios with metallicity, the very tightrelationship between the abundances of certain elements (e.g., Fe andCr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within theerror bars, the trends of the abundance ratios with metallicity areconsistent with those found in earlier literature, but in many cases thescatter around the average trends is much smaller than found in earlierstudies, which were limited to lower-quality spectra. We find that thecosmic scatter in several element ratios may be as low as 0.05 dex.The evolution of the abundance trends and scatter with decliningmetallicity provides strong constraints on the yields of the firstsupernovae and their mixing into the early ISM. The abundance ratiosfound in our sample do not match the predicted yields frompair-instability hypernovae, but are consistent with element productionby supernovae with progenitor masses up to 100 Mȯ.Moreover, the composition of the ejecta that have enriched the matterBased on observations obtained in the frame of the ESO programme ID165.N-0276(A).Full Tables 3 and 8 are available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/1117 This work hasmade use of the SIMBAD database.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are only available in electronic form, at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| Spectroscopic Binaries, Velocity Jitter, and Rotation in Field Metal-poor Red Giant and Red Horizontal-Branch Stars
We summarize 2007 radial velocity measurements of 91 metal-poor fieldred giants. Excluding binary systems with orbital solutions, ourcoverage averages 13.7 yr per star, with a maximum of 18.0 yr. We reportfour significant findings. (1) Sixteen stars are found to bespectroscopic binaries, and we present orbital solutions for 14 of them.The spectroscopic binary frequency of the metal-poor red giants, with[Fe/H]<=-1.4, for periods less than 6000 days, is 16%+/-4%, which isnot significantly different from that of comparable-metallicity fielddwarfs, 17%+/-2%. The two CH stars in our program, BD -1°2582 and HD135148, are both spectroscopic binaries. (2) Velocity jitter is presentamong about 40% of the giants with MV<=-1.4. The twobest-observed cases, HD 3008 and BD +22°2411, showpseudoperiodicities of 172 and 186 days, longer than any knownlong-period variable in metal-poor globular clusters. Photometricvariability seen in HD 3008 and three other stars showing velocityjitter hints that starspots are the cause. However, the phasing of thevelocity data with the photometry data from Hipparcos is not consistentwith a simple starspot model for HD 3008. We argue against orbitalmotion effects and radial pulsation, so rotational modulation remainsthe best explanation. The implied rotational velocities for HD 3008 andBD +22°2411, both with MV<=-1.4 and R~50Rsolar, exceed 12 km s-1. (3) Including HD 3008and BD +22°2411, we have found signs of significant excess linebroadening in eight of the 17 red giants with MV<=-1.4,which we interpret as rotation. In three cases, BD +30°2034, CD-37°14010, and HD 218732, the rotation is probably induced by tidallocking between axial rotation and the observed orbital motion with astellar companion. But this cannot explain the other five stars in oursample that display signs of significant rotation. This high frequencyof elevated rotational velocities does not appear to be caused bystellar mass transfer or mergers: there are too few main-sequencebinaries with short enough periods. We also note that the lack of anynoticeable increase in mean rotation at the magnitude level of the redgiant branch luminosity function ``bump'' argues against the rapidrotation's being caused by the transport of internal angular momentum tothe surface. Capture of a planetary-mass companion as a red giantexpands in radius could explain the high rotational velocities. (4) Wealso find significant rotation in at least six of the roughly 15 (40%)red horizontal-branch stars in our survey. It is likely that theenhanced rotation seen among a significant fraction of both blue and redhorizontal-branch stars arose when these stars were luminous red giants.Rapid rotation alone therefore appears insufficient cause to populatethe blue side of the horizontal branch. While the largest projectedrotational velocities seen among field blue and red horizontal-branchstars are consistent with their different sizes, neither are consistentwith the large values we find for the largest red giants. This suggeststhat some form of angular momentum loss (and possibly mass loss) hasbeen at work. Also puzzling is the apparent absence of rotation seen infield RR Lyrae variables. Angular momentum transfer and conservation inevolved metal-poor field stars thus pose many interesting questions forthe evolution of low-mass stars.
| Abundances of Cu and Zn in metal-poor stars: Clues for Galaxy evolution
We present new observations of copper and zinc abundances in 90metal-poor stars, belonging to the metallicity range -3<[Fe/H]<-0.5. The present study is based on high resolutionspectroscopic measurements collected at the Haute Provence Observatoire(R= 42 000, S/N>100). The trend of Cu and Zn abundances as a functionof the metallicity [Fe/H] is discussed and compared to that of otherheavy elements beyond iron. We also estimate spatial velocities andgalactic orbital parameters for our target stars in order to disentanglethe population of disk stars from that of halo stars using kinematiccriteria. In the absence of a firm a priori knowledge of thenucleosynthesis mechanisms controlling Cu and Zn production, and of therelative stellar sites, we derive constraints on these last from thetrend of the observed ratios [Cu/Fe] and [Zn/Fe] throughout the historyof the Galaxy, as well as from a few well established properties ofbasic nucleosynthesis processes in stars. We thus confirm that theproduction of Cu and Zn requires a number of different sources (neutroncaptures in massive stars, s-processing in low and intermediate massstars, explosive nucleosynthesis in various supernova types). We alsoattempt a ranking of the relative roles played by different productionmechanisms, and verify these hints through a simple estimate of thegalactic enrichment in Cu and Zn. In agreement with suggestionspresented earlier, we find evidence that type Ia Supernovae must play arelevant role, especially for the production of Cu. Based on the spectracollected with the 1.93-m telescope of Haute Provence Observatory.
| The r-Process in the Early Galaxy
We report Sr, Pd, and Ag abundances for a sample of metal-poor fieldgiants and analyze a larger sample of Y, Zr, and Ba abundances. The[Y/Zr] and [Pd/Ag] abundance ratios are similar to those measured forthe r-process-rich stars CS 22892-052 and CS 31082-001. The [Pd/Ag]ratio is larger than predicted from the solar system r-processabundances. The constant [Y/Zr] and [Sr/Y] values in the field starsplace strong limits on the contributions of the weak s-process and themain s-process to the light neutron-capture elements. Stars in theglobular cluster M15 possess lower [Y/Zr] values than the field stars.There is a large dispersion in [Y/Ba]. Because the r-process isresponsible for the production of the heavy elements in the earlyGalaxy, these dispersions require varying light-to-heavy ratios inr-process yields.
| Keck NIRSPEC Infrared OH Lines: Oxygen Abundances in Metal-poor Stars down to [Fe/H] = -2.9
Infrared OH lines at 1.5-1.7 μm in the H band were obtained with theNIRSPEC high-resolution spectrograph at the 10 m Keck Telescope for asample of seven metal-poor stars. Detailed analyses have been carriedout, based on optical high-resolution data obtained with the Fiber-fedExtended Range Optical Spectrograph at ESO. Stellar parameters werederived by adopting infrared flux method effective temperatures,trigonometric and/or evolutionary gravities, and metallicities from FeII lines. We obtain that the sample stars with metallicities[Fe/H]<-2.2 show a mean oxygen abundance [O/Fe]~0.54 for a solaroxygen abundance of ɛ(O)=8.87, or [O/Fe]~0.64 ifɛ(O)=8.77 is assumed. Observations carried out with the KeckTelescope within the Gemini-Keck agreement, and at the European SouthernObservatory.
| Abundances of 30 Elements in 23 Metal-Poor Stars
We report the abundances of 30 elements in 23 metal-poor([Fe/H]<-1.7) giants. These are based on 7774 equivalent widths andspectral synthesis of 229 additional lines. Hyperfine splitting is takeninto account when appropriate. Our choice of model atmospheres has themost influence on the accuracy of our abundances. We consider the effectof different model atmospheres on our results. In addition to the randomerrors in Teff, logg, and microturbulent velocity, there areseveral sources of systematic error. These include using Teffdetermined from Fe I lines rather than colors, ignoring non-LTE effectson the Fe I/Fe II ionization balance, using models with solar[α/Fe] ratios, and using Kurucz models with overshooting. Ofthese, only the use of models with solar [α/Fe] ratios had anegligible effect. However, while the absolute abundances can change bygreater than 0.10 dex, the relative abundances, especially betweenclosely allied atoms such as the rare earth group, often show only small(less than 0.03 dex) changes. We found that some strong lines of Fe I,Mn I, and Cr I consistently gave lower abundances by ~0.2 dex, a numberlarger than the quoted errors in the gf-values. After considering amodel with depth-dependent microturbulent velocity and a model withhotter temperatures in the upper layers, we conclude that the latter dida better job of resolving the problem and agreeing with observationalevidence for the structure of stars. The error analysis includes theeffects of correlation of Teff, logg, and ξ errors, whichis crucial for certain element ratios, such as [Mg/Fe]. The abundancespresented here are being analyzed and discussed in a separate series ofpapers.
| 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.
| 12C/13C in Metal-poor Field Halo Giants
We have estimated 12C/13C in 15 metal-poor(-2.4<=[Fe/H]<=-1.0) field halo giant stars from spectra of the13CO v=3-1 and v=2-0 band heads and surrounding12CO and 13CO R-branch lines. Our isotope ratiosare consistent with previous measurements for stars in our sample with12C/13C determined either from the infraredfirst-overtone bands of CO or from optical G-band spectra of CH and redsystem bands of CN. We have also compiled carbon isotope ratios from theliterature for a much larger sample of field and cluster red giantbranch (RGB) stars spanning a wide range of metallicities(-2.4<=[Fe/H]<=solar). Combining these data, we confirm thedecline of the isotope ratio as stars evolve up the RGB and we haveidentified a trend toward higher levels of mixing in more metal-poorstars. Standard RGB first dredge-up models do not predict the carbonisotope ratios that we observe in the more evolved (higher luminosity)metal-poor stars, but more recent models that account for other mixingmechanisms can explain these data; even for very metal-poor stars suchas those that we have observed in the Galactic halo.
| Sulphur Abundance in Very Metal-poor Stars
We have obtained high-resolution high signal-to-noise ratio spectra ofthe S I near-infrared doublet at 8694 Å in eight metal-poor starswith metallicities in the range -0.6<=[Fe/H]<=-3.0. Elementalsulphur abundances were derived for six targets, and upper limits wereset for two of the most metal-poor stars in the sample. The sulphurdoublet at 8694.62 Å has been detected in three stars (HD 19445,HD 2665, and HD 2796) with [Fe/H]<=-1.9. Our observations, combinedwith those available in the literature, indicate a monotonic increase ofthe [S/Fe] ratio as [Fe/H] decreases, reaching values of [S/Fe]~0.7-0.8below [Fe/H]=-2. We discuss plausible scenarios for the interpretationof these results. Based on observations obtained with the WilliamHerschel Telescope (WHT), operated on the island of La Palma by theIsaac Newton Group in the Spanish Observatorio del Roque de losMuchachos (ORM) of the Instituto de Astrofísica de Canarias.
| Th Ages for Metal-poor Stars
With a sample of 22 metal-poor stars, we demonstrate that theheavy-element abundance pattern (Z>=56) is the same as the r-processcontributions to the solar nebula. This bolsters the results of previousstudies that there is a universal r-process production pattern. We usethe abundance of thorium in five metal-poor stars, along with anestimate of the initial Th abundance based on the abundances of stabler-process elements, to measure their ages. We have four field red giantswith errors of 4.2 Gyr in their ages and one M92 giant with an error of5.6 Gyr, based on considering the sources of observational error only.We obtain an average age of 11.4 Gyr, which depends critically on theassumption of an initial Th/Eu production ratio of 0.496. If theuniverse is 15 Gyr old, then the (Th/Eu)0 should be 0.590, inagreement with some theoretical models of the r-process.
| Analysis of neutron capture elements in metal-poor stars
We derived model atmosphere parameters (Teff, log g, [Fe/H],Vt) for 90 metal-deficient stars (-0.5<[Fe/H]<-3),using echelle spectra from the ELODIE library (Soubiran et al.\cite{soubet98}). These parameters were analyzed and compared withcurrent determinations by other authors. The study of the followingelements was carried out: Mg, Si, Ca, Sr, Y, Ba, La, Ce, Nd, and Eu. Therelative contributions of s- and r-processes were evaluated andinterpreted through theoretical computations of the chemical evolutionof the Galaxy. The chemical evolution models (Pagel &Tautvaišienė \cite{pagta95}; Timmes et al. \cite{timet95})depict quite well the behaviour of [Si/Fe], [Ca/Fe] with [Fe/H]. Thetrend of [Mg/Fe] compares more favourably with the computations of Pagel& Tautvaišienė (\cite{pagta95}) than those of Timmes etal. (\cite{timet95}). The runs of n-capture elements vs. metallicity aredescribed well both by the model of Pagel & Tautvaišienė(\cite{pagta95}, \cite{pagta97}) and by the model of Travaglio et al.(\cite{travet99}) at [Fe/H]>-1.5, when the matter of the Galaxy issufficiently homogeneous. The analysis of n-capture element abundancesconfirms the jump in [Ba/Fe] at [Fe/H]=-2.5. Some stars from our sampleat [Fe/H]<-2.0 show a large scatter of Sr, Ba, Y, Ce. This scatter isnot caused by the errors in the measurements, and may reflect theinhomogeneous nature of the prestellar medium at early stages ofgalactic evolution. The matching of [Ba/Fe], [Eu/Fe] vs. [Fe/H] with theinhomogeneous model by Travaglio et al. (\cite{travet01a}) suggests thatat [Fe/H]<-2.5, the essential contribution to the n-rich elementabundances derives from the r-process. The main sources of theseprocesses may be low mass SN II. The larger dispersion of s-processelement abundances with respect to alpha -rich elements may arise bothfrom the birth of metal-poor stars in globular clusters with followingdifferent evolutionary paths and (or) from differences in s-elementenrichment in Galaxy populations. Based on spectra collected at theObservatoire de Haute-Provence (OHP), France
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Наблюдательные данные и астрометрия
| Созвездие: | Стрелец |
| Прямое восхождение: | 19h58m49.74s |
| Склонение: | -18°12'11.2" |
| Видимая звёздная величина: | 9.319 |
| Собственное движение RA: | 11.7 |
| Собственное движение Dec: | -91.4 |
| B-T magnitude: | 10.3 |
| V-T magnitude: | 9.4 |
Каталоги и обозначения:
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