Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
Fast Winds and Mass Loss from Metal-Poor Field Giants
Echelle spectra of the infrared He I λ10830 line were obtainedwith NIRSPEC on the Keck 2 telescope for 41 metal-deficient field giantstars including those on the red giant branch (RGB), asymptotic giantbranch (AGB), and red horizontal branch (RHB). The presence of this He Iline is ubiquitous in stars with T effgsim 4500 K andMV fainter than -1.5, and reveals the dynamics of theatmosphere. The line strength increases with effective temperature for Teffgsim 5300 K in RHB stars. In AGB and RGB stars, the linestrength increases with luminosity. Fast outflows (gsim 60 kms-1) are detected from the majority of the stars andabout 40% of the outflows have sufficient speed as to allow escape ofmaterial from the star as well as from a globular cluster. Outflowspeeds and line strengths do not depend on metallicity for our sample([Fe/H]= -0.7 to -3.0), suggesting the driving mechanism forthese winds derives from magnetic and/or hydrodynamic processes. Gasoutflows are present in every luminous giant, but are not detected inall stars of lower luminosity indicating possible variability. Mass lossrates ranging from ~3 × 10-10 to ~6 ×10-8 M sun yr-1 estimatedfrom the Sobolev approximation for line formation represent values withevolutionary significance for red giants and RHB stars. We estimate that0.2 M sun will be lost on the RGB, and the torque of thiswind can account for observations of slowly rotating RHB stars in thefield. About 0.1-0.2 M sun will be lost on the RHB itself.This first empirical determination of mass loss on the RHB maycontribute to the appearance of extended horizontal branches in globularclusters. The spectra appear to resolve the problem of missingintracluster material in globular clusters. Opportunities exist for"wind smothering" of dwarf stars by winds from the evolved population,possibly leading to surface pollution in regions of high stellardensity.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy
We examine the Pb and Th abundances in 27 metal-poor stars(-3.1< [Fe/H] <-1.4) whose very heavy metal (Z >56) enrichment was produced only by the rapid (r-) nucleosynthesisprocess. New abundances are derived from Hubble Space Telescope/SpaceTelescope Imaging Spectrograph, Keck/High Resolution EchelleSpectrograph, and Very Large Telescope/UV-Visual Echelle Spectrographspectra and combined with other measurements from the literature to forma more complete picture of nucleosynthesis of the heaviest elementsproduced in the r-process. In all cases, the abundance ratios among therare earth elements and the third r-process peak elements considered(La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solarsystem r-process abundance distribution. We compare the stellarobservations with r-process calculations within the classical"waiting-point" approximation. In these computations a superposition of15 weighted neutron-density components in the range 23 <=lognn <= 30 is fit to the r-process abundance peaks tosuccessfully reproduce both the stable solar system isotopicdistribution and the stable heavy element abundance pattern between Baand U in low-metallicity stars. Under these astrophysical conditions,which are typical of the "main" r-process, we find very good agreementbetween the stellar Pb r-process abundances and those predicted by ourmodel. For stars with anomalously high Th/Eu ratios (the so-calledactinide boost), our observations demonstrate that any nucleosyntheticdeviations from the main r-process affect—at most—only theelements beyond the third r-process peak, namely Pb, Th, and U. Ourtheoretical calculations also indicate that possible r-process abundance"losses" by nuclear fission are negligible for isotopes along ther-process path between Pb and the long-lived radioactive isotopes of Thand U.
| Calibration of Strömgren uvby-H? photometry for late-type stars - a model atmosphere approach
Context: The use of model atmospheres for deriving stellar fundamentalparameters, such as T_eff, log g, and [Fe/H], will increase as we findand explore extreme stellar populations where empirical calibrations arenot yet available. Moreover, calibrations for upcoming large satellitemissions of new spectrophotometric indices, similar to the uvby-H?system, will be needed. Aims: We aim to test the power oftheoretical calibrations based on a new generation of MARCS models bycomparisons with observational photomteric data. Methods: Wecalculated synthetic uvby-H? colour indices from synthetic spectra.A sample of 367 field stars, as well as stars in globular clusters, isused for a direct comparison of the synthetic indices versus empiricaldata and for scrutinizing the possibilities of theoretical calibrationsfor temperature, metallicity, and gravity. Results: We show thatthe temperature sensitivity of the synthetic (b-y) colour is very closeto its empirical counterpart, whereas the temperature scale based uponH? shows a slight offset. The theoretical metallicity sensitivityof the m1 index (and for G-type stars its combination withc_1) is somewhat higher than the empirical one, based upon spectroscopicdeterminations. The gravity sensitivity of the synthetic c1index shows satisfactory behaviour when compared to obervations of Fstars. For stars cooler than the sun, a deviation is significant in thec1-(b-y) diagram. The theoretical calibrations of (b-y),(v-y), and c1 seem to work well for Pop II stars and lead toeffective temperatures for globular cluster stars supporting recentclaims that atomic diffusion occurs in stars near the turnoff point ofNGC 6397. Conclusions: Synthetic colours of stellar atmospherescan indeed be used, in many cases, to derive reliable fundamentalstellar parameters. The deviations seen when compared to observationaldata could be due to incomplete linelists but are possibly also due tothe effects of assuming plane-parallell or spherical geometry and LTE.Model colours 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/498/527
| 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.
| A new stellar library in the region of the CO index at 2.3 μm. New index definition and empirical fitting functions
Context: The analysis of unresolved stellar populations demandsevolutionary synthesis models with realistic physical ingredients andextended wavelength coverage. Aims: We quantitatively describe thefirst CO bandhead at 2.3 μm to allow stellar population models toprovide improved predictions in this wavelength range. Methods: Weobserved a new stellar library with a better coverage of the stellaratmospheric parameter space than in earlier works. We performed adetailed analysis of the robustness of previous CO index definitionswith spectral resolution, wavelength calibration, signal-to-noise ratio,and flux calibration. Results: We define a new line-strength index forthe first CO bandhead at 2.3 μm, D_CO, better suited for stellarpopulation studies than previous index definitions. We derive empiricalfitting functions for the CO feature as a function of the stellarparameters (T_eff, log g and [Fe/H]), showing a detailed quantitativemetallicity dependence.
| Probing Atlas model atmospheres at high spectral resolution. Stellar synthesis and reference template validation
Aims. The fast improvement of spectroscopic observations makes mandatorya strong effort on the theoretical side to better reproduce the spectralenergy distribution (SED) of stars at high spectral resolution. In thisregard, relying on the Kurucz Atlas/Synthe original codes we computedthe Bluered library, consisting of 832 synthetic SED of stars, thatcover a large parameter space at very high spectral resolution (R = 500000) along the 3500-7000 Å wavelength range. Methods: Blueredsynthetic spectra have been used to assess in finer detail the intrinsicreliability and the performance limits of the Atlas theoreticalframework. The continuum-normalized spectra of theSun, Arcturus, andVega, plus a selected list of 45 bright stars withhigh-quality SEDs from the Prugniel & Soubiran Elodie catalog, formour sample designed to probe the global properties of synthetic spectraacross the entire range of H-R parameters. Results: Atlas modelsdisplay a better fitting performance with increasing stellartemperature. High-resolution spectra of Vega, the Sun, and Arcturus havebeen reproduced at R=100 000, respectively, within a 0.7%, 4.5%, and8.8% relative scatter in residual flux. In all the three cases, theresidual flux distribution shows a significant asymmetry (skewnessparameter γ = -2.21, -0.98, -0.67, respectively), which neatlyconfirms an overall “excess” of theoretical line blanketing.For the Sun, this apparent discrepancy is alleviated, but not recovered,by a systematic decrease (-40%) of the line oscillator strengths, log(gf), especially referring to iron transitions. Definitely, a straight“astrophysical” determination of log (gf) for eachindividual atomic transition has to be devised to overcome the problem.By neglecting overblanketing effects in theoretical models when fittinghigh-resolution continuum-normalized spectra of real stars, we lead to asystematically warmer effective temperature (between +80 and +300 K forthe solar fit) and a slightly poorer metal content.
| Automated classification of ELODIE stellar spectral library using probabilistic artificial neural networks
A Probabilistic Neural Network model has been used for automatedclassification of ELODIE stellar spectral library consisting of about2000 spectra into 158 known spectro-luminosity classes. The full spectrawith 561 flux bins and a PCA reduced set of 57, 26 and 16 componentshave been used for the training and test sessions. The results show aspectral type classification accuracy of 3.2 sub-spectral type andluminosity class accuracy of 2.7 for the full spectra and an accuracy of3.1 and 2.6 respectively with the PCA set. This technique will be usefulfor future upcoming large databases and their rapid classification.
| Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars
We report the results for rotational velocities, Vrot sin i,and macroturbulence dispersions, ζRT, for 12 metal-poorfield red giant branch (RGB) stars and 7 metal-poor field red horizontalbranch (RHB) stars. The results are based on Fourier transform analysesof absorption line profiles from high-resolution (R ≈ 120,000),high-S/N (≈215 per pixel; ≈345 per resolution element) spectraobtained with the Gecko spectrograph at the Canada-France-HawaiiTelescope (CFHT). The stars were selected from the authors' previousstudies of 20 RHB and 116 RGB stars, based primarily onlarger-than-average line-broadening values. We find thatζRT values for the metal-poor RGB stars are very similarto those for metal-rich disk giants studied earlier by Gray and hiscollaborators. Six of the RGB stars have small rotational values, lessthan 2.0 km s-1, while five show significantrotation/enhanced line broadening, over 3 km s-1. We confirmthe rapid rotation rate for RHB star HD 195636, found earlier byPreston. This star's rotation is comparable to that of the fastest knownrotating blue horizontal branch (BHB) stars, when allowance is made fordifferences in radii and moments of inertia. The other six RHB starshave somewhat lower rotation but show a trend to higher values at highertemperatures (lower radii). Comparing our results with those for BHBstars from Kinman et al., we find that the fraction of rapidly rotatingRHB stars is somewhat lower than is found among BHB stars. The number ofrapidly rotating RHB stars is also smaller than we would have expectedfrom the observed rotation of the RGB stars. We devise two empiricalmethods to translate our earlier line-broadening results intoVrot sin i for all the RGB and RHB stars they studied.Binning the RGB stars by luminosity, we find that most metal-poor fieldRGB stars show no detectable sign, on average, of rotation, which is notsurprising given the stars' large radii. However, the most luminousstars, with MV <= -1.5, do show net rotation, with meanvalues of 2-4 km s-1, depending on the algorithm employed,and also show signs of radial velocity jitter and mass loss. This"rotation" may in fact prove to be due to other line-broadening effects,such as shock waves or pulsation.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique de France, and the University of Hawaii.
| 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.
| Strömgren Photometry of Galactic Globular Clusters. I. New Calibrations of the Metallicity Index
We present a new calibration of the Strömgren metallicity indexm1 using red giant (RG) stars in four globular clusters (GCs:M92, M13, NGC 1851, 47 Tuc) with metallicity ranging from -2.2 to -0.7,marginally affected by reddening [E(B-V)<=0.04] and with accurate(u,v,b,y) photometry. The main difference between the newmetallicity-index-color (MIC) relations and similar relations availablein the literature is that we have adopted the u-y and v-y colors insteadof b-y. These colors present a stronger sensitivity to effectivetemperature, and the MIC relations show a linear slope. The differencebetween photometric estimates and spectroscopic measurements for RGs inM71, NGC 288, NGC 362, NGC 6397, and NGC 6752 is 0.04+/-0.03 dex(σ=0.11 dex). We also apply the new MIC relations to 85 field RGswith metallicity ranging from -2.4 to -0.5 and accurate reddeningestimates. We find that the difference between photometric estimates andspectroscopic measurements is -0.14+/-0.01 dex (σ=0.17 dex). Wealso provide two sets of MIC relations based on evolutionary models thathave been transformed into the observational plane by adopting eithersemiempirical or theoretical color-temperature relations. We apply thesemiempirical relations to the nine GCs and find that the differencebetween photometric and spectroscopic metallicities is 0.04+/-0.03 dex(σ=0.10 dex). A similar agreement is found for the sample of fieldRGs, with a difference of -0.09+/-0.03 dex (with σ=0.19 dex). Thedifference between metallicity estimates based on theoretical relationsand spectroscopic measurements is -0.11+/-0.03 dex (σ=0.14 dex)for the nine GCs and -0.24+/-0.03 dex (σ=0.15 dex) for the fieldRGs. Current evidence indicates that new MIC relations providemetallicities with an intrinsic accuracy better than 0.2 dex.Based in part on observations collected with the 1.54 m Danish Telescopeoperated at ESO (La Silla, Chile) and with the Nordic Optical Telescope(NOT) operated at La Palma (Spain).
| 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.
| Hubble Space Telescope Observations of Chromospheres in Metal-Deficient Field Giants
Hubble Space Telescope high-resolution spectra of metal-deficient fieldgiants more than double the stars in previous studies, span ~3 mag onthe red giant branch, and sample an abundance range [Fe/H] = -1 to -3.These stars, in spite of their age and low metallicity, possesschromospheric fluxes of Mg II (λ2800) that are within a factor of4 of Population I stars, and they give signs of a dependence on themetal abundance at the lowest metallicities. The Mg II k line widthsdepend on luminosity and correlate with metallicity. Line profileasymmetries reveal outflows that occur at lower luminosities(MV = -0.8) than detected in Ca K and Hα lines inmetal-poor giants, suggesting mass outflow occurs over a larger span ofthe red giant branch than previously thought and confirming that the MgII lines are good wind diagnostics. These results do not support amagnetically dominated chromosphere, but they appear more consistentwith some sort of hydrodynamic or acoustic heating of the outeratmospheres.
| Improved Laboratory Transition Probabilities for Hf II and Hafnium Abundances in the Sun and 10 Metal-poor Stars
Radiative lifetimes from laser-induced fluorescence measurements,accurate to ~+/-5%, are reported for 41 odd-parity levels of Hf II. Thelifetimes are combined with branching fractions measured using Fouriertransform spectrometry to determine transition probabilities for 150lines of Hf II. Approximately half of these new transition probabilitiesoverlap with recent independent measurements using a similar approach.The two sets of measurements are found to be in good agreement for linesin common. Our new laboratory data are applied to refine the hafniumphotospheric solar abundance and to determine hafnium abundances in 10metal-poor giant stars with enhanced r-process abundances. For the Sunwe derive logɛ(Hf)=0.88+/-0.08 from four lines; the uncertaintyis dominated by the weakness of the lines and their blending by otherspectral features. Within the uncertainties of our analysis, ther-process-rich stars possess constant Hf/La and Hf/Eu abundance ratios,logɛ(Hf/La)=-0.13+/-0.02(σ=0.06) andlogɛ(Hf/Eu)=+0.04+/-0.02 (σ=0.06). The observed averagestellar abundance ratio of Hf/Eu and La/Eu is larger than previousestimates of the solar system r-process-only value, suggesting asomewhat larger contribution from the r-process to the production of Hfand La. The newly determined Hf values could be employed as part of thechronometer pair, Th/Hf, to determine radioactive stellar ages.
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Medium-resolution Isaac Newton Telescope library of empirical spectra
A new stellar library developed for stellar population synthesismodelling is presented. The library consists of 985 stars spanning alarge range in atmospheric parameters. The spectra were obtained at the2.5-m Isaac Newton Telescope and cover the range λλ3525-7500 Å at 2.3 Å (full width at half-maximum) spectralresolution. The spectral resolution, spectral-type coverage,flux-calibration accuracy and number of stars represent a substantialimprovement over previous libraries used in population-synthesis models.
| 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.
| Hubble Space Telescope Observations of Heavy Elements in Metal-Poor Galactic Halo Stars
We present new abundance determinations of neutron-capture elements Ge,Zr, Os, Ir, and Pt in a sample of 11 metal-poor(-3.1<=[Fe/H]<=-1.6) Galactic halo giant stars, based on HubbleSpace Telescope UV and Keck I optical high-resolution spectroscopy. Thestellar sample is dominated by r-process-rich stars such as thewell-studied CS 22892-052 and BD +17°3248 but also includes ther-process-poor, bright giant HD 122563. Our results demonstrate thatabundances of the third r-process peak elements Os, Ir, and Pt in thesemetal-poor halo stars are very well correlated among themselves and withthe abundances of the canonical r-process element Eu (determined inother studies), thus arguing for a common origin or site for r-processnucleosynthesis of heavier (Z>56) elements. However, the large (andcorrelated) scatters of [Eu, Os, Ir, Pt/Fe] suggest that the heaviestneutron-capture r-process elements are not formed in all supernovae. Incontrast, the Ge abundances of all program stars track their Feabundances, very well. An explosive process on iron peak nuclei (e.g.,the α-rich freezeout in supernovae), rather than neutron capture,appears to have been the dominant synthesis mechanism for this elementat low metallicities: Ge abundances seem completely uncorrelated withEu. The correlation (with very small scatter) of Ge and Fe abundancessuggests that Ge must have been produced rather commonly in stars, evenat early times in the Galaxy, over a wide range of metallicity. The Zrabundances show much the same behavior as Ge with (perhaps) somewhatmore scatter, suggesting some variations in abundance with respect toFe. The Zr abundances also do not vary cleanly with Eu abundances,indicating a synthesis origin different than that of heavierneutron-capture elements. Detailed abundance distributions for CS22892-052 and BD +17°3248, combining the new elementaldeterminations for Os-Pt and recently published Nd and Ho measurements,show excellent agreement with the solar system r-process curve from theelements Ba to Pb. The lighter n-capture elements, including Ge, ingeneral fall below the same solar system r-process curve that matchesthe heavier elements.
| The Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Comparing Deep Mixing in Globular Cluster and Halo Field Giants: Carbon Abundance Data from the Literature
The behavior of carbon abundance as a function of luminosity is used tocompare the rates of deep mixing within red giants of four globularclusters and the Galactic halo field population. Measurements of [C/Fe]for the clusters M92, NGC 6397, M3, and M13 have been compiled from theliterature, together with the Gratton et al. data for halo field stars.Plots of [C/Fe] versus absolute visual magnitude show that forMV<+1.6 the rate of decline of carbon abundance withincreasing luminosity on the red giant branch isd[C/Fe]/dMV~0.22+/-0.03 among the field stars, as well as inM92, NGC 6397, and M3. Among giants fainter than MV=+1.6 thevariation of [C/Fe] with absolute magnitude is much less. The dataindicate that the rate at which deep mixing introduces carbon-depletedmaterial into the convective envelopes of field halo stars during theupper red giant branch phase of evolution is similar to that of manyglobular cluster giants. The notable exception appears to be M13, inwhich stars exhibit deep mixing at a greater rate; this may account forthe high incidence of very low oxygen abundances among the most luminousgiants in M13 in comparison to M3.
| Sodium Abundances in Stellar Atmospheres with Differing Metallicities
The non-LTE sodium abundances of 100 stars with metallicities-3<[Fe/H]<0.3 are determined using high-dispersion spectra withhigh signal-to-noise ratios. The sodium abundances [Na/Fe] obtained areclose to the solar abundance and display a smaller scatter than valuespublished previously. Giants (logg<3.8) with [Fe/H]<-1 do notdisplay overabundances of sodium, and their sodium abundances do notshow an anticorrelation with the oxygen abundance, in contrast toglobular-cluster giants. They likewise do not show sodium-abundancevariations with motion along the giant branch. No appreciable decreasein the sodium abundance was detected for dwarfs (logg>3.8) withmetallicities -2<[Fe/H]<-1. The observed relation between [Na/Fe]and [Fe/H] is in satisfactory agreement with the theoreticalcomputations of Samland, which take into account the metallicitydependence of the sodium yield and a number of other factors affectingthe distribution of elements in the Galaxy during the course of itsevolution.
| 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.
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| 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.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Szűz |
| Rektaszcenzió: | 14h05m13.03s |
| Deklináció: | -14°51'25.5" |
| Vizuális fényesség: | 7.253 |
| Távolság: | 303.03 parszek |
| RA sajátmozgás: | -92.5 |
| Dec sajátmozgás: | -45.5 |
| B-T magnitude: | 8.458 |
| V-T magnitude: | 7.353 |
Katalógusok és elnevezések:
|
INHABITED SKY
