Contents
Images
Upload your image
DSS Images Other Images
Related articles
The Structure of the Local Interstellar Medium. IV. Dynamics, Morphology, Physical Properties, and Implications of Cloud-Cloud Interactions
We present an empirical dynamical model of the LISM based on 270 radialvelocity measurements for 157 sight lines toward nearby stars. Physicalparameter measurements (i.e., temperature, turbulent velocity,depletions) are available for 90 components, or one-third of the sample,enabling initial characterizations of the physical properties of LISMclouds. The model includes 15 warm clouds located within 15 pc of theSun, each with a different velocity vector. We derive projectedmorphologies of all clouds and estimate the volume filling factor ofwarm partially ionized material in the LISM to be between ~5.5% and 19%.Relative velocities of potentially interacting clouds are oftensupersonic, consistent with heating, turbulent, and metal depletionproperties. Cloud-cloud collisions may be responsible for thefilamentary morphologies found in ~1/3 of LISM clouds, the distributionof clouds along the boundaries of the two nearest clouds (LIC and G),the detailed shape and heating of the Mic Cloud, the location of nearbyradio scintillation screens, and the location of an LISM cold cloud.Contrary to previous claims, the Sun appears to be located in thetransition zone between the LIC and G Cloud.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with programs 9525 and 10236.
| HE 0557-4840: Ultra-Metal-Poor and Carbon-Rich
We report the discovery and high-resolution, high-S/N spectroscopicanalysis of the ultra-metal-poor red giant HE 0557-4840, which is thethird most heavy-element-deficient star currently known. Its atmosphericparameters are Teff=4900 K, logg=2.2, and [Fe/H]=-4.75. Thisbrings the number of stars with [Fe/H]<-4.0 to three, and thediscovery of HE 0557-4840 suggests that the metallicity distributionfunction of the Galactic halo does not have a ``gap'' between[Fe/H]=-4.0, where several stars are known, and the two most metal-poorstars, at [Fe/H]~-5.3. HE 0557-4840 is carbon rich ([C/Fe]=+1.6), aproperty shared by all three objects with [Fe/H]<-4.0, suggestingthat the well-known increase of carbon relative to iron with decreasing[Fe/H] reaches its logical conclusion (ubiquitous carbon richness) atlowest abundance. We also present abundances (nine) and limits (nine)for a further 18 elements. For species having well-measured abundancesor strong upper limits, HE 0557-4840 is ``normal'' in comparison withthe bulk of the stellar population at [Fe/H]~-4.0, with the possibleexception of Co. We discuss the implications of these results forchemical enrichment at the earliest times, in the context of single-(``mixing and fallback'') and two-component enrichment models. Whileneither offers a clear solution, the latter appears closer to the mark.Further data are required to determine the oxygen abundance and improvethat of Co, and hence more strongly constrain the origin of this object.Based on observations collected at ANU's 2.3 m telescope on SidingSpring Mountain, Australia, and European Southern Observatory, Paranal,Chile (proposal 276.D-5041).
| Improved Laboratory Transition Probabilities for Neutral Chromium and Redetermination of the Chromium Abundance for the Sun and Three Stars
Branching fraction measurements from Fourier transform spectra inconjunction with published radiative lifetimes are used to determinetransition probabilities for 263 lines of neutral chromium. Theselaboratory values are employed to derive a new photospheric abundancefor the Sun, logε(Cr I)solar=5.64+/-0.01 (σ=0.07).The individual Cr I solar abundances do not exhibit any trends with linestrength nor with excitation energy, and there were no obviousindications of departures from LTE. In addition, oscillator strengthsfor singly ionized chromium recently reported by the FERRUM Project areused to determine logε(Cr II)solar=5.77+/-0.03(σ=0.13). Transition probability data are also applied to thespectra of three stars: HD 75732 (metal-rich dwarf), HD 140283(metal-poor subgiant), and CS 22892-052 (metal-poor giant). In all ofthe selected stars, Cr I is found to be underabundant with respect to CrII. The possible causes for this abundance discrepancy and apparentionization imbalance are discussed.
| Sulphur and zinc abundances in Galactic halo stars revisited
Aims.Based on a new set of sulphur abundances in very metal-poor starsand an improved analysis of previous data, we aim at resolving currentdiscrepancies on the trend of S/Fe vs. Fe/H and thereby gain betterinsight into the nucleosynthesis of sulphur. The trends of Zn/Fe andS/Zn will also be studied. Methods: High resolution VLT/UVES spectra of40 main-sequence stars with -3.3 < [Fe/H] < -1.0 are used toderive S abundances from the weak λ 8694.6 S I line and thestronger λ λ 9212.9,9237.5 pair of S I lines. For onestar, the S abundance is also derived from the S I triplet at 1.046μm recently observed with the VLT infrared echelle spectrographCRIRES. Fe and Zn abundances are derived from lines in the blue part ofthe UVES spectra, and effective temperatures are obtained from theprofile of the Hβ line. Results: Comparison of sulphur abundancesfrom the weak and strong S I lines provides important constraints onnon-LTE effects. The high sulphur abundances reported by others for somemetal-poor stars are not confirmed; instead, when taking non-LTEcorrections into account, the Galactic halo stars distribute around aplateau at [S/Fe] ~ +0.2 dex with a scatter of 0.07 dex only. [Zn/Fe] isclose to zero for metallicities in the range -2.0 < [Fe/H] < -1.0but increases to a level of [Zn/Fe] ~ +0.1 to +0.2 dex in the range -2.7< [Fe/H] < -2.0. At still lower metallicities [Zn/Fe] risessteeply to a value of [Zn/Fe] ~ +0.5 dex at [Fe/H] = -3.2. Conclusions:The trend of S/Fe vs. Fe/H corresponds to the trends of Mg/Fe, Si/Fe,and Ca/Fe and indicates that sulphur in Galactic halo stars has beenmade by α-capture processes in massive SNe. The observed scatterin S/Fe is much smaller than predicted from current stochastic models ofthe chemical evolution of the early Galaxy, suggesting that either themodels or the calculated yields of massive SNe should be revised. Wealso examine the behaviour of S/Zn and find that departures from thesolar ratio are significantly reduced at all metallicities if non-LTEcorrections to the abundances of these two elements are adopted. Thiseffect, if confirmed, would reduce the usefulness of the S/Zn ratio as adiagnostic of past star-formation activity, but would bring closertogether the values measured in damped Lyman-alpha systems and inGalactic stars.Based on observations collected at the European Southern Observatory atParanal, Chile (programmes No. 67.D-0106, 73.D-0024 and CRIRES scienceverification program 60.A-9072). Table 1 and Appendices are onlyavailable in electronic form at http://www.aanda.org
| Measuring the Balmer Jump and the Effective Gravity in FGK Stars
It is difficult to accurately measure the effective gravity (logg) inlate-type stars using broadband (e.g., UBV or SDSS) or intermediate-band(uvby) photometric systems, especially when the stars can cover a rangeof metallicities and reddenings. However, simple spectroscopicobservational and data reduction techniques can yield accurate valuesfor logg through comparison of the Balmer jumps of low-resolutionspectra with recent grids of synthetic flux spectra.
| The Chemical Composition of the Field Zero-Age Star HD 77407
High-resolution optical spectra of the zero age star HD77407 areanalysed and its Li, C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Co, Ni andBa contents are determined using spectral synthesis method. Thetemperature of the star is determined by fitting the Hαline wings. The parameters derived for this star areTeff=5900 K, log g=4.47 and [Fe/H] = +0.07. It is found thatthe derived iron content is slightly higher than what is given in thepublished literature. This star shows a relative overabundances of Caand Ba, and underabundances of Na, V and Ni with respect to the solarmix. Activity of the star is indicated by the filled inHα and CaII triplet line cores. It has been confirmedthat our spectroscopic approach yields fairly reliable and consistentresults for active stars.
| Explorations of the r-Processes: Comparisons between Calculations and Observations of Low-Metallicity Stars
Abundances of heavier elements (barium and beyond) in manyneutron-capture-element-rich halo stars accurately replicate the solarsystem r-process pattern. However, abundances of lighter neutron-captureelements in these stars are not consistent with the solar systempattern. These comparisons suggest contributions from two distinct typesof r-process synthesis events, a so-called main r-process for theelements above the second r-process peak and a weak r-process for thelighter neutron-capture elements. We have performed r-processtheoretical predictions to further explore the implications of the solarand stellar observations. We find that the isotopic composition ofbarium and the elemental Ba/Eu abundance ratios in r-process-richlow-metallicity stars can only be matched by computations in which theneutron densities are in the range 23<~lognn<~28,values typical of the main r-process. For r-process conditions thatsuccessfully generate the heavy element pattern extending down to A=135,the relative abundance of 129I produced in this mass regionappears to be at least ~90% of the observed solar value. Finally, in theneutron number density ranges required for production of the observedsolar/stellar third r-process-peak (A~200), the predicted abundances ofinterpeak element hafnium (Z=72, A~177-180) follow closely those ofthird-peak elements and lead. Hf, observable from the ground and closein mass number to the third r-process-peak elements, might also be usedas part of a new nuclear chronometer pair, Th/Hf, for stellar agedeterminations.
| The Last Measurements Made with the Wampler Scanner. I. An Analysis of the Consistency and Accuracy of Flux Curves for Bright Standard Stars
From 1980 through 1998, the Wampler scanner and the 0.9 m Crossleyreflector at Lick Observatory were used to obtain 137 nights offlux-curve measurements. In this paper, the resulting data are used totest, revise, and extend a set of flux curves for bright standard starspublished in 1984. Full use is made of extensive measurements of αLyr, the primary standard star. It is found that measurements madeduring spring and fall runs are highly consistent and that smallsystematic wavelength trends appearing in the standard-star data can bereadily detected and corrected. This conclusion applies for 109 Vir inparticular, and it is concluded that there is no convincing case forvariation of this key standard star after 1979. Tests of the correctedand augmented flux curves using 13 color photometry yield a number ofcases in which residuals are systematic with wavelength. However, forstars with E(B-V)<0.12 mag (including α Lyr), the revised andextended flux-curve data yield synthetic values of Strömgren u-band b-y that are highly consistent with results from filter photometry.For almost all stars measured (again including α Lyr), consistencywith filter values of (V-R)C is also obtained. Comparisons ofthe revised and augmented data with flux curves measured in the SovietUnion yield results ranging from fair agreement to marked disagreement.However, it is found that published data from additional sources includeresults that agree well with those presented here. Given this agreement,the results of the tests using filter photometry, and the character ofthe transformation coefficients for α Lyr, it is concluded thatthere is adequate support for the accuracy of the correctedstandard-star flux curves. Recommendations about future measurementsthat can be used to test this conclusion are presented.
| Spectroscopy of Globular Clusters out to Large Radius in the Sombrero Galaxy
We present new velocities for 62 globular clusters in M104 (NGC 4594,the Sombrero Galaxy), 56 from 2dF on the AAT and 6 from Hydra on WIYN.Combined with previous data, we have a total sample of 108 M104 globularcluster velocities, extending to 20' radius (~60 kpc), alongwith BVR photometry for each of these. We use this wide-field data setto study the globular cluster kinematics and dark matter content of M104out to 10' radius (30 kpc). We find no rotation in theglobular cluster system. The edge-on nature of M104 makes it unlikelythat there is strong rotation which is face-on and hence unobserved;thus, the absence of rotation over our large radial range appears to bean intrinsic feature of the globular cluster system in M104. We discussways to explain this low rotation, including the possibility thatangular momentum has been transferred to even larger radii throughgalaxy mergers. The cluster velocity dispersion is ~230 kms-1 within several arcminutes of the galaxy center, and dropsto ~150 km s-1 at ~10' radius. We derive the massprofile of M104 using our velocity dispersion profile, together with theJeans equation under the assumptions of spherical symmetry and isotropy,and find excellent agreement with the mass inferred from the stellar andgas rotation curve within 3' radius. The M/LVincreases from ~4 near the galaxy center to ~17 at 7' radius(~20 kpc, or 4 Re), thus giving strong support for thepresence of a dark matter halo in M104. More globular cluster velocitiesat larger radii are needed to further study the low rotation in theglobular cluster system, and to see if the dark matter halo in M104extends beyond a radius of 30 kpc.
| Lithium abundances in metal-poor stars
Aims.Lithium abundances for 19 metal-poor stars are determined usinghigh-resolution spectroscopy. The abundances of stars on the lithiumplateau are discussed. Methods: All abundance results are derived fromNLTE statistical equilibrium calculations and spectrum synthesismethods. Results: In agreement with previous analyses it is found thatexcitation and de-excitation due to hydrogen collisions are negligiblefor the lithium line formation process, while charge transfer reactionsare an important source of thermalization. However, the resulting NLTEeffects on the determination of lithium abundances for metal-poor starsare negligible (<0.06 dex). Conclusions: .The mean lithium abundancefor stars on the lithium plateau determined from NLTE analyses is A(Li)~ 2.26, while it is 2.21 dex when charge transfer reactions areincluded. The latter result enhances the discrepancy between theobserved lithium abundances and the primordial lithium abundance asinferred by the WMAP analysis of the cosmic microwave background. Thisdiscrepancy may be explained by metal diffusion.Based on observations collected at the Germany-Spanish AstronomicalCenter, Calar Alto, Spain.
| Chemical Abundance Analysis of the Extremely Metal-poor Star HE 1300+0157
We present a detailed chemical abundance analysis of HE 1300+0157, asubgiant with [Fe/H]=-3.9. From a high-resolution, high-S/N Subaru HDSspectrum we find the star to be enriched in C ([C/Fe]1D~+1.4)and O ([O/Fe]1D~+1.8). With the exception of these species,HE 1300+0157 exhibits an elemental abundance pattern similar to thatfound in many other very and extremely metal-poor stars. The Liabundance is lower than the Spite plateau value, in agreement withexpectation for its evolutionary status. Of particular interest, noneutron-capture elements are detected in HE 1300+0157. This type ofabundance pattern has been found by recent studies in several othermetal-poor giants. We suggest that HE 1300+0157 is an unevolved exampleof this group of stars, which exhibit high C abundances together withlow (or absent) abundances of neutron-capture elements (CEMP-no).Several potential enrichment scenarios are presented. The nondetectionof neutron-capture elements including Sr, Ba, and Pb suggests that thecarbon excess observed in HE 1300+0157 is not due to mass transferacross a binary system. Such a scenario is applied to carbon-richobjects with excesses of s-process elements. The normal observed Liabundance supports this interpretation. Most likely, the high levels ofC and O were produced prior to the birth of this star. It remainsunclear whether a single faint SN is responsible for its overallchemical pattern, or whether one requires a superposition of yields froma massive Population III object and a hypernova. These scenarios provideimportant information on the C production in the early universe and onthe formation of C-rich stars in the early Galaxy.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Isotope structure of Sm II as an indicator of r- vs. s-process nucleosynthesis
Aims.The isotope composition of samarium provides a previouslyunexploited means of determining the r- versus s-process content of e.g.metal-poor stars. The seven stable isotopes of samarium can be dividedinto two groups, where in one group the r-process is the dominantproduction mechanism, while for the second group it is the s-processthat is dominant. The center-of-gravity for spectral lines produced bythese two groups are slightly shifted with respect to each other, wherethe heavier and most abundant isotopes are mainly produced by ther-process. This relative shift makes the line position dependent uponthe content of r- and s-processed samarium. The isotope composition alsoaffects the width and the shape of the line, making it narrower andslightly more asymmetric when the composition deviates from the solarsystem composition. Methods: The behaviour of Sm II lines in stellarspectra is studied using synthetic spectrum techniques with line dataaccounting for both isotope and hyperfine structure. Isotope shifts in11 lines of Sm II have been measured from a laboratory spectrum obtainedwith a Fourier transform spectrometer. Hyperfine structure constantshave been taken from the literature. Results: The new atomic data areapplied to the spectrum of the metal-poor halo star CS 31082-001 andsuggests that this star contains predominantly r-processed material. Wealso discuss the uncertainties involved in this type of high precisionmeasurement.
| First stars VII - Lithium in extremely metal poor dwarfs
Context: .The primordial lithium abundance is a key prediction of modelsof big bang nucleosynthesis, and its abundance in metal-poor dwarfs (theSpite plateau) is an important, independent observational constraint onsuch models. Aims: .This study aims to determine the level andconstancy of the Spite plateau as definitively as possible fromhomogeneous high-quality VLT-UVES spectra of 19 of the most metal-poordwarf stars known. Methods: .Our high-resolution (R 43 000),high S/N spectra are analysed with OSMARCS 1D LTE model atmospheres andturbospectrum synthetic spectra to determine effective temperatures,surface gravities, and metallicities, as well as Li abundances for ourstars. Results: .Eliminating a cool subgiant and a spectroscopicbinary, we find 8 stars to have -3.5 < [Fe/H] < -3.0 and 9 starswith -3.0 < [Fe/H] < -2.5. Our best value for the mean level ofthe plateau is A(Li) =2.10± 0.09. The scatter around the mean isentirely explained by our estimate of the observational error and doesnot allow for any intrinsic scatter in the Li abundances. In addition,we conclude that a systematic error of the order of 200 K in any of thecurrent temperature scales remains possible. The iron excitationequilibria in our stars support our adopted temperature scale, which isbased on a fit to wings of the Hα line, and disfavour hotterscales, which would lead to a higher Li abundance, but fail to achieveexcitation equilibrium for iron. Conclusions: .We confirm thepreviously noted discrepancy between the Li abundance measured inextremely metal-poor turnoff stars and the primordial Li abundancepredicted by standard Big-Bang nucleosynthesis models adopting thebaryonic density inferred from WMAP. We discuss recent work explainingthe discrepancy in terms of diffusion and find that uncertaintemperature scales remain a major question.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Large Programme "First Stars", ID 165.N-0276(A);P.I. R. Cayrel). Tables 4-8 and Appendix A are only available inelectronic form at http://www.aanda.org
| A non-LTE study of neutral and singly-ionized calcium in late-type stars
Aims.Non-local thermodynamical equilibrium (NLTE) line formation forneutral and singly-ionized calcium is considered through a range ofspectral types when the Ca abundance varies from the solar value down to[Ca/H] = -5. We evaluate the influence of departures from LTE on Caabundance determinations and inspect the possibility of using Ca I / CaII line-strength ratios as indicators of surface gravity for extremelymetal-poor stars. Methods: .A comprehensive model atom for Ca Iand Ca II is presented. Accurate radiative and electron collisionalatomic data are incorporated. The role of inelastic collisions withhydrogen atoms in the statistical equilibrium of Ca I/II is estimatedempirically from inspection of their different influences on the Ca Iand Ca II lines in selected stars with well determined stellarparameters and high-quality observed spectra. Results: .Thedependence of NLTE effects on the atmospheric parameters is discussed.Departures from LTE significantly affect the profiles of Ca I lines overthe whole range of stellar parameters being considered. However, at[Ca/H] ≥ -2, NLTE abundance correction of individual lines have alow absolute value due to the different influence of NLTE effects online wings and the line core. At lower Ca abundances, NLTE leads tosystematically depleted total absorption in the line and positiveabundance corrections, exceeding +0.5 dex for Ca I λ 4226 at[Ca/H] = -4.9. In contrast, the NLTE effects strengthen the Ca II linesand lead to negative abundance corrections. NLTE corrections are small,≤0.02 dex, for the Ca II resonance lines, and they grow in absolutevalue with decreasing Ca abundance for the IR lines of multiplet 3d-4p,exceeding 0.4 dex in the metal-poor models with [Fe/H] ≤ -3. As atest and first application of the Ca I/II model atom, Ca abundances aredetermined on the basis of plane-parallel LTE model atmospheres for theSun, Procyon (F IV-V), and seven metal-poor stars, using high S/N andhigh-resolution spectra at visual and near-IR wavelengths. Lines of Ca Iand Ca II give consistent abundances for all objects (except Procyon)when collisions with hydrogen atoms are taken into account. The derivedabsolute solar Ca abundance (from Ca I and Ca II lines) is log\varepsilon_Ca,ȯ = 6.38 ± 0.06. For Procyon, the mean Caabundance from Ca I lines is markedly subsolar, [Ca/H] = -0.14 ±0.03. All metal-poor stars within our sample show an overabundance ofcalcium relative to iron with [Ca/Fe] abundance ratios of 0.26 to 0.46that are typical of the halo population. The W(Ca I 4226) / W(Ca II8498) equivalent width ratio is predicted to be sensitive to surfacegravity for extremely metal-poor stars, while this is not the case forthe ratio involving the Ca II resonance line(s).
| Fundamental parameters and abundances of metal-poor stars: the SDSS standard BD +17 4708
The atmospheric parameters and iron abundance of the Sloan Digital SkySurvey (SDSS) spectrophotometric standard star BD +17 4708 arecritically examined using up-to-date Kurucz model atmospheres, LTE lineformation calculations, and reliable atomic data. We findTeff=6141 ± 50 K, log g=3.87 ± 0.08, and[Fe/H]=-1.74 ± 0.09. The line-of-sight interstellar reddening,bolometric flux, limb-darkened angular diameter, stellar mass, and theabundances of Mg, Si, and Ca are also obtained: E(B-V)=0.010 ±0.003, fbol=(4.89±0.10) × 10-9 ergcm-2 s-1, θ=0.1016 ± 0.0023 mas,M=0.91 ± 0.06~M_ȯ, [Mg/Fe]=0.40 ± 0.10, [Si/Fe]=0.35± 0.11, [Ca/Fe]=0.36 ± 0.11. This star is a unique exampleof a moderately metal-poor star for which the effective temperature(Teff) can be accurately constrained from the observedspectral energy distribution (corrected for reddening). Such analysisleads to a value that is higher than most spectroscopic resultspreviously reported in the literature (~5950 K). Interstellar reddeningwas estimated using various prescriptions, including an analysis ofinterstellar lines. The surface gravity of the star was inferred fromthe fitting of the wings of the Mg I b lines. We used transitionprobabilities measured in the laboratory and reliable damping constantsfor unblended Fe lines to derive the iron abundance using both Fe I andFe II lines. We find that the ionization balance of Fe lines issatisfied only if a low Teff (~5950 K) is adopted. The meaniron abundance we obtain from the Fe II lines corresponds to A_Fe=5.77± 0.09 ([Fe/H]=-1.74 for our derived AFe,ȯ=7.51)while that from the Fe I lines is A_Fe=5.92 ± 0.11, and thereforewith our preferred Teff (6141 K), the discrepancy between themean iron abundance from Fe I and Fe II lines cannot be explained byoverionization by UV photons as the main non-LTE effect. Interestingly,the Fe I excitation balance is satisfied with a Teff onlyslightly warmer than our preferred solution and not with the lower valueof 5950 K. We also comment on non-LTE effects and the importance ofinelastic collisions with neutral H atoms in the determination of oxygenabundances in metal-poor stars from the 7774 Å O I triplet.
| The non-LTE line formation of neutral carbon in late-type stars
Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE)line formation of neutral carbon in late-type stars in order to removesome of the potential systematic errors in stellar abundance analysesemploying C i features. Methods: .The statistical equilibrium codeMULTI was used on a grid of plane-parallel 1D MARCS atmosphericmodels. Results: .Within the parameter space explored, thehigh-excitation C i lines studied are stronger in non-LTE due to thecombined effect of line source function drop and increased line opacitydue to overpopulation of the lower level for the transitions considered;the relative importance of the two effects depends on the particularcombination of T{eff}, log g, [Fe/H] and [C/Fe] and on theanalysed C i line. As a consequence, the non-LTE abundance correctionsare negative and can be substantially so, for example -0.4 dex inhalo turn-off stars at [Fe/H] -3. The magnitude of the non-LTEcorrections is rather insensitive to whether inelastic H collisions areincluded or not. Conclusions: .Our results have implications onstudies of nucleosynthetic processes and on Galactic chemical evolutionmodels. When applying our calculated corrections to recent observationaldata, the upturn in [C/O] at low metallicity might still be present(thus apparently still necessitating contributions from massive Pop. IIIstars for the carbon production), but at a lower level and possibly witha rather shallow trend of -0.2 dex/dex below [O/H] -1.
| 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.
| Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey
A project of a spectroscopic survey of Galactic structure and evolutionwith a Large sky Area Multi-Object fiber Spectroscopic Telescope(LAMOST) is presented. The spectroscopic survey consists of twoobservational modes for various targets in our Galaxy. One is a majorsurvey of the Milky Way aimed at a systematic study of the stellarabundance and Galactic chemical evolution through low resolution (R =1000-2000) spectroscopy. Another is a follow-up observation with mediumresolution (R = 10000) spectrographs aimed at detailed studies of theselected stars with different chemical composition, kinematics anddynamics.
| Lithium Isotopic Abundances in Metal-poor Halo Stars
Very high quality spectra of 24 metal-poor halo dwarfs and subgiantshave been acquired with ESO's VLT/UVES for the purpose of determining Liisotopic abundances. The derived one-dimensional, non-LTE 7Li abundances from the Li I 670.8 nm line reveal a pronounceddependence on metallicity but with negligible scatter around this trend.Very good agreement is found between the abundances from the Li I 670.8nm line and the Li I 610.4 nm line. The estimated primordial 7Li abundance is7Li/H=(1.1-1.5)×10-10, which is a factor of3-4 lower than predicted from standard big bang nucleosynthesis with thebaryon density inferred from the cosmic microwave background.Interestingly, 6Li is detected in 9 of our 24 stars at the>=2 σ significance level. Our observations suggest theexistence of a 6Li plateau at the level oflogε6Li~0.8 however, taking into accountpredictions for 6Li destruction during the pre-main-sequenceevolution tilts the plateau such that the 6Li abundancesapparently increase with metallicity. Our most noteworthy result is thedetection of 6Li in the very metal-poor star LP 815-43. Sucha high 6Li abundance during these early Galactic epochs isvery difficult to achieve by Galactic cosmic-ray spallation andα-fusion reactions. It is concluded that both Li isotopes have apre-Galactic origin. Possible 6Li production channels includeprotogalactic shocks and late-decaying or annihilating supersymmetricparticles during the era of big bang nucleosynthesis. The presence of6Li limits the possible degree of stellar 7Lidepletion and thus sharpens the discrepancy with standard big bangnucleosynthesis.Based on observations collected at the European Southern Observatory,Paranal, Chile (observing programs 65.L-0131, 68.D-0091, and273.D-5043).
| Na, Mg and Al abundances as a population discriminant for nearby metal-poor stars
Aims.Parameters for 55 nearby metal-poor stars are determined usinghigh-resolution spectroscopy. Together with similar data taken from arecent analysis, they are used to show trends of their Galacticevolution with stellar [Fe/H] or [Mg/H] abundances. The separation ofabundance ratios between disk and halo stars is used as a basiccriterion for population membership. Methods.After carefulselection of a clean subsample free of suspected or known binaries andpeculiar stars, abundances of Mg, Na and Al are based on NLTE kineticequilibrium calculations applied to spectrum synthesis methods. Results.The relation between [Na/Mg] and [Fe/H] is a continuousenrichment through all three Galactic populations spanning a range ofvalues between a metal-poor plateau at [ Na/Mg] = -0.7 and solar values.[Al/Mg] displays a step-like difference between stars of the Galactichalo with overline[Al/Mg] -0.45 and the two disk populations withoverline[Al/Mg] +0.10. [Al/Mg] ratios, together with the [Mg/Fe]ratios, asymmetric drift velocities V, and stellar evolutionary ages,make possible the individual discrimination between stars of the thickdisk and the halo. At present, this evidence is limited by the smallnumber of stars, and by the theoretical and empirical uncertainties ofstellar age determinations, but it achieves a high significance. Conclusions.While the stellar sample is not complete with respect tospace volume, the resulting abundances indicate the necessity to revisecurrent models of chemical evolution to allow for an adequate productionof Al in early stellar generations.
| Permitted Oxygen Abundances and the Temperature Scale of Metal-poor Turnoff Stars
We use high-quality VLT/UVES published data of the permitted O I tripletand Fe II lines to determine oxygen and iron abundances in unevolved(dwarfs, turnoff, subgiants) metal-poor halo stars. The calculationshave been performed both in LTE and non-LTE (NLTE), employing effectivetemperatures obtained with the new infrared flux method (IRFM)temperature scale by Ramírez & Meléndez, and surfacegravities from Hipparcos parallaxes and theoretical isochrones. A newlist of accurate transition probabilities for Fe II lines, tied to theabsolute scale defined by laboratory measurements, has been used.Interstellar absorption has been carefully taken into account byemploying reddening maps, stellar energy distributions andStrömgren photometry. We find a plateau in the oxygen-to-iron ratioover more than 2 orders of magnitude in iron abundance(-3.2<[Fe/H]<-0.7), with a mean [O/Fe]=0.5 dex (σ=0.1 dex),independent of metallicity, temperature, and surface gravity. The flat[O/Fe] ratio is mainly due to the use of adequate NLTE corrections andthe new IRFM temperature scale, which, for metal-poor F/early G dwarfsis hotter than most Teff scales used in previous studies ofthe O I triplet. According to the new IRFM Teff scale, thetemperatures of turnoff halo stars strongly depend on metallicity, aresult that is in excellent qualitative and quantitative agreement withstellar evolution calculations, which predict that the Teffof the turnoff at [Fe/H]=-3 is about 600-700 K higher than that at[Fe/H]=-1. Recent determinations of Hα temperatures in turnoffstars are in excellent relative agreement with the new IRFMTeff scale in the metallicity range -2.7<[Fe/H]<-1,with a zero-point difference of only 61 K.
| Oxygen abundances in metal-poor subgiants as determined from [O I], O I and OH lines
The debate on the oxygen abundances of metal-poor stars has its originin contradictory results obtained using different abundance indicators.To achieve a better understanding of the problem we have acquired highquality spectra with the Ultraviolet and Visual Echelle Spectrograph atVLT, with a signal-to-noise of the order of 100 in the near ultravioletand 500 in the optical and near infrared wavelength range. Threedifferent oxygen abundance indicators, OH ultraviolet lines around 310.0nm, the [O i] line at 630.03 nm and the O i lines at 777.1-5 nm wereobserved in the spectra of 13 metal-poor subgiants with-3.0≤[Fe/H]≤-1.5. Oxygen abundances were obtained from theanalysis of these indicators which was carried out assuming localthermodynamic equilibrium and plane-parallel model atmospheres.Abundances derived from O i were corrected for departures from localthermodynamic equilibrium. Stellar parameters were computed usingT_eff-vs.-color calibrations based on the infrared flux method andBalmer line profiles, Hipparcos parallaxes and Fe II lines. [O/Fe]values derived from the forbidden line at 630.03 nm are consistent withan oxygen/iron ratio that varies linearly with [Fe/H] as[O/Fe]=-0.09(±0.08)[Fe/H]+0.36(±0.15). Values based on theO i triplet are on average 0.19±0.22 dex(s.d.) higher than thevalues based on the forbidden line while the agreement between OHultraviolet lines and the forbidden line is much better with a meandifference of the order of -0.09±0.25 dex(s.d.). In general, ourresults follow the same trend as previously published results with theexception of the ones based on OH ultraviolet lines. In that case ourresults lie below the values which gave rise to the oxygen abundancedebate for metal-poor stars.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| An Atlas of K-Line Spectra for Cool Magnetic CP Stars: The Wing-Nib Anomaly (WNA)
We present a short atlas illustrating the unusual Ca II K-line profilesin upper main-sequence stars with anomalous abundances. Slopes of theprofiles for 10 cool, magnetic chemically peculiar (CP) stars changeabruptly at the very core, forming a deep ``nib.'' The nibs show thesame or nearly the same radial velocity as the other atomic lines. Thenear wings are generally more shallow than in normal stars. In threemagnetic CP stars, the K lines are too weak to show this shape, althoughthe nibs themselves are arguably present. The Ca II H lines also showdeep nibs, but the profiles are complicated by the nearby, strongHɛ absorption. The K-line structure is nearly unchanged withphase in β CrB and α Cir. Calculations, including NLTE, showthat other possibilities in addition to chemical stratification mayyield niblike cores.
| An X-Ray Search for Compact Central Sources in Supernova Remnants. II. Six Large-Diameter SNRs
We present the second in a series of studies in which we have searchedfor undiscovered neutron stars in supernova remnants (SNRs). This paperdeals with the six largest SNRs in our sample, too large for Chandra orXMM-Newton to cover in a single pointing. These SNRs are nearby, withtypical distances of <1 kpc. We therefore used the ROSAT BrightSource Catalog and past observations in the literature to identify X-raypoint sources in and near the SNRs. Out of 54 sources, we wereimmediately able to identify optical/IR counterparts to 41 from existingdata. We obtained Chandra snapshot images of the remaining 13 sources.Of these, 10 were point sources with readily identified counterparts,two were extended, and one was not detected in the Chandra observationbut is likely a flare star. One of the extended sources may be a pulsarwind nebula, but if so it is probably not associated with the nearbySNR. We are then left with no identified neutron stars in these six SNRsdown to luminosity limits of ~1032 ergs s-1. Theselimits are generally less than the luminosities of typical neutron starsof the same ages, but are compatible with some lower luminosity sourcessuch as the neutron stars in the SNRs CTA 1 and IC 443.
| HE 1327-2326, an Unevolved Star with [Fe/H]<-5.0. I. A Comprehensive Abundance Analysis
We present the elemental abundances of HE 1327-2326, the mostiron-deficient star known, determined from a comprehensive analysis ofspectra obtained with the Subaru Telescope High Dispersion Spectrograph.HE 1327-2326 is either in its main-sequence or subgiant phase ofevolution. Its non-LTE-corrected iron abundance is [Fe/H]=-5.45, 0.2 dexlower than that of HE 0107-5240, the previously most iron-poor objectknown, and more than 1 dex lower than those of all other metal-poorstars. Both HE 1327-2326 and HE 0107-5240 exhibit extremely largeoverabundances of carbon ([C/Fe]~+4). The combination of extremely highcarbon abundance with outstandingly low iron abundance in these objectsclearly distinguishes them from other metal-poor stars. The large carbonexcesses in these two stars are not the result of a selection effect.There also exist important differences between HE 1327-2326 and HE0107-5240. While the former shows remarkable overabundances of the lightelements (N, Na, Mg, and Al), the latter shows only relatively smallexcesses of N and Na. The neutron-capture element Sr is detected in HE1327-2326, but not in HE 0107-5240 its Sr abundance is significantlyhigher than the upper limit for HE 0107-5240. The Li I λ6707line, which is detected in most metal-poor dwarfs and warm subgiantshaving the same temperature as HE 1327-2326, is not found in thisobject. The upper limit of its Li abundance [logε(Li)<1.5] isclearly lower than the Spite plateau value. These data provide newconstraints on models of nucleosynthesis processes in thefirst-generation objects that were responsible for metal enrichment atthe earliest times. We discuss possible scenarios to explain theobserved abundance patterns.Based on data collected with the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| The Oxygen Abundance of HE 1327-2326
From a newly obtained VLT/UVES spectrum, we have determined the oxygenabundance of HE 1327-2326, the most iron-poor star known to date.Ultraviolet OH lines yield one-dimensional LTE abundances of[O/Fe]OH =3.7 (subgiant case) and [O/Fe]OH=3.4(dwarf case). Using a correction of -1.0 dex to account forthree-dimensional (3D) effects on OH line formation, the abundances arelowered to [O/Fe]=2.8 and [O/Fe]=2.5, respectively, which we adopt.Without 3D corrections, the UV OH-based abundance would be indisagreement with the upper limits derived from the O I triplet lines:[O/Fe]trip<2.8 (subgiant) and [O/Fe]trip<3.0(dwarf). We also correct the previously determined carbon and nitrogenabundances for 3D effects. Knowledge of the O abundance of HE 1327-2326has implications for the interpretation of its abundance pattern. Alarge O abundance is in accordance with HE 1327-2326 being an earlyPopulation II star that formed from material chemically enriched by afirst-generation supernova. Our derived abundances, however, do notexclude other possibilities such as a Population III scenario.Based on observations collected at the European Southern Observatory,Paranal, Chile (proposal 075.D-0048).
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Chemical abundances of very metal-poor stars
High-resolution and high signal-to-noise ratio spectra of 32 verymetal-poor stars were obtained with the Coudé echellespectrograph mounted on the 2.16-m telescope at the NationalAstronomical Observatories (Xinglong, China). Equivalent widths of FeI,FeII, OI, NaI, MgI, AlI, SiI, SiII, KI, CaI, ScII, TiI, VI, CrI, MnI,NiI, CuI and BaII lines were measured. Stellar effective temperatureswere determined by colour indices. Stellar surface gravities werecalculated from Hipparcos parallaxes and stellar evolutionary tracks.Photospheric abundances of 16 elements were derived by localthermodynamical equilibrium analysis. Stellar space motions (U, V, W)and Galactic orbital parameters were calculated. Based on kinematics,sample stars were separated into dissipative collapse and accretioncomponents of halo population. The global kinematics of the twocomponents were analysed. Element abundances were discussed as functionsof metallicities. The results of oxygen and α-elements abundanceconfirmed the previous works. The [K/Fe] shows a gradual systematicincrease toward a lower metallicity, such as in the case ofα-elements. The [Ba/Fe] trend suggests that the s-processdominated Ba production at least for the metal-poor stars with[Fe/H]> -2.0.
| 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.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Libra |
| Right ascension: | 15h43m03.10s |
| Declination: | -10°56'00.6" |
| Apparent magnitude: | 7.223 |
| Distance: | 57.339 parsecs |
| Proper motion RA: | -1116.1 |
| Proper motion Dec: | -301.8 |
| B-T magnitude: | 7.771 |
| V-T magnitude: | 7.269 |
Catalogs and designations:
|
