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Diversity of multiwavelength emission bumps in the GRB 100219A afterglow
Context. Multi-wavelength observations of gamma-ray burst (GRB)afterglows provide important information about the activity of theircentral engines and their environments. In particular, the shorttimescale variability, such as bumps and/or rebrightening featuresvisible in the multi-wavelength light curves, is still poorlyunderstood. Aims: We analyze the multi-wavelength observations ofthe GRB 100219A afterglow at redshift 4.7. In particular, we attempt toidentify the physical origin of the late achromatic flares/bumpsdetected in the X-ray and optical bands. Methods: We presentground-based optical photometric data and Swift X-ray observations onGRB 100219A. We analyzed the temporal behavior of the X-ray and opticallight curves, as well as the X-ray spectra. Results: The earlyflares in the X-ray and optical light curves peak simultaneously atabout 1000 s after the burst trigger, while late achromatic bumps in theX-ray and optical bands appear at about 2 × 104 s afterthe burst trigger. These are uncommon features in the afterglowphenomenology. Considering the temporal and spectral properties, weargue that both optical and X-ray emissions come from the samemechanism. The late flares/bumps may be produced by late internal shocksfrom long-lasting activity of the central engine. An off-axis origin fora structured jet model is also discussed to interpret the bump shapes.The early optical bump can be interpreted as the afterglow onset, whilethe early X-ray flare could be caused by the internal activity. GRB100219A exploded in a dense environment as revealed by the strongattenuation of X-ray emission and the optical-to-X-ray spectral energydistribution.
| A Simple Multistage Closed-(Box+Reservoir) Model of Chemical Evolution
Simple closed-box (CB) models of chemical evolution are extended on tworespects, namely (i) simple closed-(box+reservoir) (CBR) models allowinggas outflow from the box into the reservoir (Hartwick 1976) or gasinflow into the box from the reservoir (Caimmi 2007) with rateproportional to the star formation rate, and (ii) simple multistageclosed-(box+reservoir) (MCBR) models allowing different stages ofevolution characterized by different inflow or outflow rates. Thetheoretical differential oxygen abundance distribution (TDOD) predictedby the model maintains close to a continuous broken straight line. Anapplication is made where a fictitious sample is built up from twodistinct samples of halo stars and taken as representative of the innerGalactic halo. The related empirical differential oxygen abundancedistribution (EDOD) is represented, to an acceptable extent, as acontinuous broken line for two viable [O/H]-[Fe/H] empirical relations. The slopes and the intercepts of the regression lines are determined,and then used as input parameters to MCBR models. Within the errors(mpsigma), regression line slopes correspond to a large inflow duringthe earlier stage of evolution and to low or moderate outflow during thesubsequent stages. A possible inner halo - outer (metal-poor) bulgeconnection is also briefly discussed. Quantitative results cannot beconsidered for applications to the inner Galactic halo, unless selectioneffects and disk contamination are removed from halo samples, anddiscrepancies between different oxygen abundance determination methodsare explained.
| Beryllium and Alpha-element Abundances in a Large Sample of Metal-poor Stars
The light elements, Li, Be, and B, provide tracers for many aspects ofastronomy including stellar structure, Galactic evolution, andcosmology. We have made observations of Be in 117 metal-poor starsranging in metallicity from [Fe/H] = -0.5 to -3.5 with KeckI/HIRES. Our spectra are high resolution (~42,000) and high signal tonoise (the median is 106 per pixel). We have determined the stellarparameters spectroscopically from lines of Fe I, Fe II, Ti I, and Ti II.The abundances of Be and O were derived by spectrum synthesistechniques, while abundances of Fe, Ti, and Mg were found from manyspectral line measurements. There is a linear relationship between[Fe/H] and A(Be) with a slope of +0.88 ± 0.03 over three ordersof magnitude in [Fe/H]. We find that Be is enhanced relative to Fe;[Be/Fe] is +0.40 near [Fe/H] ~-3.3 and drops to 0.0 near [Fe/H]~-1.7. For the relationship between A(Be) and [O/H], we find agradual change in slope from 0.69 ± 0.13 for the Be-poor/O-poorstars to 1.13 ± 0.10 for the Be-rich/O-rich stars. Inasmuch asthe relationship between [Fe/H] and [O/H] seems robustly linear (slope =+0.75 ± 0.03), we conclude that the slope change in Be versus Ois due to the Be abundance. Much of the Be would have been formed in thevicinity of Type II supernova (SN II) in the early history of the Galaxyand by Galactic cosmic-ray (GCR) spallation in the later eras. AlthoughBe is a by-product of CNO, we have used Ti and Mg abundances asalpha-element surrogates for O in part because O abundances are rathersensitive to both stellar temperature and surface gravity. We find thatA(Be) tracks [Ti/H] very well with a slope of 1.00 ± 0.04. Italso tracks [Mg/H] very well with a slope of 0.88 ± 0.03. We havekinematic information on 114 stars in our sample and they divide equallyinto dissipative and accretive stars. Almost the full range of [Fe/H]and [O/H] is covered in each group. There are distinct differences inthe relationships of A(Be) and [Fe/H] and of A(Be) and [O/H] for thedissipative and the accretive stars. It is likely that the formation ofBe in the accretive stars was primarily in the vicinity of SN II, whilethe Be in the dissipative stars was primarily formed by GCR spallation.We find that Be is not as good a cosmochronometer as Fe. We have found aspread in A(Be) that is valid at the 4? level between [O/H] =-0.5 and -1.0, which corresponds to -0.9 and-1.6 in [Fe/H].
| The Stellar Abundances for Galactic Archaeology (SAGA) data base - II. Implications for mixing and nucleosynthesis in extremely metal-poor stars and chemical enrichment of the Galaxy
We discuss the characteristics of known extremely metal-poor (EMP) starsin the Galaxy using the Stellar Abundances for Galactic Archaeology(SAGA) data base. We find the transition of the initial mass function tobe at [Fe/H]˜-2 from the viewpoint of the distribution of carbonabundance and the frequency of carbon-enhanced stars. Analyses ofcarbon-enhanced stars in our sample suggest that nucleosynthesis inasymptotic giant branch (AGB) stars can contribute to carbon enrichmentin a different way depending on whether the metallicity is above orbelow [Fe/H]˜-2.5, which is consistent with the current models ofstellar evolution at low metallicity. For observed EMP stars, we confirmthat some, though not all, observed stars might have undergone at leasttwo types of extra mixing to change their surface abundances. One is thedepletion of lithium abundance during the early phase of the red giantbranch; the other is a decrease of the C/N ratio by one order ofmagnitude during the red giant branch phase. Observed small scatters ofabundances for ?-elements and iron-group elements suggest that thechemical enrichment of our Galaxy takes place in a well-mixedinterstellar medium. The abundance trends of ?-elements are highlycorrelated with each other including ?-enhanced and depletedstars, while the abundances of iron-group elements are subject todifferent slopes relative to the iron abundance. This implies that thesupernova yields of ?-elements are almost independent of mass andmetallicity, while those of iron-group elements have a metallicitydependence or mass dependence on the variable initial mass function. Theoccurrence of the hot-bottom burning for M? 5 M&sun; isconsistent with an initial mass function of the Galaxy peaked at˜10-12 M&sun;, compatible with the statistics ofcarbon-enhanced stars with and without s-process element enhancement andnitrogen-enhanced stars. For s-process elements, we find not only apositive correlation between carbon and s-process element abundances,but also an increasing slope of the abundance ratio between them withincreasing mass number of s-process elements. The dominant site of thes-process is still an open question because none of the known mechanismsfor the s-process is able to account for this observed correlation. Inspite of the evidence of AGB evolution in observed abundances of EMPstars, any evidence of binary mass transfer is elusive by pursuing theeffect of dilution in the convective envelope. We find the dependence ofsulphur and vanadium abundances on the effective temperatures, inaddition to the previously reported trends for silicon, scandium,titanium, chromium and cobalt.
| The Hamburg/ESO R-process Enhanced Star survey (HERES). VI. The Galactic chemical evolution of silicon
Aims: To obtain detailed silicon abundances of metal-poor stars,we aim to explore the correlation between the abundance ratios and thestellar parameters and the chemical enrichment of the interstellarmedium (ISM). Methods: We determined the silicon abundances of253 metal-poor stars in the metallicity range -4 < [Fe/H] < -1.5,based on non-local thermodynamic equilibrium (NLTE) line formationcalculations of neutral silicon and high-resolution spectra obtainedwith VLT-UT2/UVES. Results: The Teff dependence of[Si/Fe] noticed in previous investigations is diminished in ourabundance analysis owing to the inclusion of NLTE effects. An increasingslope of [Si/Fe] towards decreasing metallicity is present in ourresults, in agreement with Galactic chemical evolution models. Intrinsicscatter of [Si/Fe] in our sample is small. We identified two dwarfs with[Si/Fe] ~ +1.0: HE 0131-3953, and HE 1430-1123. These main-sequenceturnoff stars are also carbon-enhanced. They may have been pre-enrichedby sub-luminous supernovae. Conclusions: The small intrinsicscatter of [Si/Fe] in our sample may imply that these stars formed in aregion where the yields of type II supernovae were mixed into a largevolume, or that the formation of these stars was strongly clustered,even if the ISM was enriched by single SNa II in a small mixing volume.Based on observations collected at the European Southern Observatory,Paranal, Chile (Proposal numbers 170.D-0010, and 280.D-5011).Table 3 isonly available in electronic form at http://www.aanda.org
| Multiplicity and Period Distribution of Population II Field Stars in Solar Vicinity
We examine a sample of 223 F, G, and early K metal-poor subdwarfs ([m/H]< -1) with high proper motions (?>0farcs2yr-1) at distances of up to 250 pc from the Sun. Bymeans of our own speckle interferometric observations conducted on the 6m Bolshoi Azimuthal Telescope of the Special Astrophysical Observatoryof the Russian Academy of Sciences, and the spectroscopic and visualdata taken from the literature, we determine the frequency of binary andmultiple systems in this sample. The ratio of single, binary, triple,and quadruple systems among 221 primary components of the sample is147:64:9:1. We show that the distribution of orbital periods of binaryand multiple subdwarfs is asymmetric in the range of up to P =1010 days, and has a maximum at P =102-103 days, which differs from the distribution,obtained for the thin disk G dwarfs. We estimated the number ofundetected companions in our sample. Comparing the frequency of binarysubdwarfs in the field and in the globular clusters, we show that theprocess of halo field star formation by the means of destruction ofglobular clusters is very unlikely in our Galaxy. We discuss themultiplicity of old metal-poor stars in nearby stellar streams.
| Cu I resonance lines in turn-off stars of NGC 6752 and NGC 6397. Effects of granulation from CO5BOLD models
Context. Copper is an element whose interesting evolution withmetallicity is not fully understood. Observations of copper abundancesrely on a very limited number of lines, the strongest are the Cu I linesof Mult. 1 at 324.7 nm and 327.3 nm which can be measured even atextremely low metallicities. Aims: We investigate the quality ofthese lines as abundance indicators. Methods: We measure theselines in two turn-off (TO) stars in the Globular Cluster NGC 6752 andtwo TO stars in the Globular Cluster NGC 6397 and derive abundances with3D hydrodynamical model atmospheres computed with the CO5BOLD code.These abundances are compared to the Cu abundances measured in giantstars of the same clusters, using the lines of Mult. 2 at 510.5 nm and578.2 nm. Results: The abundances derived from the lines of Mult.1 in TO stars differ from the abundances of giants of the same clusters.This is true both using CO5BOLD models and using traditional 1D modelatmospheres. The LTE 3D corrections for TO stars are large, while theyare small for giant stars. Conclusions: The Cu I resonance linesof Mult. 1 are not reliable abundance indicators. It is likely thatdepartures from LTE should be taken into account to properly describethese lines, although it is not clear if these alone can account for theobservations. An investigation of these departures is indeed encouragedfor both dwarfs and giants. Our recommendation to those interested inthe study of the evolution of copper abundances is to rely on themeasurements in giants, based on the lines of Mult. 2. We caution,however, that NLTE studies may imply a revision in all the Cuabundances, both in dwarfs and giants.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (Programmes 71.D-0155, 75.D-0807, 76.B-0133).
| Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy
Aims: We investigate statistical equilibrium of Cr in theatmospheres of late-type stars to ascertain whether the systematicabundance discrepancy between Cr I and Cr II lines, as often found inprevious work, is due to deviations from local thermodynamic equilibrium(LTE). Furthermore, we attempt to interpret the Non-LTE (NLTE) trend of[Cr/Fe] with [Fe/H] using chemical evolution models for the solarneighborhood. Methods: NLTE calculations are performed for themodel of the Cr atom, comprising 340 levels and 6806 transitions intotal. We use the quantum-mechanical photoionization cross-sections ofNahar (2009) and investigate the sensitivity of the model to uncertaincross-sections for H I collisions. NLTE line formation is performed forthe MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with-3.2 < [Fe/H] < -0.5, and Cr abundances are derived by comparingthe synthetic and observed flux spectra. Results: We achieve goodionization equilibrium of Cr for models with different stellarparameters, if inelastic collisions with H I atoms are neglected. Thesolar NLTE abundance based on Cr I lines is 5.74 dex with ? = 0.05dex, which is ~0.1 dex higher than the LTE abundance. For the metal-poorstars, the NLTE abundance corrections to Cr I lines range from +0.3 to+0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range ofmetallicities analyzed here, which is consistent with current views onthe production of these iron peak elements in supernovae. Conclusions: The tendency of Cr to become deficient with respect to Fein metal-poor stars is an artifact caused by the neglect of NLTE effectsin the line formation of Cr i, and has no relation to any peculiarphysical conditions in the Galactic ISM or deficiencies ofnucleosynthesis theory.Table 4 is only available in electronic form at http://www.aanda.org
| Galactic evolution of oxygen. OH lines in 3D hydrodynamical model atmospheres
Context. Oxygen is the third most common element in the Universe. Themeasurement of oxygen lines in metal-poor unevolved stars, in particularnear-UV OH lines, can provide invaluable information about theproperties of the Early Galaxy. Aims: Near-UV OH lines constitutean important tool to derive oxygen abundances in metal-poor dwarf stars.Therefore, it is important to correctly model the line formation of OHlines, especially in metal-poor stars, where 3D hydrodynamical modelscommonly predict cooler temperatures than plane-parallel hydrostaticmodels in the upper photosphere. Methods: We have made use of agrid of 52 3D hydrodynamical model atmospheres for dwarf stars computedwith the code CO5BOLD, extracted from the more extendedCIFIST grid. The 52 models cover the effective temperature range5000-6500 K, the surface gravity range 3.5-4.5 and the metallicity range-3 < [Fe/H] < 0. Results: We determine 3D-LTE abundancecorrections in all 52 3D models for several OH lines and ion{Fe}{i}lines of different excitation potentials. These 3D-LTE corrections aregenerally negative and reach values of roughly -1 dex (for the OH 3167with excitation potential of approximately 1 eV) for the highertemperatures and surface gravities. Conclusions: We apply these3D-LTE corrections to the individual O abundances derived from OH linesof a sample the metal-poor dwarf stars reported in Israelian et al.(1998, ApJ, 507, 805), Israelian et al. (2001, ApJ, 551, 833) andBoesgaard et al. (1999, AJ, 117, 492) by interpolating the stellarparameters of the dwarfs in the grid of 3D-LTE corrections. The new3D-LTE [O/Fe] ratio still keeps a similar trend as the 1D-LTE, i.e.,increasing towards lower [Fe/H] values. We applied 1D-NLTE correctionsto 3D ion{Fe}{i} abundances and still see an increasing [O/Fe] ratiotowards lower metallicites. However, the Galactic [O/Fe] ratio must berevisited once 3D-NLTE corrections become available for OH and Fe linesfor a grid of 3D hydrodynamical model atmospheres.
| Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion
We present NLTE Li abundances for 88 stars in the metallicity range -3.5< [Fe/H] < -1.0. The effective temperatures are based on theinfrared flux method with improved E(B-V) values obtained mostly frominterstellar Na I D lines. The Li abundances were derived through MARCSmodels and high-quality UVES+VLT, HIRES+Keck and FIES+NOT spectra, andcomplemented with reliable equivalent widths from the literature. Theless-depleted stars with [Fe/H] < -2.5 and [Fe/H] > -2.5 fall intotwo well-defined plateaus of ALi = 2.18 (? = 0.04) andALi = 2.27 (? = 0.05), respectively. We show that thetwo plateaus are flat, unlike previous claims for a steep monotonicdecrease in Li abundances with decreasing metallicities. At allmetallicities we uncover a fine-structure in the Li abundances of Spiteplateau stars, which we trace to Li depletion that depends on bothmetallicity and mass. Models including atomic diffusion and turbulentmixing seem to reproduce the observed Li depletion assuming a primordialLi abundance ALi = 2.64, which agrees well with currentpredictions (ALi = 2.72) from standard Big Bangnucleosynthesis. Adopting the Kurucz overshooting model atmospheresincreases the Li abundance by +0.08 dex to ALi = 2.72, whichperfectly agrees with BBN+WMAP.Based in part on observations obtained at the W. M. Keck Observatory,the Nordic Optical Telescope on La Palma, and on data from theHIRES/Keck archive and the European Southern Observatory ESO/ST-ECFScience Archive Facility.Table 1 is only available in electronic form athttp://www.aanda.org
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| Stellar archaeology: Exploring the Universe with metal-poor stars
The abundance patterns of the most metal-poor stars in the Galactic haloand small dwarf galaxies provide us with a wealth of information aboutthe early Universe. In particular, these old survivors allow us to studythe nature of the first stars and supernovae, the relevantnucleosynthesis processes responsible for the formation and evolution ofthe elements, early star- and galaxy formation processes, as well as theassembly process of the stellar halo from dwarf galaxies a long timeago. This review presents the current state of the field of ``stellararchaeology'' - the diverse use of metal-poor stars to explore thehigh-redshift Universe and its constituents. In particular, theconditions for early star formation are discussed, how these ultimatelyled to a chemical evolution, and what the role of the most iron-poorstars is for learning about Population III supernovae yields. Rapidneutron-capture signatures found in metal-poor stars can be used toobtain stellar ages, but also to constrain this complex nucleosynthesisprocess with observational measurements. Moreover, chemical abundancesof extremely metal-poor stars in different types of dwarf galaxies canbe used to infer details on the formation scenario of the halo and therole of dwarf galaxies as Galactic building blocks. I conclude with anoutlook as to where this field may be heading within the next decade. Atable of ˜ 1000 metal-poor stars and their abundances ascollected from the literature is provided in electronic format.
| A Non-local Thermodynamic Equilibrium Analysis of Boron Abundances in Metal-poor Stars
The non-local thermodynamic equilibrium (NLTE) line formation of neutralboron in the atmospheres of cool stars are investigated. Our resultsconfirm that NLTE effects for the B I resonance lines, which are due toa combination of overionization and optical pumping effects, are mostimportant for hot, metal-poor, and low-gravity stars; however, theamplitude of departures from local thermodynamic equilibrium (LTE) foundby this work is smaller than that of previous studies. In addition, ourcalculation shows that the line formation of B I will get closer to LTEif the strength of collisions with neutral hydrogen increases, which iscontrary to the result of previous studies. The NLTE line formationresults are applied to the determination of boron abundances for asample of 16 metal-poor stars with the method of spectrum synthesis ofthe B I 2497 Å resonance lines using the archived HST/GHRSspectra. Beryllium and oxygen abundances are also determined for thesestars with the published equivalent widths of the Be II 3131 Åresonance and O I 7774 Å triplet lines, respectively. Theabundances of the nine stars which are not depleted in Be or B showthat, no matter what the strength of collisions with neutral hydrogenmay be, both Be and B increase with O quasilinearly in the logarithmicplane, which confirms the conclusions that Be and B are mainly producedby the primary process in the early Galaxy. The most noteworthy resultof this work is that B increases with Fe or O at a very similar speedas, or a bit faster than, Be does, which is in accord with thetheoretical models. The B/Be ratios remain almost constant over themetallicity range investigated here. Our average B/Be ratio falls in theinterval [13 ± 4, 17 ± 4], which is consistent with thepredictions of the spallation process. The contribution of B from the?-process may be required if the 11B/10Bisotopic ratios in metal-poor stars are the same as the meteoric value.An accurate measurement of the 11B/10B ratios inmetal-poor stars is crucial to understanding the production history ofboron.Based on observations made with ESO telescopes and NASA/ESA Hubble SpaceTelescope obtained from the ESO/ST-ECF Science Archive Facility; basedon spectral data retrieved from the ELODIE archive at Observatoire deHaute-Provence (OHP).
| StarCAT: A Catalog of Space Telescope Imaging Spectrograph Ultraviolet Echelle Spectra of Stars
StarCAT is a catalog of high resolution ultraviolet spectra of objectsclassified as "stars," recorded by Space Telescope Imaging Spectrograph(STIS) during its initial seven years of operations (1997-2004). StarCATis based on 3184 echelle observations of 545 distinct targets, with atotal exposure duration of 5.2 Ms. For many of the objects, broadultraviolet coverage has been achieved by splicing echellegrams taken intwo or more FUV (1150-1700 Å) and/or NUV (1600-3100 Å)settings. In cases of multiple pointings on conspicuously variablesources, spectra were separated into independent epochs. Otherwise,different epochs were combined to enhance the signal-to-noise ratio(S/N). A post-facto correction to the {\sf calstis} pipeline data setscompensated for subtle wavelength distortions identified in a previousstudy of the STIS calibration lamps. An internal "fluxing" procedureyielded coherent spectral energy distributions (SEDs) for objects withbroadly overlapping wavelength coverage. The best StarCAT materialachieves 300 m s-1 internal velocity precision;absolute accuracy at the 1 km s-1 level; photometricaccuracy of order 4%; and relative flux precision several times better(limited mainly by knowledge of SEDs of UV standard stars). WhileStarCAT represents a milestone in the large-scale post-processing ofSTIS echellegrams, a number of potential improvements in the underlying"final" pipeline are identified.
| An absolutely calibrated Teff scale from the infrared flux method. Dwarfs and subgiants
Various effective temperature scales have been proposed over the years.Despite much work and the high internal precision usually achieved,systematic differences of order 100 K (or more) among various scales arestill present. We present an investigation based on the infrared fluxmethod aimed at assessing the source of such discrepancies and pin downtheir origin. We break the impasse among different scales by using alarge set of solar twins, stars which are spectroscopically andphotometrically identical to the Sun, to set the absolute zero point ofthe effective temperature scale to within few degrees. Our newlycalibrated, accurate and precise temperature scale applies to dwarfs andsubgiants, from super-solar metallicities to the most metal-poor starscurrently known. At solar metallicities our results validatespectroscopic effective temperature scales, whereas for [Fe/H]? -2.5our temperatures are roughly 100 K hotter than those determined frommodel fits to the Balmer lines and 200 K hotter than those obtained fromthe excitation equilibrium of Fe lines. Empirical bolometric correctionsand useful relations linking photometric indices to effectivetemperatures and angular diameters have been derived. Our results takefull advantage of the high accuracy reached in absolute calibration inrecent years and are further validated by interferometric angulardiameters and space based spectrophotometry over a wide range ofeffective temperatures and metallicities.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/512/A54
| Chemical Abundances of Outer Halo Stars in the Milky Way
We present the chemical abundances of 57 metal-poor ([Fe/H] 5 kpc above andbelow the Galactic plane. High-resolution (R ˜ 50000-55000), highsignal-to-noise (S/N > 100) spectra for the sample stars obtainedwith Subaru/HDS were used to derive the chemical abundances of Na, Mg,Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, and Ba with an LTE abundance analysiscode. The resulting abundance data were combined with those presented inthe literature that mostly targeted at smaller Zmax stars,and both data were used to investigate any systematic trends in detailedabundance patterns depending on their kinematics. It was shown that, inthe metallicity range of ?2 < [Fe/H] < ?1, the [Mg/Fe]ratios for stars with Zmax > 5 kpc are systematicallylower (˜0.1 dex) than those with a smaller Zmax. Forthis metallicity range, a modest degree of depression in the [Si/Fe] andthe [Ca/Fe] ratios was also observed. This result of lower [?/Fe]for the assumed outer halo stars is consistent with previous studiesthat found a signature of lower [?/Fe] ratios for stars withextreme kinematics. The distribution of the [Mg/Fe] ratios for the outerhalo stars partly overlaps with that for stars belonging to the MilkyWay dwarf satellites in the metallicity interval of ?2 < [Fe/H]< ?1 and spans a range intermediate between the distributionsfor the inner halo stars and the stars belonging to the satellites. Ourresults confirm the inhomogeneous nature of the chemical abundanceswithin the Milky Way stellar halo, depending on the kinematic propertiesof the constituent stars, as suggested by earlier studies. Possibleimplications for the formation of the Milky Way halo and its relevanceto the suggested dual nature of the halo are discussed.
| NLTE analysis of CoI/CoII lines in spectra of cool stars with new laboratory hyperfine splitting constants
The analysis of stellar abundances for odd-Z Fe-peak elements requiresaccurate non-local thermodynamic equilibrium (NLTE) modelling ofspectral lines fully taking into account the hyperfine structure (HFS)splitting of lines. Here, we investigate the statistical equilibrium ofCo in the atmospheres of cool stars and the influence of NLTE and HFS onthe formation of Co lines and abundances. Significant departures fromLTE level populations are found for CoI number densities of excitedstates in CoII also differ from LTE at low metallicity. The NLTE levelpopulations are used to determine the abundance of Co in solarphotosphere, log? = 4.95 +/- 0.04dex, which is in agreement withthat in CI meteorites within the combined uncertainties. The spectrallines of CoI were calculated using the results of recent measurements ofhyperfine interaction constants by UV Fourier transform spectrometry.For CoII, the first laboratory measurements of HFS A and B factors wereperformed. These highly accurate A factor measurements (errors of theorder of 3-7 per cent) allow, for the first time, reliable modelling ofCoII lines in the solar and stellar spectra and, thus, a test of theCoI/CoII ionization equilibrium in stellar atmospheres. A differentialabundance analysis of Co is carried out for 18 stars in the metallicityrange -3.12 < [Fe/H] < 0. The abundances are derived by the methodof spectrum synthesis. At low [Fe/H], NLTE abundance corrections for CoIlines are as large as +0.6,..., + 0.8dex. Thus, LTE abundances of Co inmetal-poor stars are severely underestimated. The stellar NLTEabundances determined from the single UV line of CoII are lower by~0.5-0.6dex. The discrepancy might be attributed to possible blends thathave not been accounted for in the solar CoII line and its erroneousoscillator strength. The increasing [Co/Fe] trend in metal-poor stars,as calculated from the CoI lines under NLTE, can be explained if Co isoverproduced relative to Fe in massive stars. The models of Galacticchemical evolution are wholly inadequate to describe this trendsuggesting that the problem is in supernova yields.Based on observations collected at the European Southern Observatory,Chile, 67.D-0086A, and the Calar Alto Observatory, Spain.E-mail: mbergema@mpa-garching.mpg.de (MB); j.pickering@imperial.ac.uk(JCP); gehren@usm.lmu.de (TG)
| A holistic approach to carbon-enhanced metal-poor stars
Context. Carbon-enhanced metal-poor (CEMP) stars are known to haveproperties that reflect the nucleosynthesis of the first low- andintermediate-mass stars, because most have been polluted by anow-extinct AGB star. Aims: By considering abundances in thevarious CEMP subclasses separately, we try to derive parameters (such asmetallicity, mass, temperature, and neutron source) characterising AGBnucleosynthesis from the specific signatures imprinted on theabundances, and separate them from the impact of thermohaline mixing,first dredge-up, and dilution associated with the mass transfer from thecompanion. Methods: To place CEMP stars in a broader context, wecollect abundances for about 180 stars of various metallicities (fromsolar to [Fe/H] =-4), luminosity classes (dwarfs and giants), andabundance patterns (e.g. C-rich and poor, Ba-rich and poor), from bothour own sample and the literature. Results: We first show thatthere are CEMP stars that share the properties of CEMP-s stars andCEMP-no stars (which we refer to as CEMP-low-s stars). We also show thatthere is a strong correlation between Ba and C abundances in the s-onlyCEMP stars. This represents a strong detection of the operation of the13C neutron source in low-mass AGB stars. For the CEMP-rsstars (seemingly enriched with elements from both the s- andr-processes), the correlation of the N abundances with abundances ofheavy elements from the 2nd and 3rd s-process peaks bears instead thesignature of the 22Ne neutron source. Since CEMP-rs starsalso exhibit O and Mg enhancements, we conclude that extremely hotconditions prevailed during the thermal pulses of the contaminating AGBstars. We also note that abundances are not affected by the evolution ofthe CEMP-rs star itself (especially by the first dredge-up). Thisimplies that mixing must have occurred while the star was on the mainsequence, and that a large amount of matter must have been accreted soas to trigger thermohaline mixing. Finally, we argue that most CEMP-nostars (with neutron-capture element abundances comparable to non-CEMPstars) are likely the extremely metal-poor counterparts of CEMPneutron-capture-rich stars. We also show that the C enhancement inCEMP-no stars declines with metallicity at extremely low metallicity([Fe/H] < -3.2). This trend is not predicted by any of the currentAGB models.Tables 1-4 are only available in electronic form at http://www.aanda.org
| The Lick AGN Monitoring Project: Photometric Light Curves and Optical Variability Characteristics
The Lick AGN Monitoring Project targeted 13 nearby Seyfert 1 galaxieswith the intent of measuring the masses of their central black holesusing reverberation mapping. The sample includes 12 galaxies selected tohave black holes with masses roughly in the range106-107 M sun, as well as thewell-studied active galactic nucleus (AGN) NGC 5548. In conjunction witha spectroscopic monitoring campaign, we obtained broadband B and Vimages on most nights from 2008 February through 2008 May. The imagingobservations were carried out by four telescopes: the 0.76 m KatzmanAutomatic Imaging Telescope, the 2 m Multicolor Active Galactic NucleiMonitoring telescope, the Palomar 60 inch (1.5 m) telescope, and the0.80 m Tenagra II telescope. Having well-sampled light curves over thecourse of a few months is useful for obtaining the broad-linereverberation lag and black hole mass, and also allows us to examine thecharacteristics of the continuum variability. In this paper, we discussthe observational methods and the photometric measurements, and presentthe AGN continuum light curves. We measure various variabilitycharacteristics of each of the light curves. We do not detect anyevidence for a time lag between the B- and V-band variations, and we donot find significant color variations for the AGNs in our sample.
| The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo
We determine the metallicity distribution function (MDF) of the Galactichalo by means of a sample of 1638 metal-poor stars selected from theHamburg/ESO objective-prism survey (HES). The sample was corrected forminor biases introduced by the strategy for spectroscopic follow-upobservations of the metal-poor candidates, namely “best andbrightest stars first”. Comparison of the metallicities [Fe/H] ofthe stars determined from moderate-resolution (i.e., R˜ 2000)follow-up spectra with results derived from abundance analyses based onhigh-resolution spectra (i.e., R>20 000) shows that the [Fe/H]estimates used for the determination of the halo MDF are accurate towithin 0.3 dex, once highly C-rich stars are eliminated. We determinedthe selection function of the HES, which must be taken into account fora proper comparison between the HES MDF with MDFs of other stellarpopulations or those predicted by models of Galactic chemical evolution.The latter show a reasonable agreement with the overall shape of the HESMDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007)with a critical metallicity for low-mass star formation ofZ_cr=10-3.4~Z&sun; reproduces the sharp drop at[Fe/H] ˜ -3.6 present in the HES MDF. Although currently about tenstars at [Fe/H] < -3.6 are known, the evidence for the existence of atail of the halo MDF extending to [Fe/H] ˜ -5.5 is weak from thesample considered in this paper, because it only includes two stars[Fe/H] < -3.6. Therefore, a comparison with theoretical models has toawait larger statistically complete and unbiased samples. A comparisonof the MDF of Galactic globular clusters and of dSph satellites to theGalaxy shows qualitative agreement with the halo MDF, derived from theHES, once the selection function of the latter is included. However,statistical tests show that the differences between these are stillhighly significant.Based on observations collected at Las Campanas Observatory, PalomarObservatory, Siding Spring Observatory, and the European SouthernObservatory (Proposal IDs 69.D-0130, 170.D-0010, 073.D-0555, and081.D-0596).
| Beryllium, Oxygen, and Iron Abundances in Extremely Metal-Deficient Stars
The abundance of beryllium in the oldest, most metal-poor stars acts asa probe of early star formation and Galactic chemical evolution. We haveanalyzed high-resolution, high signal-to-noise ratio Keck/HIRES spectraof 24 stars with [Fe/H] from -2.3 to -3.5 in order todetermine the history of Be abundance and explore the possibility of aBe plateau. We have determined stellar parameters of our samplespectroscopically, using equivalent widths of Fe I, Ti I, and Ti IIlines. We have determined O abundances from three OH features whichoccur in the same spectral region; this region is relatively uncrowdedand has a well determined continuum in these very/extremely metal-poorstars. We have supplemented this sample with reanalyzed spectra of 25stars from previous observations so that our total sample ranges in[Fe/H] from -0.5 to -3.5. Our results indicate that therelationship between Be and [Fe/H] continues to lower metallicities witha slope of 0.92 ± 0.04. Although there is no indication of aplateau with constant Be abundance, the four lowest metallicity stars(below [Fe/H] of -3.0) do show a Be enhancement relative to Fe atthe 1σ level. A single relationship between Be and [O/H] has aslope of 1.21 ± 0.08, but there is also a good fit with twoslopes: 1.59 above [O/H] = -1.6 and 0.74 for stars with [O/H]below -1.6. This change in slope could result from a change in thedominant production mechanism for Be. In the era of the formation of themore metal-poor stars, Be would be formed by acceleration of CNO atomsin the vicinity of SN II and in later times by high-energy cosmic-raysbombarding CNO in the ambient interstellar gas. We find an excellentcorrelation between [Fe/H] and [O/H] and show that [O/Fe] is near +1.0at [Fe/H] = -3.5 declining to 0 at [Fe/H] = 0.
| Statistical equilibrium of silicon in the atmospheres of metal-poor stars
Aims: The statistical equilibrium of neutral and ionized silicon in theatmospheres of metal-poor stars is discussed. Non-local thermodynamicequilibrium effects (NLTE) are investigated and the silicon abundancesin metal-poor stars determined. Methods: We have used highresolution, high signal to noise ratio spectra from the UVESspectragraph at the ESO VLT telescope. Line formation calculations of Sii and Si ii in the atmospheres of metal-poor stars are presented foratomic models of silicon including 174 terms and 1132 line transitions.Recent improved calculations of Si i and Si ii photoionizationcross-sections are taken into account, and the influence of thefree-free quasi-molecular absorption in the Ly? wing isinvestigated by comparing theoretical and observed fluxes of metal-poorstars. All abundance results are derived from LTE and NLTE statisticalequilibrium calculations and spectrum synthesis methods. Results:It is found that the extreme ultraviolet radiation is very important formetal-poor stars, especially for the high temperature, very metal-poorstars. The radiative bound-free cross-sections also play a veryimportant role for these stars. Conclusions: NLTE effects for Siare found to be important for metal-poor stars, in particular for warmmetal-poor stars. It is found that these effects depend on thetemperature. For warm metal-poor stars, the NLTE abundance correctionreaches ~0.2 dex relative to standard LTE calculations. Our resultsindicate that Si is overabundant for metal-poor stars.Based on observations obtained in the frame of the ESO programme ID165.N-0276(A).
| Lithium Abundances of Extremely Metal-Poor Turnoff Stars
We have determined Li abundances for eleven metal-poor turnoff stars,among which eight have [Fe/H] <-3, based on LTE analyses ofhigh-resolution spectra obtained with the High Dispersion Spectrographon the Subaru Telescope. The Li abundances for four of these eight starsare determined for the first time by this study. Effective temperaturesare determined by a profile analysis of H? and H?. Whileseven stars have Li abundances as high as the Spite Plateau value, theremaining four objects with [Fe/H] <-3 have A(Li) =log (Li/H)+ 12lsim 2.0, confirming the existence of extremely metal-poor (EMP) turnoffstars having low Li abundances, as reported by previous work. Theaverage of the Li abundances for stars with [Fe/H]<-3 is lower by 0.2dex than that of the stars with higher metallicity. No clear constrainton the metallicity dependence or scatter of the Li abundances is derivedfrom our measurements for the stars with [Fe/H]<-3. Correlations ofthe Li abundance with effective temperatures, with abundances of Na, Mg,and Sr, and with the kinematical properties are investigated, but noclear correlation is seen in the EMP star sample.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity
Aims: A detailed study is presented, including estimates of the impacton elemental abundance analysis, of the non-local thermodynamicequilibrium (non-LTE) formation of the high-excitation neutral oxygen777 nm triplet in model atmospheres representative of stars withspectral types F to K. Methods: We have applied the statisticalequilibrium code MULTI to a number of plane-parallel MARCS atmosphericmodels covering late-type stars (4500 ? T_eff ? 6500 K, 2 ? logg ? 5 [cgs], and -3.5 ? [Fe/H] ? 0). The atomic model employedincludes, in particular, recent quantum-mechanical electron collisiondata. Results: We confirm that the O i triplet lines form undernon-LTE conditions in late-type stars, suffering negative abundancecorrections with respect to LTE. At solar metallicity, the non-LTEeffect, mainly attributed in previous studies to photon losses in thetriplet itself, is also driven by an additional significant contributionfrom line opacity. At low metallicity, the very pronounced departuresfrom LTE are due to overpopulation of the lower level (3s ^5S^o) of thetransition. Large line opacity stems from triplet-quintet intersystemelectron collisions, a form of coupling previously not considered orseriously underestimated. The non-LTE effects generally become severefor models (both giants and dwarfs) with higher T_eff. Interestingly, inmetal-poor turn-off stars, the negative non-LTE abundance correctionstend to rapidly become more severe towards lower metallicity. Whenneglecting H collisions, they amount to as much as |?log?_O| ~ 0.9 dex and ~1.2 dex, respectively at [Fe/H] = -3 and[Fe/H] = -3.5. Even when such collisions are included, the LTE abundanceremains a serious overestimate, correspondingly by |?log?_O| ~ 0.5 dex and ~0.9 dex at such low metallicities. Althoughthe poorly known inelastic hydrogen collisions thus remain an importantuncertainty, the large metallicity-dependent non-LTE effects seem topoint to a resulting “low” (compared to LTE) [O/Fe] inmetal-poor halo stars. Conclusions: Our results may be importantin solving the long-standing [O/Fe] debate. When applying the derivednon-LTE corrections, the LTE oxygen abundance inferred from the 777 nmpermitted triplet will be decreased substantially at low metallicity. Ifthe classical Drawin formula is employed for O+H collisions, the derived[O/Fe] trend becomes almost flat below [Fe/H] ~ -1, in better agreementwith recent literature estimates generally obtained from other oxygenabundance indicators. A value of [O/Fe] ? +0.5 may therefore beappropriate, as suggested by standard theoretical models of type IIsupernovae nucleosynthetic yields. If neglecting impacts with H atomsinstead, [O/Fe] decreases towards lower [Fe/H], which would open newquestions. Our tests using ATLAS model atmospheres show that, thoughnon-LTE corrections for metal-poor dwarfs are smaller (by ~0.2 dex whenadopting efficient H collisions) than in the MARCS case, our mainconclusions are preserved, and that the LTE approach tends to seriouslyoverestimate the O abundance at low metallicity. However, in order tofinally reach consistency between oxygen abundances from the differentavailable spectral features, it is of high priority to reduce the largeuncertainty regarding H collisions, to undertake a full investigation ofthe interplay of non-LTE and 3D effects, and to clarify the issue of thetemperature scale at low metallicity.
| The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars
Aims: We present new measurements of the abundances of carbon and oxygenderived from high-excitation C i and O i absorption lines in metal-poorhalo stars, with the aim of clarifying the main sources of these twoelements in the early stages of the chemical enrichment of the Galaxy.Methods: We target 15 new stars compared to our previous study,with an emphasis on additional C/O determinations in the crucialmetallicity range -3 ⪉ [Fe/H]⪉ -2. The stellar effectivetemperatures were estimated from the profile of the Hβ line.Departures from local thermodynamic equilibrium were accounted for inthe line formation for both carbon and oxygen. The non-LTE effects arevery strong at the lowest metallicities but, contrary to what hassometimes been assumed in the past due to a simplified assessment, ofdifferent degrees for the two elements. In addition, for the 28 starswith [Fe/H] < -1 previously analysed, stellar parameters werere-derived and non-LTE corrections applied in the same fashion as forthe rest of our sample, giving consistent abundances for 43 halo starsin total. Results: The new observations and non-LTE calculationsstrengthen previous suggestions of an upturn in C/O towards lowermetallicity (particularly for [O/H] ⪉ -2). The C/O values derivedfor these very metal-poor stars are, however, sensitive to excitationvia the still poorly quantified inelastic H collisions. While these donot significantly affect the non-LTE results for C i, they greatlymodify the O i outcome. Adopting the H collisional cross-sectionsestimated from the classical Drawin formula leads to [C/O] ≈ 0 at[O/H] ≈ -3. To remove the upturn in C/O, near-LTE formation for O ilines would be required, which could only happen if the H collisionalefficiency with the Drawin recipe is underestimated by factors of up toseveral tens of times, a possibility which we consider unlikely. Conclusions: The high C/O values derived at the lowest metallicitiesmay be revealing the fingerprints of Population III stars or may signalrotationally-aided nucleosynthesis in more normal Population II stars.Based on data collected with the European Southern Observatory's VeryLarge Telescope (VLT) at the Paranal, Chile (programmes No. 67.D-0106and 73.D-0024) and with the Magellan Telescope at Las CampanasObservatory, Chile.
| UBVRI Photometric Standard Stars Around the Celestial Equator: Updates and Additions
New broadband UBVRI photoelectric observations on theJohnson-Kron-Cousins photometric system have been made of 202 starsaround the sky, and centered at the celestial equator. These starsconstitute both an update of and additions to a previously publishedlist of equatorial photometric standard stars. The list is capable ofproviding, for both celestial hemispheres, an internally consistenthomogeneous broadband standard photometric system around the sky. Whenthese new measurements are included with those previously published byLandolt (1992), the entire list of standard stars in this paperencompasses the magnitude range 8.90 < V < 16.30, and the colorindex range -0.35 < (B - V) < +2.30.
| A candidate tidal disruption event in the Galaxy cluster Abell 3571
Context: Tidal disruption events are possible sources of temporarynuclear activity in galactic nuclei and can be considered as goodindicators of the existence of supermassive black holes in the centersof galaxies. Aims: A new X-ray source has been detectedserendipitously with ROSAT in a PSPC pointed observation of the galaxycluster A3571. Given the strong flux decay of the object in subsequentdetections, the tidal disruption scenario is investigated as a possibleexplanation of the event. Methods: We followed the evolution ofthe X-ray transient with ROSAT, XMM-Newton and Chandra for a totalperiod of ~13 years. We also obtained 7-band optical/NIR photometry withGROND at the ESO/MPI 2.2 m telescope. Results: We report a verylarge decay of the X-ray flux of the ROSAT source identified with thegalaxy LEDA 095953, a member of the cluster Abell 3571. We measured amaximum 0.3-2.4 keV luminosity Log (L_X) = 42.8 erg s-1. Thehigh state of the source lasted at least 150 ks; afterwardsLX declined as ~t-2. The spectrum of the brightestepoch is consistent with a black body with temperature kT˜ 0.12keV. Conclusions: The total energy released by this event in 10yr was estimated to be ?~E > 2 × 1050 erg. Weinterpret this event as a tidal disruption of a solar type star by thecentral supermassive black hole (i.e. ~107~M&sun;)of the galaxy.
| 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.
| Lithium abundances of halo dwarfs based on excitation temperature. I. Local thermodynamic equilibrium
Context: The discovery of the Spite plateau in the abundances of7Li for metal-poor stars led to the determination of anobservationally deduced primordial lithium abundance. However, after thesuccess of the Wilkinson Microwave Anisotropy Probe (WMAP) indetermining the baryon density, Ω_Bh2, there was adiscrepancy between observationally determined and theoreticallydetermined abundances in the case of 7Li. One of the mostimportant uncertain factors in the calculation of the stellar7Li abundance is the effective temperature, T_eff. Aims: We use sixteen metal-poor halo dwarfs to calculate new T_effvalues using the excitation energy method. With this temperature scalewe then calculate new Li abundances for this group of stars in anattempt to resolve the 7Li discrepancy. Methods: Usinghigh signal-to-noise (S/N ≈ 100) spectra of 16 metal-poor halodwarfs, obtained with the UCLES spectrograph on the AAT, measurements ofequivalent widths from a set of unblended Fe I lines are made. Theseequivalent widths are then used to calculate new T_eff values with theuse of the single line radiative transfer program WIDTH6, where we haveconstrained the gravity using either theoretical isochrones or theHipparcos parallax, rather than the ionization balance. The lithiumabundances of the stars are calculated with these temperatures. Results: The physical parameters are derived for the 16 programmestars, and two standards. These include T_eff, log g, [Fe/H],microturbulence and 7Li abundances. A comparison between thetemperature scale of this work and those adopted by others has beenundertaken. We find good consistency with the temperatures derived fromthe Hα line by Asplund et al. (2006, ApJ, 644, 229), but not withthe hotter scale of Meléndez & Ramírez (2004, ApJ,615, L33). We also present results of the investigation into whether anytrends between 7Li and metallicity or temperature are presentin these metal-poor stars.Appendix A is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/493/601
| Speckle interferometry of metal-poor stars in the solar neighborhood. II
The results of speckle interferometric observations of 115 metal-poorstars ([m/H] < ‑1) within 250 pc from the Sun and with propermotions µ ≳ 0.2″/yr, made with the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences,are reported. Close companions with separations ranging from0.034″ to 1″ were observed for 12 objects—G76-21,G59-1, G63-46, G135-16, G168-42, G141-47, G142-44, G190-10, G28-43,G217-8, G130-7, and G89-14—eight of them are astrometricallyresolved for the first time. The newly resolved systems include onetriple star—G190-10. If combined with spectroscopic and visualdata, our results imply a single:binary:triple:quadruple star ratio of147:64:9:1 for a sample of 221 primary components of halo and thick-diskstars.
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | בתולה |
| התרוממות ימנית: | 13h40m02.49s |
| סירוב: | -00°02'18.8" |
| גודל גלוי: | 12.014 |
| תנועה נכונה: | -226.8 |
| תנועה נכונה: | -83.3 |
| B-T magnitude: | 11.746 |
| V-T magnitude: | 11.992 |
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