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Публикации по объекту
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).
| Galactic model parameters for field giants separated from field dwarfs by their 2MASS and V apparent magnitudes
We present a method which separates field dwarfs and field giants bytheir 2MASS and V apparent magnitudes. This method is based onspectroscopically selected standards and is hence reliable. We appliedit to stars in two fields, SA 54 and SA 82, and we estimated a full setof Galactic model parameters for giants including their total localspace density. Our results are in agreement with the ones given in therecent literature.
| The lithium content of the Galactic Halo stars
Thanks to the accurate determination of the baryon density of theuniverse by the recent cosmic microwave background experiments, updatedpredictions of the standard model of Big Bang nucleosynthesis now yieldthe initial abundance of the primordial light elements withunprecedented precision. In the case of ^7Li, the CMB+SBBN value issignificantly higher than the generally reported abundances for Pop IIstars along the so-called Spite plateau. In view of the crucialimportance of this disagreement, which has cosmological, galactic andstellar implications, we decided to tackle the most critical issues ofthe problem by revisiting a large sample of literature Li data in halostars that we assembled following some strict selection criteria on thequality of the original analyses. In the first part of the paper wefocus on the systematic uncertainties affecting the determination of theLi abundances, one of our main goal being to look for the "highestobservational accuracy achievable" for one of the largest sets of Liabundances ever assembled. We explore in great detail the temperaturescale issue with a special emphasis on reddening. We derive four sets ofeffective temperatures by applying the same colour {T}_eff calibrationbut making four different assumptions about reddening and determine theLTE lithium values for each of them. We compute the NLTE corrections andapply them to the LTE lithium abundances. We then focus on our "best"(i.e. most consistent) set of temperatures in order to discuss theinferred mean Li value and dispersion in several {T}_eff and metallicityintervals. The resulting mean Li values along the plateau for [Fe/H]≤ 1.5 are A(Li)_NLTE = 2.214±0.093 and 2.224±0.075when the lowest effective temperature considered is taken equal to 5700K and 6000 K respectively. This is a factor of 2.48 to 2.81 (dependingon the adopted SBBN model and on the effective temperature range chosento delimit the plateau) lower than the CMB+SBBN determination. We findno evidence of intrinsic dispersion. Assuming the correctness of theCMB+SBBN prediction, we are then left with the conclusion that the Liabundance along the plateau is not the pristine one, but that halo starshave undergone surface depletion during their evolution. In the secondpart of the paper we further dissect our sample in search of newconstraints on Li depletion in halo stars. By means of the Hipparcosparallaxes, we derive the evolutionary status of each of our samplestars, and re-discuss our derived Li abundances. A very surprisingresult emerges for the first time from this examination. Namely, themean Li value as well as the dispersion appear to be lower (althoughfully compatible within the errors) for the dwarfs than for the turnoffand subgiant stars. For our most homogeneous dwarfs-only sample with[Fe/H] ≤ 1.5, the mean Li abundances are A(L)_NLTE = 2.177±0.071 and 2.215±0.074 when the lowest effective temperatureconsidered is taken equal to 5700 K and 6000 K respectively. This is afactor of 2.52 to 3.06 (depending on the selected range in {T}_eff forthe plateau and on the SBBN predictions we compare to) lower than theCMB+SBBN primordial value. Instead, for the post-main sequence stars thecorresponding values are 2.260±0.1 and 2.235±0.077, whichcorrespond to a depletion factor of 2.28 to 2.52. These results,together with the finding that all the stars with Li abnormalities(strong deficiency or high content) lie on or originate from the hotside of the plateau, lead us to suggest that the most massive of thehalo stars have had a slightly different Li history than their lessmassive contemporaries. In turn, this puts strong new constraints on thepossible depletion mechanisms and reinforces Li as a stellartomographer.
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are only available in electronic form, at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| The Liverpool-Edinburgh high proper motion survey
We present a catalogue of 6206 stars which have proper motions exceeding0.18arcsec yr-1 with an R-band faint magnitude limit of 19.5mag. This catalogue has been produced using SuperCOSMOS digitized R-bandESO and UK Schmidt Plates in 131 Schmidt fields covering more than 3 000square degrees (>7.5% of the whole sky) at the South Galactic Cap.The survey is >= 90% complete within the nominal limits of the LuytenTwo Tenths Catalogue of mR <= 18 .5 mag and 0.2 <= mu<=2.5arcsec yr-1, and is >= 80% complete formR <= 19 .5 mag and mu <=2 .5arcsec yr-1.Catalogue 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/397/575
| O/Fe in metal-poor main sequence and subgiant stars
A study of the O/Fe ratio in metal-poor main sequence and subgiant starsis presented using the \ion{Oi},6300 Å line, the O I 7774 Åtriplet, and a selection of weak Fe Ii lines observed on high-resolutionspectra acquired with the VLT UVES spectrograph. The \ion{Oi line isdetected in the spectra of 18 stars with -2.4 < [Fe/H] < -0.5, andthe triplet is observed for 15 stars with Fe/H ranging from -1.0 to-2.7. The abundance analysis was made first using standard modelatmospheres taking into account non-LTE effects on the triplet: the\ion{Oi} line and the triplet give consistent results with [O/Fe]increasing quasi-linearly with decreasing [Fe/H] reaching [O/Fe] =~ +0.7at [Fe/H] = -2.5. This trend is in reasonable agreement with otherresults for [O/Fe] in metal-poor dwarfs obtained using standardatmospheres and both ultraviolet and infrared OH lines. There is alsobroad agreement with published results for [O/Fe] for giants obtainedusing standard model atmospheres and the \ion{Oi} line, and the OHinfrared lines, but the O I lines give higher [O/Fe] values which may,however, fall into place when non-LTE effects are considered. Whenhydrodynamical model atmospheres representing stellar granulation indwarf and subgiant stars replace standard models, the [O/Fe] from the\ion{Oi} and Fe Ii lines is decreased by an amount which increases withdecreasing [Fe/H]. These 3D effects on [O/Fe] is compounded by theopposite behaviour of the \ion{Oi} (continuous opacity effect) and Fe Iilines (excitation effect). The [O/Fe] vs. [Fe/H] relation remainsquasi-linear extending to [O/Fe] =~ +0.5 at [Fe/H] = -2.5, but with atendency of a plateau with [O/Fe] =~ +0.3 for -2.0 < [Fe/H] <-1.0, and a hint of cosmic scatter in [O/Fe] at [Fe/H] =~ -1.0. Use ofthe hydrodynamical models disturbs the broad agreement between theoxygen abundances from the \ion{Oi} , O I, and OH lines, but 3D non-LTEeffects may serve to erase these differences. The [O/Fe] values from the\ion{Oi} line and the hydrodynamical model atmospheres for dwarfs andsubgiant stars are lower than the values for giants using standard modelatmospheres and the \ion{Oi}, and O I lines. Based on observationscollected at the European Southern Observatory, Chile (ESO Nos.65.L-0131, 65.L-0507, and 67.D-0439).
| Three-dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for theestimation of atmospheric parameters (Teff, logg, and [Fe/H])for Galactic F- and G-type stars. The ANNs are fed withmedium-resolution (Δλ~1-2 Å) non-flux-calibratedspectroscopic observations. From a sample of 279 stars with previoushigh-resolution determinations of metallicity and a set of (external)estimates of temperature and surface gravity, our ANNs are able topredict Teff with an accuracy ofσ(Teff)=135-150 K over the range4250<=Teff<=6500 K, logg with an accuracy ofσ(logg)=0.25-0.30 dex over the range 1.0<=logg<=5.0 dex, and[Fe/H] with an accuracy σ([Fe/H])=0.15-0.20 dex over the range-4.0<=[Fe/H]<=0.3. Such accuracies are competitive with theresults obtained by fine analysis of high-resolution spectra. It isnoteworthy that the ANNs are able to obtain these results withoutconsideration of photometric information for these stars. We have alsoexplored the impact of the signal-to-noise ratio (S/N) on the behaviorof ANNs and conclude that, when analyzed with ANNs trained on spectra ofcommensurate S/N, it is possible to extract physical parameter estimatesof similar accuracy with stellar spectra having S/N as low as 13. Takentogether, these results indicate that the ANN approach should be ofprimary importance for use in present and future large-scalespectroscopic surveys.
| Revised Magnesium Abundances in Galactic Halo and Disk Stars
A differential analysis of the magnesium abundances in 61 F-K dwarfs andsubgiants with metallicities -2.6<[Fe/H]<+0.2 is performed basedon published observational data. Fundamental parameters for 36 stars aredetermined: T eff from V-K and V-R; logg from HIPPARCOS parallaxes, and[Fe/H] and ξt from Fe II lines. The computations allow for non-LTEeffects in the formation of the Mg I lines. For most of the stars, thestandard errors in the Mg abundances do not exceed 0.07 dex. Themetallicity dependence of [Mg/Fe] is analyzed. Magnesium shows aconstant overabundance relative to Fe of 0.46±0.06 dex formetallicities -2.6<[Fe/H]<-0.7 Mg. The Mg overabundance decreasesabruptly to +0.27 dex at [Fe/H]⋍-0.7. At highermetallicities, the Mg abundance smoothly decreases to the solar value at[Fe/H]=0.0. Halo stars with metallicities [Fe/H]<-1.0 exhibit lowerMg overabundances ( ) compared to the [Mg/Fe] values for other starswith similar [Fe/H].
| Non-LTE Abundances and Consequences for the Evolution of the α-Elements in the Galaxy
Abundances of α-elements such as Ca and Mg in disk and halo starsare usually derived from equivalent width lines measured onhigh-resolution spectra and assuming local thermodynamic equilibrium(LTE). In this paper, we present non-LTE differential abundances derivedby computing the statistical equilibrium of Ca I and Mg I atoms, usinghigh-resolution equivalent widths available in the literature for 252dwarf to subgiant stars. These non-LTE abundances, combined with recentdetermination of non-LTE abundances of iron, seem to remove thedispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic halo anddisk phases, revealing new and surprising structures. These results haveimportant consequences for chemical evolution models of the Galaxy. Inaddition, non-LTE abundance ratios for stars belonging to the M92cluster apparently have the same behavior. More high-resolutionobservations, mainly of globular clusters, are urgently needed toconfirm our results.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| Abundances of light elements in metal-poor stars. III. Data analysis and results
We present the results of the analysis of an extensive set of new andliterature high quality data concerning Fe, C, N, O, Na, and Mg. Thisanalysis exploited the T_eff scale determined in Gratton et al. (1996a),and the non-LTE abundance corrections computed in Gratton et al.(1999a). Results obtained with various abundance indices are discussedand compared. Detailed comparison with models of galactic chemicalevolution will be presented in future papers of this series. Our non-LTEanalysis yields the same O abundances from both permitted and forbiddenlines for stars with T_eff >4600 K, in agreement with King (1993),but not with other studies using a lower T_eff -scale for subdwarfs.However, we obtain slightly smaller O abundances for the most luminousmetal-poor field stars than for fainter stars of similar metallicities,an effect attributed to inadequacies of the adopted model atmospheres(Kurucz 1992, with overshooting) for cool stars. We find a nearlyconstant O overundance in metal-poor stars ([Fe/H]<-0.8), at a meanvalue of 0.46+/- 0.02 dex (sigma =0.12, 32 stars), with only a gentleslope with [Fe/H] ( ~ -0.1); this result is different from the steeperslope recently obtained using OH band in the near UV. If only bonafideunmixed stars are considered, C abundances scale with Fe ones (i.e.[C/Fe]~ 0) down to [Fe/H] ~ -2.5. Due to our adoption of a differentT_eff scale, we do not confirm the slight C excess in the most metalpoor disk dwarfs (-0.8<[Fe/H]<-0.4) found in previousinvestigations. Na abundances scale as Fe ones in the high metallicityregime, while metal-poor stars present a Na underabundance. None of thefield stars analyzed belong to the group of O-poor and Na-rich starsobserved in globular clusters. Na is deficient with respect to Mg inhalo and thick disk stars; within these populations, Na deficiency maybe a slow function of [Mg/H]. Solar [Na/Mg] ratios are obtained for thindisk stars. Tables~ 2 to 9 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strabg.fr/Abstract.html
| Stars in the Galactic Halo
Not Available
| Stellar Iron Abundances: Non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II inthe atmosphere of late-type stars. We used atomic models for Fe I and FeII having, respectively, 256 and 190 levels, as well as 2117 and 3443radiative transitions. Photoionization cross sections are from the IronProject. These atomic models were used to investigate non-LTE (NLTE)effects in iron abundances of late-type stars with different atmosphericparameters. We found that most Fe I lines in metal-poor stars are formedin conditions far from LTE. We derived metallicity corrections of about0.3 dex with respect to LTE values for the case of stars with[Fe/H]~-3.0. Fe II is found not to be affected by significant NLTEeffects. The main NLTE effect invoked in the case of Fe I isoverionization by ultraviolet radiation; thus classical ionizationequilibrium is far from being satisfied. An important consequence isthat surface gravities derived by LTE analysis are in error and shouldbe corrected before final abundance corrections. This apparently solvesthe observed discrepancy between spectroscopic surface gravities derivedby LTE analyses and those derived from Hipparcos parallaxes. A table ofNLTE [Fe/H] and log g values for a sample of metal-poor late-type starsis given.
| Estimation of Stellar Metal Abundance. II. A Recalibration of the Ca II K Technique, and the Autocorrelation Function Method
We have recalibrated a method for the estimation of stellar metalabundance, parameterized as [Fe/H], based on medium-resolution (1-2Å) optical spectra (the majority of which cover the wavelengthrange 3700-4500 Å). The equivalent width of the Ca II K line (3933Å) as a function of [Fe/H] and broadband B-V color, as predictedfrom spectrum synthesis and model atmosphere calculations, is comparedwith observations of 551 stars with high-resolution abundances availablefrom the literature (a sevenfold increase in the number of calibrationstars that were previously available). A second method, based on theFourier autocorrelation function technique first described by Ratnatunga& Freeman, is used to provide an independent estimate of [Fe/H], ascalibrated by comparison with 405 standard-star abundances.Metallicities based on a combination of the two techniques for dwarfsand giants in the color range 0.30<=(B-V)_0<=1.2 exhibit anexternal 1 sigma scatter of approximately 0.10-0.20 dex over theabundance range -4.0<=[Fe/H]<=0.5. Particular attention has beengiven to the determination of abundance estimates at the metal-rich endof the calibration, where our previous attempt suffered from aconsiderable zero-point offset. Radial velocities, accurate toapproximately 10 km s^-1, are reported for all 551 calibration stars.
| Photometric Abundance Calibration of delta Scuti Stars Using HK Photometry
The hk index has been used as a metallicity indicator for RR Lyraevariable stars. It is now being applied to the shorter period deltaScuti variables. Employing spectroscopic abundances of stars withpublished hk values and photometric indices calculated from stellaratmosphere models, a three-dimensional interpolation is used todetermine [Fe/H] from intrinsic b-y, c_1, and hk values. The resulting[Fe/H], log g, and T_eff values for 10 delta Scuti stars are presented.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Abundances of light elements in metal-poor stars. I. Atmospheric parameters and a new T_eff_ scale
We present atmospheric parameters for about 300 stars of differentchemical composition, whose spectra will be used to study the galacticenrichment of Fe and light elements. These parameters were derived usingan homogeneous iterative procedure, which considers new calibrations ofcolour-T_eff_ relations for F, G and K-type stars based on Infrared FluxMethod (IRFM) and interferometric diameters for population I stars, andthe Kurucz (1992) model atmospheres. We found that these calibrationsyield a self-consistent set of atmospheric parameters forT_eff_>4400K, representing a clear improvement over results obtainedwith older model atmospheres. Using this T_eff_ -scale and Feequilibrium of ionization, we obtained very low gravities (implyingluminosities incompatible with that expected for RGB stars) formetal-poor stars cooler than 4400K; this might be due either to amoderate Fe overionization (expected from statistical equilibriumcalculations) or to inadequacy of Kurucz models to describe theatmospheres of very cool giants. Our T_eff_ scale is compared with otherscales recently used for metal-poor stars; it agrees well with thoseobtained using Kurucz (1992) models, but it gives much larger T_eff_'sthan those obtained using OSMARCS models (Edvardsson et al. 1993). Thisdifference is attributed to the different treatment of convection in thetwo sets of models. For the Sun, the Kurucz (1992) model appears to bepreferable to the OSMARCS ones because it better predicts the solar limbdarkening; furthermore, we find that our photometric T_eff_ 's formetal-poor stars agree well with both direct estimates based on theIRFM, and with T_eff_'s derived from Hα wings when using Kuruczmodels.
| Spread of the lithium abundance in halo stars.
The observed scatter of the lithium abundance around a "plateau" isrevisited for three samples of stars for which the temperature has beendetermined from either the excitation temperature, or the dereddenedcolor (b-y)_o_ or the profile of the Halpha_wings. Systematicdifferences are noted between the three methods of temperaturedetermination. From sample to sample the rms observed scatter of thelithium abundance varies from 0.06dex to 0.08dex (to be compared to thevalue 0.13 previously found by Thorburn (1994), for another sample ofstars). We show that in all cases but one, this scatter is fullyexplained by the temperature and equivalent width errors. The intrinsicscatter, if real, is small.
| Ca II H and K Filter Photometry on the UVBY System. II. The Catalog of Observations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2828T&db_key=AST
| Empirical Calibration of Metallicity Indices for Single Stellar Populations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....109.2218I&db_key=AST
| New proper motion determination of Luyten catalogue stars (LTT) south of declination -40deg and right ascension between 16h and 24h.
Data are given for 285 LTT stars found in 23 areas, covering 25 squaredegrees each, south of declination -40deg and right ascension between16h and 24h. Four stars present differences in proper motion >=0.10arcsec, eleven present differences in position angle >20deg and onepresents those differences in both values.
| Kinematics of metal-poor stars in the galaxy
We discuss the kinematic properties of a sample of 1936 Galactic stars,selected without kinematic bias, and with abundances (Fe/H) is less thanor equal to -0.6. The stars selected for this study all have measuredradial velocities, and the majority have abundances determined fromspectroscopic or narrow-/intermediate-band photometric techniques. Incontrast to previous examinations of the kinematics of the metal-poorstars in the Galaxy, our sample contains large numbers of stars that arelocated at distances in excess of 1 kpc from the Galactic plane. Thus, amuch clearer picture of the nature of the metal-deficient populations inthe Galaxy can now be drawn.
| The primordial abundance of Li-6 and Be-9
Light element (Li-6, Li-7 and Be-9) depletion isochrones for halo starshave been calculated with standard stellar evolution models. Thesemodels include the latest available opacities and are computed throughthe subgiant branch. If Li-6 is not produced in appreciable amounts bystellar flares, then the detection of Li-6 in HD 84937 by Smith,Lambert, and Nissen is compatible with standard stellar evolution andstandard big bang nucleosynthesis only if HD 84937 is a subgiant. Thepresent parallax is consistent with HD 84937 being a subgiant star atthe 2.5 sigma level. The most metal poor star with a measured Be-9abundance is HD 140283, which is a relatively cool subgiant. Standardstellar evolution predicts that Be-9 will have been depleted in thisstar by approx. 0.3 dex (for Teff = 5640 K). Revising theabundance upward changes the oxygen-beryllium relation, suggestingincompatibility with standard cosmic-ray production models, and hence,standard big bang nucleosynthesis. However, an increase in the derivedtemperature of HD 1402283 to 5740 K would result in little depletion ofBe-9 and agreement with standard big bang nucleosynthesis.
| Chemical composition and atmospheric parameters of metal-poor halo stars
The abundance of oxygen in 9 metal-poor stars is derived from highresolution observations of OH lines in the spectral region 3138-3155 A.The same spectra were previously used to determine the berylliumabundance (Gilmore et al. 1992). In addition, Mg, Ca, Ti, Cr, and Feabundances are derived from accurate equivalent widths of weak atomicabsorption lines in the 5100-6100 A spectral region. Atmosphericparameters, T_eff_ and g, are determined from the Stroemgren photometricindices b-y and c_1_. New accurate oscillator strengths for the FeI andFeII lines allow an important check of these parameters as well as theassumption of LTE; the FeI excitation balance is consistent with thephotometric temperatures, whereas the FeII/FeI ionization balancesuggests an over-ionization of iron amounting to about 0.15 dex withrespect to LTE. The derived abundance ratios between theα-elements and Fe are nearly constant in the metallicity range-3.2 <~[Fe/H]<~-1.8. Any possible scatter of Mg/Fe, Ca/Fe andTi/Fe in the galactic halo is less than 0.06 dex and an upper limit forthe scatter of O/Fe is 0.15 dex. This suggests that the IMF was similarin different regions of the inner halo and that the mixing ofnucleosynthesis products was very efficient in galactic regions oftypically globular cluster masses.
| The boron-to-beryllium ratio in halo stars - A signature of cosmic-ray nucleosynthesis in the early Galaxy
We discuss Galactic cosmic-ray (GCR) spallation production of Li, Be,and B in the early Galaxy with particular attention to the uncertaintiesin the predictions of this model. The observed correlation between theBe abundance and the metallicity in metal-poor Population II starsrequires that Be was synthesized in the early Galaxy. We show that theobservations and such Population II GCR synthesis of Be arequantitatively consistent with the big bang nucleosynthesis productionof Li-7. We find that there is a nearly model independent lower bound toB/Be of about 7 for GCR synthesis. Recent measurements of B/Be about 10in HD 140283 are in excellent agreement with the predictions ofPopulation II GCR nucleosynthesis. Measurements of the boron abundancein additional metal-poor halo stars is a key diagnostic of the GCRspallation mechanism. We also show that Population II GCR synthesis canproduce amounts of Li-6 which may be observed in the hottest halo stars.
| Photoelectric beta photometry of 118 stars with V in the range 14-15 and B - V not greater than 0.650 at the south galactic pole
Observations of a sample of 118 stars at the south galactic pole arepresented. Diagrams of instrumental vs standard beta values for thestandard stars may indicate that the transformation must be divided intotwo regions with slightly different slopes A and intercepts B. Thenarrow bandpass of the filter set employed has an FWHM of 27 A and iscentered at 4860 A. A radial velocity of 100 km/s will reduce thebeta-index of a sharp beta line with 0.005-0.010 m relative to a zerovelocity star. It is argued that the two transformations are a result ofslightly different bandwidths of the narrow-band filters used for theoriginal measurements of the present secondary standards and in thepresent observing run. The present narrow band's FWHM is smaller by 3 A.
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Is beryllium in metal-poor stars of galactic or cosmological origin?
Standard Big Bang nucleosynthesis predicts a very small primordialabundance of beryllium. Observations of nine very metal-poor starsindicate a beryllium abundance roughly proportional to the oxygenabundance, a trend that can be explained in terms of galactic chemicalevolution. Combining this rate of beryllium production with recentobservations of boron and lithium in similar stars yields an upper limitto the primordial beryllium abundance several orders of magnitudegreater than the cosmological prediction, a result that can be explainedby cosmic-ray activity in the early Galaxy.
| Early type high-velocity stars in the solar neighborhood. IV - Four-color and H-beta photometry
Results are presented from photometric obaservations in the Stromgrenuvby four-color and H-beta systems of early-type high-velocity stars inthe solar neighborhood. Several types of photometrically peculiar starsare selected on the basis of their Stromgren indices and areprovisionally identified as peculiar A stars, field horizontal-branchstars, metal-poor stars near the Population II and old-disk turnoffs,metal-poor blue stragglers, or metallic-line A stars. Numerousphotometrically normal stars were also found.
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Группы:
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Наблюдательные данные и астрометрия
| Созвездие: | Журавль |
| Прямое восхождение: | 22h33m47.05s |
| Склонение: | -42°03'13.8" |
| Видимая звёздная величина: | 9.725 |
| Собственное движение RA: | -61.8 |
| Собственное движение Dec: | -252.7 |
| B-T magnitude: | 10.152 |
| V-T magnitude: | 9.761 |
Каталоги и обозначения:
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