Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Spectroscopic Studies of Extremely Metal-poor Stars with the Subaru High Dispersion Spectrograph. V. The Zn-enhanced Metal-poor Star BS 16920-017
We report Zn abundances for 18 very metal-poor stars studied in ourprevious work, covering the metallicity range -3.2< [Fe/H] <-2.5.The [Zn/Fe] values of most stars show an increasing trend withdecreasing [Fe/H] in this metallicity range, confirming the resultsfound by previous studies. However, the extremely metal-poor star BS16920-017([Fe/H] =-3.2) exhibits a significantly high [Zn/Fe] ratio([Zn/Fe] = +1.0). Comparison of the chemical abundances of this objectwith HD 4306, which has similar atmospheric parameters to BS 16920-017,clearly demonstrates a deficiency of ? elements andneutron-capture elements in this star, along with enhancements of Mn andNi, as well as Zn. The association with a hypernova explosion that hasbeen proposed to explain the high Zn abundance ratios found in extremelymetal-poor stars is a possible explanation, although further studies arerequired to fully interpret the abundance pattern of this object.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Extremely metal-poor stars in SDSS fields
Some insight on the first generation of stars can be obtained from thechemical composition of their direct descendants, extremely metal-poorstars (EMP), with metallicity less than or equal to 1/1000 of the solarmetallicity. Such stars are exceedingly rare, the most successfulsurveys, for this purpose, have so far provided only about 100 starswith 1/1 000 the solar metallicity and 4 stars with about 1/10 000 ofthe solar metallicity. The Sloan Digital Sky Survey has the potential toprovide a large number of candidates of extremely low metallicity.X-shooter has the unique capability of performing the necessaryfollow-up spectroscopy providing accurate metallicities and abundanceratios for several elements (Mg, Al, Ca, Ti, Cr, Sr, ...) for EMPcandidates. We here report on the results for the first two starsobserved in the course of our Franco-Italian X-shooter GTO. The twostars were targeted to be of metallicity around -3.0, the analysis ofthe X-shooter spectra showed them to be of metallicity around -2.0, butwith a low ? to iron ratio, which explains the underestimate ofthe metallicity from the SDSS spectra. The efficiency of X-shooterallows an in situ study of the outer halo, for the two stars studiedhere we estimate distances of 3.9 and 9.1 kpc, these are likely the mostdistant dwarf stars studied in detail to date.Based on spectra obtained with X-shooter at the 8.2-m Kueyen ESOtelescope, GTO programmes 085.D-0194 and 086.D.0094.
| 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
| The Weak s-Process in Massive Stars and its Dependence on the Neutron Capture Cross Sections
The slow neutron capture process in massive stars (weak s process)produces most of the s-process isotopes between iron and strontium.Neutrons are provided by the 22Ne(?,n)25Mgreaction, which is activated at the end of the convective He-burningcore and in the subsequent convective C-burning shell. Thes-process-rich material in the supernova ejecta carries the signature ofthese two phases. In the past years, new measurements of neutron capturecross sections of isotopes beyond iron significantly changed thepredicted weak s-process distribution. The reason is that the variationof the Maxwellian-averaged cross sections (MACS) is propagated toheavier isotopes along the s path. In the light of these results, wepresent updated nucleosynthesis calculations for a 25 M sunstar of Population I (solar metallicity) in convective He-burning coreand convective C-burning shell conditions. In comparison with previoussimulations based on the Bao et al. compilation, the new measurement ofneutron capture cross sections leads to an increase of s-process yieldsfrom nickel up to selenium. The variation of the cross section of oneisotope along the s-process path is propagated to heavier isotopes,where the propagation efficiency is higher for low cross sections. New74Ge, 75As, and 78Se MACS result in ahigher production of germanium, arsenic, and selenium, thereby reducingthe s-process yields of heavier elements by propagation. Results arereported for the He core and for the C shell. In shell C-burning, thes-process nucleosynthesis is more uncertain than in the He core, due tohigher MACS uncertainties at higher temperatures. We also analyze theimpact of using the new lower solar abundances for CNO isotopes on thes-process predictions, where CNO is the source of 22Ne, andwe show that beyond Zn this is affecting the s-process yields more thannuclear or stellar model uncertainties considered in this paper. Inparticular, using the new updated initial composition, we obtain a highs-process production (overproduction higher than 16O, ~100)for Cu, Ga, Ge, and As. Using the older abundances by Anders &Grevesse, also Se, Br, Kr, and Rb are efficiently produced. Our resultshave important implications in explaining the origin of copper in thesolar abundance distribution, pointing to a prevailing contribution fromthe weak s-process in agreement with spectroscopic observations andGalactic chemical evolution calculations. Because of the improvement dueto the new MACS for nickel and copper isotopes, the nucleosynthesis ofcopper is less affected by nuclear uncertainties compared to heaviers-process elements. An experimental determination of the 63NiMACS is required for a further improvement of the abundance predictionof copper. The available spectroscopic observations of germanium andgallium in stars are also discussed, where most of the cosmic abundancesof these elements derives from the s-process in massive stars.
| Kinematics of high proper motion stars determined from high resolution spectra in ground-based ultraviolet
Radial velocities for 15 stars with high proper motions were measured asa result of spectral observations, conducted with the NES echellespectrograph of the 6-m BTA telescope in the wavelength range of3550-5100 Å with a spectral resolution of R=60000. Thestandard deviation of the measured velocity does not exceed ? ?0.9 km/s for the stars with metallicity [Fe/H]? ?1, and? ? 1.1 km/s for [Fe/H]? ?1. The heliocentricvelocities measured with high accuracy in combination withtrigonometrical parallaxes and proper motions from the HIPPARCOS catalogallowed us to determine the distances and parameters of the galacticorbits of the stars under study. In general they are located within 100pc; the binarity of several program stars is confirmed.
| High-Resolution Spectroscopy of Extremely Metal-Poor Stars in the Least Evolved Galaxies: Ursa Major II and Coma Berenices
We present spectra of six metal-poor stars in two of the ultra-faintdwarf galaxies orbiting the Milky Way (MW), Ursa Major II, and ComaBerenices obtained with the Keck/High Resolution Echelle Spectrometer(HIRES). These observations include the first high-resolutionspectroscopic observations of extremely metal-poor ([Fe/H] < -3.0)stars not belonging to the MW halo field star population. We obtainabundance measurements and upper limits for 26 elements between carbonand europium. The entire sample of stars spans a range of -3.2 <[Fe/H] < -2.3, and we confirm that each galaxy contains a largeintrinsic spread of Fe abundances. A comparison with MW halo stars ofsimilar metallicities reveals substantial agreement between theabundance patterns of the ultra-faint dwarf galaxies and the MW halo forthe light, ?, and iron-peak elements (C to Zn). This agreementcontrasts with the results of earlier studies of more metal-rich stars(-2.5 lsim [Fe/H] lsim -1.0) in more luminous dwarf spheroidal galaxies,which found significant abundance discrepancies with respect to the MWhalo data. The abundances of neutron-capture elements (Sr to Eu) in theultra-faint dwarf galaxies are extremely low, consistent with the mostmetal-poor halo stars, but not with the typical halo abundance patternat [Fe/H] gsim -3.0. Not only are our results broadly consistent with agalaxy formation model that predicts that massive dwarf galaxies are thesource of the metal-rich component ([Fe/H]> - 2.5) of the MW halo,but they also suggest that the faintest known dwarfs may be the primarycontributors to the metal-poor end of the MW halo metallicitydistribution.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the California Institute of Technology and theUniversity of California, and the National Aeronautics and SpaceAdministration.
| Searching for the metal-weak thick disc in the solar neighbourhood
An abundance analysis is presented of 60 metal-poor stars drawn fromcatalogues of nearby stars provided by Arifyanto et al. and Schuster etal. In an attempt to isolate a sample of metal-weak thick disc stars, weapplied the kinematic criteria Vrot >=100kms-1,|ULSR| <= 140kms-1 (LSR -local standard of rest) and |WLSR| <= 100kms-1.14 stars satisfying these criteria and having [Fe/H] <= -1.0 areincluded in the sample of 60 stars. Eight of the 14 have [Fe/H] >=-1.3 and may be simply thick disc stars of slightly lower than average[Fe/H]. The other six have [Fe/H] from -1.3 to -2.3 and are eithermetal-weak thick disc stars or halo stars with kinematics mimickingthose of the thick disc. The sample of 60 stars is completed by eightthick disc stars, 20 stars of a hybrid nature (halo or thick disc stars)and 18 stars with kinematics distinctive of the halo.
| The Highly Unusual Chemical Composition of the Hercules Dwarf Spheroidal Galaxy
We report on the abundance analysis of two red giants in the faintHercules dwarf spheroidal (dSph) galaxy. These stars show a remarkabledeficiency in the neutron-capture elements, while the hydrostatic?-elements (O, Mg) are strongly enhanced. Our data indicate[Ba/Fe] and [Mg/Fe] abundance ratios of <~-2 and ~+0.8 dex,respectively, with essentially no detection of other n-capture elements.In contrast to the only other dSph star with similar abundance patterns,Dra 119, which has a very low metallicity at [Fe/H] = -2.95 dex, ourobjects, at [Fe/H] ~ -2.0 dex, are only moderately metal-poor. Themeasured ratio of hydrostatic/explosive ?-elements indicates thathigh-mass (~35 Msolar) Type II supernovae progenitors are themain, if not only, contributors to the enrichment of this galaxy. Thissuggests that star formation and chemical enrichment in the ultrafaintdSphs proceeds stochastically and inhomogeneously on small scales, orthat the IMF was strongly skewed to high-mass stars. The neutron capturedeficiencies and the [Co/Fe] and [Cr/Fe] abundance ratios in our starsare similar to those in the extremely low metallicity Galactic halo.This suggests that either our stars are composed mainly of the ejectafrom the first, massive, Population III stars (but at moderately high[Fe/H]), or that SN ejecta in the Hercules galaxy were diluted with ~30times less hydrogen than typical for extreme metal-poor stars.This Letter includes data gathered with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| Uncovering the Chemical Signature of the First Stars in the Universe
The chemical abundance patterns observed in metal-poor Galactic halostars contain the signature of the first supernovae, and thus allow usto probe the first stars that formed in the universe. We construct atheoretical model for the early chemical enrichment history of the MilkyWay, aiming in particular at the contribution from pair-instabilitysupernovae (PISNe). These are a natural consequence of currenttheoretical models for primordial star formation at the highest masses.However, no metal-poor star displaying the distinct PISN signature hasyet been observed. We here argue that this apparent absence of any PISNsignature is due to an observational selection effect. Whereas mostsurveys traditionally focus on the most metal-poor stars, we predictthat early PISN enrichment tends to ``overshoot,'' reaching enrichmentlevels of [Ca/H]~=-2.5 that would be missed by current searches. Weutilize existing observational data to place constraints on theprimordial initial mass function (IMF). The number fraction of PISNe inthe primordial stellar population is estimated to be <0.07, or<~40% by mass, assuming that metal-free stars have masses in excessof 10 Msolar. We further predict, based on theoreticalestimates for the relative number of PISNe, that the expected fractionof second-generation stars below [Ca/H]=-2 with a dominant (i.e.,>90%) contribution from PISNe is merely ~10-4 to5×10-4. The corresponding fraction of stars formed fromgas exclusively enriched by PISNe is a factor of ~4 smaller. With theadvent of next-generation telescopes and new, deeper surveys, we shouldbe able to test these predictions.
| Chemical Abundances and Kinematics in Globular Clusters and Local Group Dwarf Galaxies and Their Implications for Formation Theories of the Galactic Halo
We review Galactic halo formation theories and supporting evidence, inparticular, kinematics and detailed chemical abundances of stars in somerelevant globular clusters as well as Local Group dwarf galaxies. Outerhalo red HB clusters tend to have large eccentricities and inhabit thearea of the Lee diagram populated by dwarf spheroidal stars, favoring anextragalactic origin. Old globular clusters show the full range ofeccentricities, while younger ones seem to have preferentially higheccentricities, again hinting at their extragalactic origin. However,the three outer halo second parameter clusters with well-determinedorbits indicate they come from three independent systems. We comparedetailed abundances of a variety of elements between the halo and alldwarf galaxies studied to date, including both dwarf spheroidals andirregulars. The salient feature is that halo abundances are essentiallyunique. In particular, the general ? vs. [Fe/H] pattern of 12 ofthe 13 galaxies studied are similar to each other and very differentfrom the Milky Way. Sgr appears to be the only possible exception. Atthe metal-poor end the extragalactic sample is only slightly deficientcompared to the halo but begins to diverge by [Fe/H] ~ -2 and thedifference is particularly striking for stars with [Fe/H] ~ -1. OnlySgr, the most massive dSph, has some stars similar in ?-abundanceto Galactic stars at intermediate metallicities, even the most extremelow-? subset most likely to have been accreted. It appears veryunlikely that a significant fraction of the metal-rich halo could havecome from disrupted dSphs of low mass. However, at least some of themetal-poor halo may have come from typical dSphs, and a portion of theintermediate metallicity and metal-rich halo may have come from verymassive systems like Sgr. This argues against the standard hierarchicalgalaxy formation scenario and the Searle-Zinn paradigm for the formationof the Galactic halo via accretion of ``fragments'' composed of starslike those we see in typical present-day dSphs. The chemical differencesbetween the dwarfs and the halo are due to a combination of a low starformation efficiency and a high galactic wind efficiency in the former.AGB stars are also more important in the chemical evolution of thedwarfs. The formation problem may be solved if the majority of halostars formed within a few, very massive satellites accreted very early.However, any such satellites must either be accreted much earlier thanpostulated, before the onset of SNe Ia, or star formation must beprevented to occur in them until only shortly before they are accreted.The intrinsic scatter in many elements, particularly the?-elements, indicates that the halo was also mixed on asurprisingly short timescale, a further problem for hierarchicalformation theories.
| Speckle interferometry of metal-poor stars in the solar neighborhood. I
We report the results of speckle-interferometric observations of 109high proper-motion metalpoor stars made with the 6-m telescope of theSpecial Astrophysical Observatory of the Russian Academy of Sciences. Weresolve eight objects—G102-20, G191-55, BD+19° 1185A, G89-14,G87-45, G87-47, G111-38, and G114-25—into individual componentsand we are the first to astrometrically resolve seven of these stars.New resolved systems included two triple (G111-38, G87-47) and onequadruple (G89-14) star. The ratio ofsingle-to-binary-to-triple-to-quadruple systems among the stars of oursample is equal to 71:28:6:1.
| New Distant Companions to Known Nearby Stars. II. Faint Companions of Hipparcos Stars and the Frequency of Wide Binary Systems
We perform a search for faint, common proper motion companions ofHipparcos stars using the recently published Lépine-Shara ProperMotion-North catalog of stars with proper motionμ>0.15'' yr-1. Our survey uncovers a totalof 521 systems with angular separations3''<Δθ<1500'', with 15 triplesand 1 quadruple. Our new list of wide systems with Hipparcos primariesincludes 130 systems identified here for the first time, including 44 inwhich the secondary star has V>15.0. Our census is statisticallycomplete for secondaries with angular separations20''<Δθ<300'' and apparentmagnitudes V<19.0. Overall, we find that at least 9.5% of nearby(d<100 pc) Hipparcos stars have distant stellar companions withprojected orbital separations s>1000 AU. We observe that thedistribution in orbital separations is consistent with Öpik's law,f(s)ds~s-1ds, only up to a separation s~4000 AU, beyond whichit follows a more steeply decreasing power law f(s)ds~s-ldswith l=1.6+/-0.1. We also find that the luminosity function of thesecondaries is significantly different from that of the single stars'field population, showing a relative deficiency in low-luminosity(8
| Medium-resolution Isaac Newton Telescope library of empirical spectra - II. The stellar atmospheric parameters
We present a homogeneous set of stellar atmospheric parameters(Teff, logg, [Fe/H]) for MILES, a new spectral stellarlibrary covering the range λλ 3525-7500Å at2.3Å (FWHM) spectral resolution. The library consists of 985 starsspanning a large range in atmospheric parameters, from super-metal-rich,cool stars to hot, metal-poor stars. The spectral resolution, spectraltype coverage and number of stars represent a substantial improvementover previous libraries used in population synthesis models. Theatmospheric parameters that we present here are the result of aprevious, extensive compilation from the literature. In order toconstruct a homogeneous data set of atmospheric parameters we have takenthe sample of stars of Soubiran, Katz & Cayrel, which has very welldetermined fundamental parameters, as the standard reference system forour field stars, and have calibrated and bootstrapped the data fromother papers against it. The atmospheric parameters for our clusterstars have also been revised and updated according to recent metallicityscales, colour-temperature relations and improved set of isochrones.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Medium-resolution Isaac Newton Telescope library of empirical spectra
A new stellar library developed for stellar population synthesismodelling is presented. The library consists of 985 stars spanning alarge range in atmospheric parameters. The spectra were obtained at the2.5-m Isaac Newton Telescope and cover the range λλ3525-7500 Å at 2.3 Å (full width at half-maximum) spectralresolution. The spectral resolution, spectral-type coverage,flux-calibration accuracy and number of stars represent a substantialimprovement over previous libraries used in population-synthesis models.
| Chemical Compositions of Red Giant Stars in Old Large Magellanic Cloud Globular Clusters
We have observed 10 red giant stars in four old Large Magellanic Cloudglobular clusters with the high-resolution spectrograph MIKE on theMagellan Landon Clay 6.5 m telescope. The stars in our sample have up to20 elemental abundance determinations for the α-, iron peak, andneutron-capture element groups. We have also derived abundances for thelight odd-Z elements Na and Al. We find NGC 2005 and NGC 2019 to be moremetal-rich than previous estimates from the Ca II triplet, and we derive[Fe/H] values closer to those obtained from the slope of the red giantbranch. However, we confirm previous determinations for Hodge 11 and NGC1898 to within 0.2 dex. The LMC cluster [Mg/Fe] and [Si/Fe] ratios arecomparable to the values observed in old Galactic globular clusterstars, as are the abundances [Y/Fe], [Ba/Fe], and [Eu/Fe]. The LMCclusters do not share the low-Y behavior observed in some dwarfspheroidal galaxies. [Ca/Fe], [Ti/Fe], and [V/Fe] in the LMC, however,are significantly lower than what is seen in the Galactic globularcluster system. Neither does the behavior of [Cu/Fe] as a function of[Fe/H] in our LMC clusters match the trend seen in the Galaxy, stayinginstead at a constant value of roughly -0.8. Because not all[α/Fe] ratios are suppressed, these abundance ratios cannot beattributed solely to the injection of Type Ia supernova material andinstead reflect the differences in star formation history of the LMCversus the Milky Way. An extensive numerical experimental study wasperformed, varying both input parameters and stellar atmosphere models,to verify that the unusual abundance ratios derived in this study arenot the result of the adopted atomic parameters, stellar atmospheres, orstellar parameters. We conclude that many of the abundances in the LMCglobular clusters we observed are distinct from those observed in theMilky Way, and these differences are intrinsic to the stars in thosesystems.
| Relics of Metal-free Low-Mass Stars Exploding as Thermonuclear Supernovae
Renewed interest in the first stars that were formed in the universe hasled to the discovery of extremely iron-poor stars. Since severalcompeting scenarios exist, our understanding of the mass range thatdetermines the observed elemental abundances remains unclear. In thisstudy, we consider three well-studied metal-poor stars in terms of thetheoretical supernova (SN) model. Our results suggest that the observedabundance patterns in the metal-poor star BD +80 245 and the pair ofstars HD 134439/40 agree strongly with the theoretical possibility thatthese stars inherited their heavy-element abundance patterns from SNeinitiated by thermonuclear runaways in the degenerate carbon-oxygencores of primordial asymptotic giant branch stars with masses of ~3.5-5Msolar. Recent theoretical calculations have predicted thatsuch SNe could be originated from metal-free stars in theintermediate-mass range. On the other hand, intermediate-mass starscontaining some metals would end their lives as white dwarfs afterexpelling their envelopes in the wind due to intense momentum transportfrom outgoing photons to heavy elements. This new pathway for theformation of SNe requires that stars be formed from the primordial gas.Thus, we suggest that stars of a few solar masses were formed from theprimordial gas and that some of them caused thermonuclear explosionswhen the mass of their degenerate carbon-oxygen cores increased to theChandrasekhar limit without experiencing efficient mass loss.
| 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.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| Draco 119: A Remarkable Heavy-Element-deficient Giant
We report the abundance analysis of new high signal-to-noise ratio (S/N)spectra of the most metal-poor ([Fe/H]=-2.95) star currently known to bea member of a dwarf galaxy, the Draco dSph red giant D119. No absorptionlines for elements heavier than Ni are detected in two Keck HIRESspectra covering the 3850-6655 Å wavelength range, a phenomenonnot previously noted in any other metal-poor star. We present upperlimits for several heavy-element abundances. The most stringent limits,based on the nondetection of Sr II and Ba II lines, indicate that thetotal s- and r-process enrichment of D119 is at least 100 times smallerthan Galactic stars of similar metallicity. The light-element abundancesare consistent with the star having formed out of material enrichedprimarily by massive Type II supernovae (M>20-25Msolar).If this is the case, we are forced to conclude that massive, metal-poorType II supernovae did not contribute to the r-process in theproto-Draco environment. We compare the abundance pattern observed inD119 to current predictions of prompt enrichment and pair-instabilitysupernovae and find that the model predictions fail by an order ofmagnitude or more for many elements.Based on data obtained at the W. M. Keck Observatory, which is operatedas a scientific partnership among the California Institute ofTechnology, the University of California, and NASA, and was madepossible by the generous financial support of the W. M. Keck Foundation.
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| A CCD imaging search for wide metal-poor binaries
We explored the regions within a radius of 25 arcsec around 473 nearby,low-metallicity G- to M-type stars using (VR)I optical filters andsmall-aperture telescopes. About 10% of the sample was searched up toangular separations of 90 arcsec. We applied photometric and astrometrictechniques to detect true physical companions to the targets. The greatmajority of the sample stars was drawn from the Carney-Latham surveys;their metallicities range from roughly solar to [Fe/H] = -3.5 dex. OurI-band photometric survey detected objects that are between 0 and 5 magfainter (completeness) than the target stars; the maximum dynamicalrange of our exploration is 9 mag. We also investigated the literature,and inspected images from the Digitized Sky Surveys to complete oursearch. By combining photometric and proper motion measurements, weretrieved 29 previously known companions, and identified 13 new propermotion companions. Near-infrared 2MASS photometry is provided for thegreat majority of them. Low-resolution optical spectroscopy (386-1000nm) was obtained for eight of the new companion stars. Thesespectroscopic data confirm them as cool, late-type, metal-depleteddwarfs, with spectral classes from esdK7 to sdM3. After comparison withlow-metallicity evolutionary models, we estimate the masses of theproper motion companion stars to be in the range 0.5-0.1Mȯ. They are moving around their primary stars atprojected separations between 32 and 57 000 AU. These orbitalsizes are very similar to those of solar-metallicity stars of the samespectral types. Our results indicate that about 15% of the metal-poorstars have stellar companions in wide orbits, which is in agreement withthe binary fraction observed among main sequence G- to M-type stars andT Tauri stars.Based on observations made with the IAC80 telescope operated on theisland of Tenerife by the Instituto de Astrofísica de Canarias inthe Spanish Observatorio del Teide; also based on observations made withthe 2.2 m telescope of the German-Spanish Calar Alto Observatory(Almería, Spain), the William Herschel Telescope (WHT) operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos (ORM) of the Instituto deAstrofísica de Canarias; and the Telescopio Nazionale Galileo(TNG) at the ORM.The complete Table 1 is 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/419/167
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| Disk and Halo Wide Binaries from the Revised Luyten Catalog: Probes of Star Formation and MACHO Dark Matter
We present a catalog of 1147 candidate common proper motion binariesselected from the revised New Luyten Two-Tenths Catalog (NLTT). Amongthese, we identify 999 genuine physical pairs using the measuredproper-motion difference and the relative positions of each binary'scomponents on a reduced proper motion (RPM) diagram. The RPM positionsalso serve to classify them as either disk main-sequence pairs (801),halo subdwarf (116) pairs, or pairs containing at least one white dwarf(82). The disk and halo samples are complete to separations ofΔθ=500'' and Δθ=900'',which correspond to ~0.1 and ~1 pc, respectively. At wide separations,both distributions are well described by single power lawsdN/dΔθ~(Δθ)-α, withα=1.67+/-0.07 for the disk and α=1.55+/-0.10 for the halo.The fact that these distributions have similar slopes (and similarnormalizations as well) argues for similarity of the star formationconditions of these two populations. The fact that the halo binariesobey a single power law out to ~1 pc permits strong constraints on halodark matter candidates. At somewhat closer separations(10''<~Δθ<~25''), the diskdistribution shows a pronounced flattening, which is detected at veryhigh statistical significance and is not due to any obvious systematiceffect. We also present a list of 11 previously unknown halo stars withparallaxes that are recognized here as companions of Hipparcos stars.
| Accretion of Dust Grains as a Possible Origin of Metal-poor Stars with Low ?/Fe Ratios
The origin of low ?/Fe ratios in some metal-poor stars, theso-called low-? stars, is discussed. It is found that most oflow-? stars in the Galaxy are on the main sequence. This stronglysuggests that these stars suffered from external pollution. It is alsofound that the Zn/Fe abundance ratios of low-? stars both in theGalaxy and in dwarf spheroidal galaxies are lower than the average valueof Galactic halo stars, whereas damped Ly? absorbers have higherratios. This implies that some low-? stars accreted matter thatwas depleted from gas onto dust grains. To explain the features in theselow-? stars, we have proposed that metal-poor stars harboringplanetary systems are the origin of these low-? stars. Starsengulfing a small fraction of planetesimals enhance the surface contentof Fe to exhibit low ?/Fe ratios on their surfaces, while they areon the main sequence, because dwarfs have shallow surface convectionzones where the engulfed matter is mixed. After the stars leave the mainsequence, the surface convection zones become deeper, reducing theenhancement of Fe. Eventually, when the stars ascend to the tip of thered giant branch, they engulf giant planets to become low-? starsagain as observed in dwarf spheroidal galaxies. We predict thatlow-? stars with low Mn/Fe ratios harbor planetary systems.
| Lithium Abundance of Metal-poor Stars
High-resolution, high signal-to-noise ratio spectra have been obtainedfor 32 metal-poor stars. The equivalent widths of Li λ6708Åwere measured and the lithium abundances were derived. The averagelithium abundance of 21 stars on the lithium plateau is 2.33±0.02dex. The Lithium plateau exhibits a marginal trend along metallicity,dA(Li)/d[Fe/H] = 0.12±0.06, and no clear trend with the effectivetemperature. The trend indicates that the abundance of lithium plateaumay not be primordial and that a part of the lithium was produced inGalactic Chemical Evolution (GCE).
| Oxygen Abundances in Metal-poor Stars
We present oxygen abundances derived from both the permitted andforbidden oxygen lines for 55 subgiants and giants with [Fe/H] valuesbetween -2.7 and solar with the goal of understanding the discrepancy inthe derived abundances. A first attempt, using Teff valuesfrom photometric calibrations and surface gravities from luminositiesobtained agreement between the indicators for turn-off stars, but thedisagreement was large for evolved stars. We find that the difference inthe oxygen abundances derived from the permitted and forbidden lines ismost strongly affected by Teff, and we derive a newTeff scale based on forcing the two sets of lines to give thesame oxygen abundances. These new parameters, however, do not agree withother observables, such as theoretical isochrones or Balmer-line profilebased Teff determinations. Our analysis finds thatone-dimensional, LTE analyses (with published non-LTE corrections forthe permitted lines) cannot fully resolve the disagreement in the twoindicators without adopting a temperature scale that is incompatiblewith other temperature indicators. We also find no evidence ofcircumstellar emission in the forbidden lines, removing such emission asa possible cause for the discrepancy.
| Chemical Substructure in the Milky Way Halo: A New Population of Old Stars
We report the results of a coherent study of a new class of halo starsdefined on the basis of the chemical compositions of three metal-poorobjects ([Fe/H]~=-2) that exhibit unusually low abundances ofα-element (Mg, Si, Ca) and neutron-capture (Sr, Y, Ba) material.Our analyses confirm and expand on earlier reports of atypical α-and neutron-capture abundances in BD +80°245, G4-36, and CS22966-043. We also find that the latter two stars exhibit unusualrelative abundance enhancements within the iron peak (Cr, Mn, Ni, Zn),along with what may be large abundances of Ga, an element not previouslyreported as being observed in any metal-poor star. These results providefurther evidence that chemical enrichment and star formation historiesvaried from region to region within the Milky Way halo. Comparing thechemical abundances of the newly identified stellar population tosupernova model yields, we derive supernova ratios of Type Ia versusType II events in the range of0.6<~(NIa/NII)NewPop<~1.3. Forthe Sun, we derive0.18+/-0.01<(NIa/NII)solar<0.25+/-0.06,supernova ratios in good agreement with values found in the literature.Given the relatively low metallicity and relatively highIa/NII> ratios of the low-α stars studiedhere, these objects may have been born from material produced in theyields of the earliest Type Ia supernova events. We also report theresults of a preliminary attempt to employ the observed chemicalabundances of low-metallicity stars in the identification, and possiblecosmic evolution, of Type Ia supernova progenitors, and we discuss thelimitations of current model yields.Based on data acquired at the following facilities: McDonaldObservatory, which is operated by the University of Texas at Austin; LasCampanas Observatory, which is operated by the Observatories of theCarnegie Institution of Washington; W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA; and Kitt PeakNational Observatory, National Optical Astronomy Observatory, which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under cooperative agreement with the National ScienceFoundation (NSF).
| Completeness of USNO-B for High Proper Motion Stars
I test the completeness of USNO-B detections of high proper motion(μ>180 mas yr-1) stars and the accuracy of itsmeasurements by comparing them to the revised New Luyten Two-Tenthscatalog of Salim & Gould. For 14.5~20 mas yr-1) may actuallyhave still larger errors than tabulated.
| Mapping the Galactic Halo. VI. Spectroscopic Measures of Luminosity and Metallicity
We present our calibration of spectroscopic measures of luminosity andmetallicity for halo giant candidates and give metallicities anddistances for our first sample of spectroscopically confirmed giants.These giants have distances ranging from 15 to 83 kpc. As surveys reachfarther into the Galaxy's halo with K giant samples, identification ofgiants becomes more difficult. This is because the numbers of foregroundhalo K dwarfs rise for V magnitudes of 19-20, typical for halo giants at~100 kpc. Our photometric survey uses the strength of the Mg b/H featurenear 5170 Å to weed K dwarfs out of the disk and thick disk, butwe need spectroscopic measures of the strength of the Ca II K, Ca Iλ4227, and Mg b/H features to distinguish between the verymetal-poor dwarfs and halo giants. Using a full error analysis of ourspectroscopic measures, we show why a signal-to-noise ratio of ~15pixel-1 at Ca I λ4227 and ~10 at Ca II K is needed forreliable luminosity discrimination. We use the Ca II K and Mg b featuresto measure metallicity in our halo giants, with typical errors (randomplus systematic) of 0.3 dex for [Fe/H] values from -0.8 to -3.0.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Zurafa |
| Sað Açýklýk: | 08h11m06.24s |
| Yükselim: | +79°54'29.6" |
| Görünürdeki Parlaklýk: | 9.98 |
| özdevim Sað Açýklýk: | 135 |
| özdevim Yükselim: | -365.1 |
| B-T magnitude: | 10.684 |
| V-T magnitude: | 10.039 |
Kataloglar ve belirtme:
|
