תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
The NIR Ca ii triplet at low metallicity. Searching for extremely low-metallicity stars in classical dwarf galaxies
The NIR Ca ii triplet absorption lines have proven to be an importanttool for quantitative spectroscopy of individual red giant branch starsin the Local Group, providing a better understanding of metallicities ofstars in the Milky Way and dwarf galaxies and thereby an opportunity toconstrain their chemical evolution processes. An interesting puzzle inthis field is the significant lack of extremely metal-poor stars, below[Fe/H] = -3, found in classical dwarf galaxies around the MilkyWay using this technique. The question arises whether these stars arereally absent, or if the empirical Ca ii triplet method used to studythese systems is biased in the low-metallicity regime. Here we presentresults of synthetic spectral analysis of the Ca ii triplet, that isfocused on a better understanding of spectroscopic measurements oflow-metallicity giant stars. Our results start to deviate strongly fromthe widely-used and linear empirical calibrations at [Fe/H] <-2. We provide a new calibration for Ca ii triplet studies whichis valid for -0.5 ? [Fe/H] ? -4. We subsequently applythis new calibration to current data sets and suggest that the classicaldwarf galaxies are not so devoid of extremely low-metallicity stars aswas previously thought.Using observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile proposal171.B-0588.
| Study of isotopic fractions and abundances of the neutron-capture elements in HD 175305
The chemical abundances of metal-poor stars are excellent sources ofinformation for setting new constraints on models of Galactic chemicalevolution at low metallicities. In this paper we present an attempt tofit the elemental abundances observed in the bright, metal-poor giant HD175305, and derive isotopic fractions using a parametric model. Theobserved abundances can be well matched by the combined contributionsfrom s- and r-process material. The component coefficients of the r- ands-processes are C1 = 3.220 and C3 = 1.134,respectively. The Sm isotopic fraction in this star where the observedneutron-capture elements are produced is predicted to bef152+154 = 0.582, which suggests that, even though ther-process is predominantly responsible for the synthesis of theneutron-capture elements in the early Galaxy, the onset of the s-processhad already occurred at this metallicity of [Fe/H] = -1.6.
| Chemical composition of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy
Context: Individual stars in dwarf spheroidal galaxies around the MilkyWay Galaxy have been studied both photometrically and spectroscopically.Extremely metal-poor stars among them are very valuable because theyshould record the early enrichment in the Local Group. However, ourunderstanding of these stars is very limited because detailed chemicalabundance measurements are needed from high resolution spectroscopy. Aims: To constrain the formation and chemical evolution of dwarfgalaxies, metallicity and chemical composition of extremely metal-poorstars are investigated. Methods: Chemical abundances of sixextremely metal-poor ([Fe/H] < -2.5) stars in the Sextans dwarfspheroidal galaxy are determined based on high resolution spectroscopy(R=40 000) with the Subaru Telescope High Dispersion Spectrograph. Results: (1) The Fe abundances derived from the high resolutionspectra are in good agreement with the metallicity estimated from the Catriplet lines in low resolution spectra. The lack of stars with [Fe/H]? -3 in Sextans, found by previous estimates from the Ca triplet, isconfirmed by our measurements, although we note that high resolutionspectroscopy for a larger sample of stars will be necessary to estimatethe true fraction of stars with such low metallicity. (2) While oneobject shows an overabundance of Mg (similar to Galactic halo stars),the Mg/Fe ratios of the remaining five stars are similar to the solarvalue. This is the first time that low Mg/Fe ratios at such lowmetallicities have been found in a dwarf spheroidal galaxy. No evidencefor over-abundances of Ca and Ti are found in these five stars, thoughthe measurements for these elements are less certain. Possiblemechanisms to produce low Mg/Fe ratios, with respect to that of Galactichalo stars, are discussed. (3) Ba is under-abundant in four objects,while the remaining two stars exhibit large and moderate excesses ofthis element. The abundance distribution of Ba in this galaxy is similarto that in the Galactic halo, indicating that the enrichment of heavyelements, probably by the r-process, started at metallicities [Fe/H]? -2.5, as found in the Galactic halo.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.Table [see full text] is only available in electronic form athttp://www.aanda.org
| Chemical Evolution of Zinc in the Galaxy
Not Available
| Explosive Nucleosynthesis of Weak R-Process Elements in Extremely Metal-Poor Core-Collapse Supernovae
There have been attempts to fit the abundance patterns of extremelymetal-poor (EMP) stars with supernova (SN) nucleosynthesis models forthe elements lighter than Zn. Observations have revealed the presence ofEMP stars with a peculiarly high ratio of "weak r-process elements" Sr,Y, and Zr. Although several possible processes were suggested for theorigin of these elements, a complete solution for reproducing thoseratios has not yet been found. In order to reproduce the abundancepatterns of such stars, we investigate a model with neutron-rich matterejection from the inner region of the conventional mass-cut. We findthat explosive nucleosynthesis in a high energy SN (or "hypernova") canreproduce the high abundances of Sr, Y, and Zr but that the enhancementsof Sr, Y, and Zr are not achieved by nucleosynthesis in a normal SN. Ourresults imply that, if these elements are ejected from a normal SN,nucleosynthesis in higher entropy flow than that of the SN shock isrequired.
| 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.
| Nucleosynthesis modes in the high-entropy-wind of type II supernovae
The exact conditions for the supernova high-entropy wind (HEW) as one ofthe favored sites for the rapid neutron-capture (r-) process stillcannot be reproduced selfconsistently in present hydrodynamicsimulations. Therefore, we have performed large-scale networkcalculations within a parameterized HEW model to constrain the necessaryconditions for a full r-process, and to compare our results with recentastronomical observations. A superposition of entropy trajectories withmodel-inherent weightings results in an excellent reproduction of theoverall solar-system isotopic abundances (Nr,?) of the“main” r-process elements beyond Sn. For the lighterr-elements, our model supports earlier qualitative ideas about amultiplicity of nucleosynthesis processes in the Fe-group region. In thehigh-entropy-wind scenario, these suggestions are quantified, and theorigin of the “missing” abundances to Nr,?is determined to be a rapid primary charged-particle (?-) process,thus excluding a classical “weak” neutron-capture component.This explains the recent halo-star observations of a non-correlation ofCu Ge and Sr Zr with metallicity [Fe/H] and r-process enrichment [Eu/H].Moreover, for the first time a partial correlation with the“main” r-process is identified for Ru and Pd.
| The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) Project. I. The Lithium-, s-, and r-enhanced Metal-poor Giant HKII 17435-00532
We present the first detailed abundance analysis of the metal-poor giantHKII 17435-00532. This star was observed as part of the University ofTexas long-term project Chemical Abundances of Stars in the Halo (CASH).A spectrum was obtained with the High Resolution Spectrograph (HRS) onthe Hobby-Eberly Telescope with a resolving power of R~15,000. Ouranalysis reveals that this star may be located on the red giant branch,red horizontal branch, or early asymptotic giant branch. We find thatthis metal-poor ([Fe/H]=-2.2) star has an unusually high lithiumabundance [log?(Li)=+2.1], mild carbon ([C/Fe]=+0.7) and sodium([Na/Fe]=+0.6) enhancement, as well as enhancement of both s-process([Ba/Fe]=+0.8) and r-process ([Eu/Fe]=+0.5) material. The high Liabundance can be explained by self-enrichment through extra mixing thatconnects the convective envelope with the outer regions of the H-burningshell. If so, HKII 17435-00532 is the most metal-poor star in which thisshort-lived phase of Li enrichment has been observed. The Na andn-capture enrichment can be explained by mass transfer from a companionthat passed through the thermally pulsing AGB phase of evolution withonly a small initial enrichment of r-process material present in thebirth cloud. Despite the current nondetection of radial velocityvariations (over ~180 days), it is possible that HKII 17435-00532 is ina long-period or highly inclined binary system, similar to other starswith similar n-capture enrichment patterns.Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| Europium, Samarium, and Neodymium Isotopic Fractions in Metal-Poor Stars
We have derived isotopic fractions of europium, samarium, and neodymiumin two metal-poor giants with differing neutron-capture nucleosynthetichistories. These isotopic fractions were measured from newhigh-resolution (R~120,000), high signal-to-noise ratio (S/N ~ 160-1000)spectra obtained with the 2d-coudé spectrograph of McDonaldObservatory's 2.7 m Smith telescope. Synthetic spectra were generatedusing recent high-precision laboratory measurements of hyperfine andisotopic subcomponents of several transitions of these elements andmatched quantitatively to the observed spectra. We interpret ourisotopic fractions by the nucleosynthesis predictions of the stellarmodel, which reproduces s-process nucleosynthesis from the physicalconditions expected in low-mass, thermally pulsing stars on the AGB, andthe classical method, which approximates s-process nucleosynthesis by asteady neutron flux impinging on Fe-peak seed nuclei. Our Eu isotopicfraction in HD 175305 is consistent with an r-process origin by theclassical method and is consistent with either an r- or an s-processorigin by the stellar model. Our Sm isotopic fraction in HD 175305suggests a predominantly r-process origin, and our Sm isotopic fractionin HD 196944 is consistent with an s-process origin. The Nd isotopicfractions, while consistent with either r-process or s-process origins,have very little ability to distinguish between any physical values forthe isotopic fraction in either star. This study for the first timeextends the n-capture origin of multiple rare earths in metal-poor starsfrom elemental abundances to the isotopic level, strengthening ther-process interpretation for HD 175305 and the s-process interpretationfor HD 196944.
| Rotation and Macroturbulence in Metal-Poor Field Red Giant and Red Horizontal Branch Stars
We report the results for rotational velocities, Vrot sin i,and macroturbulence dispersions, ζRT, for 12 metal-poorfield red giant branch (RGB) stars and 7 metal-poor field red horizontalbranch (RHB) stars. The results are based on Fourier transform analysesof absorption line profiles from high-resolution (R ≈ 120,000),high-S/N (≈215 per pixel; ≈345 per resolution element) spectraobtained with the Gecko spectrograph at the Canada-France-HawaiiTelescope (CFHT). The stars were selected from the authors' previousstudies of 20 RHB and 116 RGB stars, based primarily onlarger-than-average line-broadening values. We find thatζRT values for the metal-poor RGB stars are very similarto those for metal-rich disk giants studied earlier by Gray and hiscollaborators. Six of the RGB stars have small rotational values, lessthan 2.0 km s-1, while five show significantrotation/enhanced line broadening, over 3 km s-1. We confirmthe rapid rotation rate for RHB star HD 195636, found earlier byPreston. This star's rotation is comparable to that of the fastest knownrotating blue horizontal branch (BHB) stars, when allowance is made fordifferences in radii and moments of inertia. The other six RHB starshave somewhat lower rotation but show a trend to higher values at highertemperatures (lower radii). Comparing our results with those for BHBstars from Kinman et al., we find that the fraction of rapidly rotatingRHB stars is somewhat lower than is found among BHB stars. The number ofrapidly rotating RHB stars is also smaller than we would have expectedfrom the observed rotation of the RGB stars. We devise two empiricalmethods to translate our earlier line-broadening results intoVrot sin i for all the RGB and RHB stars they studied.Binning the RGB stars by luminosity, we find that most metal-poor fieldRGB stars show no detectable sign, on average, of rotation, which is notsurprising given the stars' large radii. However, the most luminousstars, with MV <= -1.5, do show net rotation, with meanvalues of 2-4 km s-1, depending on the algorithm employed,and also show signs of radial velocity jitter and mass loss. This"rotation" may in fact prove to be due to other line-broadening effects,such as shock waves or pulsation.Based on observations obtained at the Canada-France-Hawaii Telescope(CFHT) which is operated by the National Research Council of Canada, theInstitut National des Sciences de l'Univers of the Centre National de laRecherche Scientifique de France, and the University of Hawaii.
| Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants
We present the parameters of 891 stars, mostly clump giants, includingatmospheric parameters, distances, absolute magnitudes, spatialvelocities, galactic orbits and ages. One part of this sample consistsof local giants, within 100 pc, with atmospheric parameters eitherestimated from our spectroscopic observations at high resolution andhigh signal-to-noise ratio, or retrieved from the literature. The otherpart of the sample includes 523 distant stars, spanning distances up to1 kpc in the direction of the North Galactic Pole, for which we haveestimated atmospheric parameters from high resolution but lowsignal-to-noise Echelle spectra. This new sample is kinematicallyunbiased, with well-defined boundaries in magnitude and colours. Werevisit the basic properties of the Galactic thin disk as traced byclump giants. We find the metallicity distribution to be different fromthat of dwarfs, with fewer metal-rich stars. We find evidence for avertical metallicity gradient of -0.31 dex kpc-1 and for atransition at ~4-5 Gyr in both the metallicity and velocities. Theage-metallicity relation (AMR), which exhibits a very low dispersion,increases smoothly from 10 to 4 Gyr, with a steeper increase for youngerstars. The age-velocity relation (AVR) is characterized by thesaturation of the V and W dispersions at 5 Gyr, and continuous heatingin U.
| Line Broadening in Field Metal-Poor Red Giant and Red Horizontal Branch Stars
We report 349 radial velocities for 45 metal-poor field red giant branch(RGB) and red horizontal branch (RHB) stars, with time coverage rangingfrom 1 to 21 years. We have identified one new spectroscopic binary, HD4306, and one possible such system, HD 184711. We also provide 57 radialvelocities for 11 of the 91 stars reported in our previous work. All butone of the 11 stars had been found to have variable radial velocities.New velocities for the long-period spectroscopic binaries BD-1 2582 andHD 108317 have extended the time coverage to 21.7 and 12.5 years,respectively, but in neither case have we yet completed a full orbitalperiod. As was found in the previous study, radial velocity "jitter" ispresent in many of the most luminous stars. Excluding stars showingspectroscopic binary orbital motion, all 7 of the red giants withestimated MV values more luminous than -2.0 display jitter,as well as 3 of the 14 stars with -2.0 < MV <= -1.4. Wehave also measured the line broadening in all the new spectra, usingsynthetic spectra as templates. Comparison with results fromhigh-resolution and higher signal-to-noise (S/N) spectra employed byother workers shows good agreement down to line-broadening levels of 3km s-1, well below our instrumental resolution of 8.5 kms-1. As the previous work demonstrated, the majority of themost luminous red giants show significant line broadening, as do many ofthe red horizontal branch stars, and we briefly discuss possible causes.The line broadening appears related to velocity jitter, in that bothappear primarily among the highest luminosity red giants.
| Nucleosynthesis in the Early Galaxy
Recent observations of r-process-enriched metal-poor star abundancesreveal a nonuniform abundance pattern for elements Z<=47. Based onnoncorrelation trends between elemental abundances as a function of Eurichness in a large sample of metal-poor stars, it is shown that themixing of a consistent and robust light element primary process (LEPP)and the r-process pattern found in r-II metal-poor stars explains suchapparent nonuniformity. Furthermore, we derive the abundance pattern ofthe LEPP from observation and show that it is consistent with a missingcomponent in the solar abundances when using a recent s-process model.As the astrophysical site of the LEPP is not known, we explore thepossibility of a neutron-capture process within a site-independentapproach. It is suggested that scenarios with neutron densitiesnn<=1013 cm-3 or in the rangenn>=1024 cm-3 best explain theobservations.
| Halo Star Streams in the Solar Neighborhood
We have assembled a sample of halo stars in the solar neighborhood tolook for halo substructure in velocity and angular momentum space. Oursample (231 stars) includes red giants, RR Lyrae variable stars, and redhorizontal branch stars within 2.5 kpc of the Sun with [Fe/H] less than-1.0. It was chosen to include stars with accurate distances, spacevelocities, and metallicities, as well as well-quantified errors. Withour data set, we confirm the existence of the streams found by Helmi andcoworkers, which we refer to as the H99 streams. These streams have adouble-peaked velocity distribution in the z-direction (out of theGalactic plane). We use the results of modeling of the H99 streams byHelmi and collaborators to test how one might use vz velocityinformation and radial velocity information to detect kinematicsubstructure in the halo. We find that detecting the H99 streams withradial velocities alone would require a large sample (e.g.,approximately 150 stars within 2 kpc of the Sun and within 20° ofthe Galactic poles). In addition, we use the velocity distribution ofthe H99 streams to estimate their age. From our model of the progenitorof the H99 streams, we determine that it was accreted between 6 and 9Gyr ago. The H99 streams have [α/Fe] abundances similar to otherhalo stars in the solar neighborhood, suggesting that the gas thatformed these stars were enriched mostly by Type II supernovae. We havealso discovered in angular momentum space two other possiblesubstructures, which we refer to as the retrograde and progradeoutliers. The retrograde outliers are likely to be halo substructure,but the prograde outliers are most likely part of the smooth halo. Theretrograde outliers have significant structure in the vφdirection and show a range of [α/Fe], with two having low[α/Fe] for their [Fe/H]. The fraction of substructure stars in oursample is between 5% and 7%. The methods presented in this paper can beused to exploit the kinematic information present in future largedatabases like RAVE, SDSS-II/SEGUE, and Gaia.
| Neutron-Capture Elements in the Very Metal-poor Star HD 88609: Another Star with Excesses of Light Neutron-Capture Elements
We obtained a high-resolution, high-signal-to-noise UV-blue spectrum ofthe extremely metal-poor red giant HD 88609 to determine the abundancesof heavy elements. Nineteen neutron-capture elements are detected in thespectrum. Our analysis revealed that this object has large excesses oflight neutron-capture elements, while heavy neutron-capture elements aredeficient. The abundance pattern shows a continuously decreasing trendas a function of atomic number, from Sr to Yb, which is quite differentfrom those in stars with excesses of r-process elements. Such anabundance pattern is very similar to that of HD 122563, which wasstudied in our previous work. The results indicate that the abundancepattern found in the two stars could represent the pattern produced bythe nucleosynthesis process that provided light neutron-capture elementsin the very early Galaxy.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Measuring the Balmer Jump and the Effective Gravity in FGK Stars
It is difficult to accurately measure the effective gravity (logg) inlate-type stars using broadband (e.g., UBV or SDSS) or intermediate-band(uvby) photometric systems, especially when the stars can cover a rangeof metallicities and reddenings. However, simple spectroscopicobservational and data reduction techniques can yield accurate valuesfor logg through comparison of the Balmer jumps of low-resolutionspectra with recent grids of synthetic flux spectra.
| 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.
| 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.
| Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman
We develop and test a method for the estimation of metallicities([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhancedmetal-poor (CEMP) stars based on the application of artificial neuralnetworks, regressions, and synthesis models to medium-resolution (1-2Å) spectra and J-K colors. We calibrate this method by comparisonwith metallicities and carbon abundance determinations for 118 starswith available high-resolution analyses reported in the recentliterature. The neural network and regression approaches make use of apreviously defined set of line-strength indices quantifying the strengthof the Ca II K line and the CH G band, in conjunction with J-K colorsfrom the Two Micron All Sky Survey Point Source Catalog. The use ofnear-IR colors, as opposed to broadband B-V colors, is required becauseof the potentially large affect of strong molecular carbon bands onbluer color indices. We also explore the practicality of obtainingestimates of carbon abundances for metal-poor stars from the spectralinformation alone, i.e., without the additional information provided byphotometry, as many future samples of CEMP stars may lack such data. Wefind that although photometric information is required for theestimation of [Fe/H], it provides little improvement in our derivedestimates of [C/Fe], and hence, estimates of carbon-to-iron ratios basedsolely on line indices appear sufficiently accurate for most purposes.Although we find that the spectral synthesis approach yields the mostaccurate estimates of [C/Fe], in particular for the stars with thestrongest molecular bands, it is only marginally better than is obtainedfrom the line index approaches. Using these methods we are able toreproduce the previously measured [Fe/H] and [C/Fe] determinations withan accuracy of ~0.25 dex for stars in the metallicity interval-5.5<=[Fe/H]<=-1.0 and with 0.2<=(J-K)0<=0.8. Athigher metallicity, the Ca II K line begins to saturate, especially forthe cool stars in our program, and hence, this approach is not useful insome cases. As a first application, we estimate the abundances of [Fe/H]and [C/Fe] for the 56 stars identified as possibly carbon-rich, relativeto stars of similar metal abundance, in the sample of ``strong G-band''stars discussed by Beers, Preston, and Shectman.
| Survey for Transiting Extrasolar Planets in Stellar Systems. II. Spectrophotometry and Metallicities of Open Clusters
We present metallicity estimates for seven open clusters based onspectrophotometric indices from moderate-resolution spectroscopy.Observations of field giants of known metallicity provide a correlationbetween the spectroscopic indices and the metallicity of open clustergiants. We use χ2 analysis to fit the relation ofspectrophotometric indices to metallicity in field giants. The resultingfunction allows an estimate of the target-cluster giants' metallicitieswith an error in the method of +/-0.08 dex. We derive the followingmetallicities for the seven open clusters: NGC 1245, [M/H]=-0.14+/-0.04NGC 2099, [M/H]=+0.05+/-0.05 NGC 2324, [M/H]=-0.06+/-0.04 NGC 2539,[M/H]=-0.04+/-0.03 NGC 2682 (M67), [M/H]=-0.05+/-0.02 NGC 6705,[M/H]=+0.14+/-0.08 NGC 6819, [M/H]=-0.07+/-0.12. These metallicityestimates will be useful in planning future extrasolar planet transitsearches, since planets may form more readily in metal-richenvironments.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra
The LTE abundances of sulfur (S) of 21 metal-poor stars and one normalstar were explored in the metallicity range of -3 < [Fe/H] ≤ 0,based on the equivalent widths of the S I (1) 9212, 9237Å and S I(6) 8693, 9894Å lines measured on high-resolution spectra, whichwere observed by the OAO 1.88-m telescope equipped with HIDES. Our mainresults are: (1) The abundances derived from the S I (6) lines areconsistent with those from the S I (1) lines among our sample stars inthe range of [Fe/H] > -2 with an average difference of +0.03 ±0.05 dex, whereas a significant discrepancy is observed in the range of[Fe/H] ≤ -2. (2) The behavior of [S(6)/Fe], versus [Fe/H] of ourhalo sample stars exhibits a nearly flat trend with an average of +0.62± 0.09 dex in the range of -3 < [Fe/H] < -1.25, and shows adistribution around +0.29 dex in -1.25 ≤ [Fe/H] ≤ -0.7. Oursample stars with -1.25 ≤ [Fe/H] ≤ -0.5 follow an increasingtrend with decreasing [Fe/H]. The behavior of [S(1)/Fe] of our samplestars also shows essentially the same trend as [S(6)/Fe], though it isquantitatively different. (3) The S behavior in the range of -3 <[Fe/H] ≤ 0 inferred from the abundances of multiplets 6 and 1 arequalitatively consistent with each other, and may be represented by acombination of a nearly flat trend and a linearly increasing trend withdecreasing [Fe/H]. A transition of the trend is likely to occur at[Fe/H] -1.5 dex.
| 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.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. I. Observational Data
We have obtained high-resolution (R~=50,000 or 90,000), high-quality(S/N>~100) spectra of 22 very metal-poor stars ([Fe/H]<~-2.5) withthe High Dispersion Spectrograph fabricated for the 8.2 m SubaruTelescope. The spectra cover the wavelength range from 3500 to 5100Å equivalent widths are measured for isolated lines of numerouselemental species, including the α-elements, the iron-peakelements, and the light and heavy neutron-capture elements. Errors inthe measurements and comparisons with previous studies are discussed.These data will be used to perform detailed abundance analyses in thefollowing papers of this series. Radial velocities are also reported andare compared with previous studies. At least one moderatelyr-process-enhanced metal-poor star, HD 186478, exhibits evidence of asmall-amplitude radial velocity variation, confirming the binary statusnoted previously. During the course of this initial program, we havediscovered a new moderately r-process-enhanced, very metal-poor star, CS30306-132 ([Fe/H]=-2.4 [Eu/Fe]=+0.85), which is discussed in detail inthe companion paper.Based on data collected at Subaru Telescope, which is operated by theNational Astronomical Observatory of Japan.
| Spectroscopic Studies of Extremely Metal-Poor Stars with the Subaru High Dispersion Spectrograph. II. The r-Process Elements, Including Thorium
We have obtained high-resolution, high signal-to-noise near-UV-bluespectra of 22 very metal-poor stars ([Fe/H]<-2.5) with the SubaruHigh Dispersion Spectrograph and measured the abundances of elementsfrom C to Th. The metallicity range of the observed stars is-3.2<[Fe/H]<-2.4. As found by previous studies, the star-to-starscatter in the measured abundances of neutron-capture elements in thesestars is very large, much greater than could be assigned toobservational errors, in comparison with the relatively small scatter inthe α- and iron-peak elements. In spite of the large scatter inthe ratios of the neutron-capture elements relative to iron, theabundance patterns of heavy neutron-capture elements (56<=Z<~72)are quite similar within our sample stars. The Ba/Eu ratios in the 11very metal-poor stars in our sample in which both elements have beendetected are nearly equal to that of the solar system r-processcomponent. Moreover, the abundance patterns of the heavy neutron-captureelements (56<=Z<=70) in seven objects with clear enhancements ofthe neutron-capture elements are similar to that of the solar systemr-process component. These results prove that heavy neutron-captureelements in these objects are primarily synthesized by the r-process. Incontrast, the abundance ratios of the light neutron-capture elements(38<=Z<=46) relative to the heavier ones (56<=Z<=70) exhibita large dispersion. Our inspection of the correlation between Sr and Baabundances in very metal-poor stars reveals that the dispersion of theSr abundances clearly decreases with increasing Ba abundance. This trendis naturally explained by hypothesizing the existence of two processes,one that produces Sr without Ba and another that produces Sr and Ba insimilar proportions. This result should provide a strong constraint onthe origin of the light neutron-capture elements at low metallicity. Wehave identified a new highly r-process element enhanced, metal-poorstar, CS 22183-031, a giant with [Fe/H]=-2.93 and [Eu/Fe]=+1.2. We alsoidentified a new, moderately r-process-enhanced, metal-poor star, CS30306-132, a giant with [Fe/H]=-2.42 and [Eu/Fe]=+0.85. The abundanceratio of the radioactive element Th (Z=90) relative to the stablerare-earth elements (e.g., Eu) in very metal-poor stars has been used asa cosmochronometer by a number of previous authors. Thorium is detectedin seven stars in our sample, including four objects for which thedetection of Th has already been reported. New detections of thoriumhave been made for the stars HD 6268, HD 110184, and CS 30306-132. TheTh/Eu abundance ratios [log(Th/Eu)], are distributed over the range-0.10 to -0.59, with typical errors of 0.10 to 0.15 dex. In particular,the ratios in two stars, CS 31082-001 and CS 30306-132, aresignificantly higher than the ratio in the well-studied object CS22892-052 and those of other moderately r-process-enhanced metal-poorstars previously reported. Since these very metal-poor stars arebelieved to be formed in the early Galaxy, this result suggests that theabundance ratios between Th and stable rare-earth elements such as Eu,both of which are presumably produced by r-process nucleosynthesis, mayexhibit real star-to-star scatter, with implications for (1) theastrophysical sites of the r-process, and (2) the use of Th/Eu as acosmochronometer.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| 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.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
קטלוגים וכינוים:
|
