내용
사진
사진 업로드
DSS Images Other Images
관련 글
Potassium Abundances in Red Giants of Mildly to Very Metal-Poor Globular Clusters
A non-LTE analysis of K I resonance lines at 7664.91 and 7698.97 A wascarried out for 15 red giants belonging to three globular clusters ofdifferent metallicity (M 4, M 13, and M 15) along with two referenceearly-K giants (rho Boo and alpha Boo), in order to check whether the Kabundances are uniform within a cluster and to investigate the behaviorof [K/Fe] ratio at the relevant metallicity range of -2.5 <[Fe/H]< -1. We confirmed that [K/H] (as well as [Fe/H]) is almosthomogeneous within each cluster to a precision of < ~0.1 dex, thoughdubiously large deviations are exceptionally seen for two peculiar starsshowing signs of considerably increased turbulence in the upperatmosphere. The resulting [K/Fe] ratios are mildly supersolar by a fewtenths of dex for three clusters, tending to gradually increase from~+0.1-0.2 at [Fe/H] ~-1 to ~+0.3 at [Fe/H] ~ -2.5. This result connectsreasonably well with the [K/Fe] trend of disk stars (-1 < [Fe/H]) andthat of extremely metal-poor stars (-4 <[Fe/H] < -2.5). That is,[K/Fe] appears to continue a gradual increase from [Fe/H]~0 toward alower metallicity regime down to [Fe/H]~-3, where a broad maximum of[K/Fe]~+0.3-0.4 is attained, possibly followed by a slight downturn at[Fe/H]<~-3.
| Evolution of the barium abundance in the early Galaxy from a NLTE analysis of the Ba lines in a homogeneous sample of EMP stars
Context: Barium is a key element in constraining the evolution of the(not well understood) r-process in the first galactic stars andcurrently the Ba abundances in these very metal-poor stars were mostlymeasured under the Local Thermodynamical Equilibrium (LTE) assumption,which may lead in general to an underestimation of Ba. Aims: We presenthere determinations of the barium abundance taking into account thenon-LTE (NLTE) effects in a sample of extremely metal-poor stars (EMPstars): 6 turnoff stars and 35 giants. Methods: The NLTE profiles ofthe three unblended Ba II lines (4554 Å, 5853 Å, 6496Å) have been computed. The computations were made with a modifiedversion of the MULTI code, applied to an atomic model of the Ba atomwith 31 levels of Ba I, 101 levels of Ba II, and compared to theobservations. Results: The ratios of the NLTE abundances of bariumrelative to Fe are slightly shifted towards the solar ratio. In the plotof [Ba/Fe] versus [Fe/H], the slope of the regression line is slightlyreduced as is the scatter. In the interval -3.3 <[Fe/H] < -2.6,[Ba/Fe] decreases with a slope of about 1.4 and a scatter close to 0.44.For [Fe/H] <-3.3 the number of stars is not sufficient to decidewhether [Ba/Fe] keeps decreasing (and then CD-38:245 should beconsidered as a peculiar “barium-rich star”) or if a plateauis reached as soon as [Ba/Fe] ≈ -1. In both cases the scatter remainsquite large, larger than what can be accounted for by the measurementand determination errors, suggesting the influence of a complex processof Ba production, and/or inefficient mixing in the early Galaxy.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory (Large Programme “First Stars”, ID165.N-0276; P.I.: R. Cayrel.
| The Abundance Spread in the Boötes I Dwarf Spheroidal Galaxy
We present medium-resolution spectra of 16 radial velocity red-giantmembers of the low-luminosity Boötes I dwarf spheroidal (dSph)galaxy that have sufficient S/N for abundance determination, based onthe strength of the Ca II K line. Assuming [Ca/Fe] ~ 0.3, the abundancerange in the sample is Δ[Fe/H] ~ 1.7 dex, with one star having[Fe/H] = -3.4. The dispersion is σ([Fe/H]) = 0.45 +/- 0.08-similarto those of the Galaxy's more luminous dSph systems and ωCentauri. This suggests that the large mass (>~107Msolar) normally assumed to foster self-enrichment and theproduction of chemical abundance spreads was provided by the nonbaryonicmaterial in Boötes I.
| NLTE determination of the aluminium abundance in a homogeneous sample of extremely metal-poor stars
Aims. Aluminium is a key element to constrain the models of the chemicalenrichment and the yields of the first supernovae. But obtaining preciseAl abundances in extremely metal-poor (EMP) stars requires that thenon-LTE effects be carefully taken into account. Methods: The NLTEprofiles of the blue resonance aluminium lines have been computed in asample of 53 extremely metal-poor stars with a modified version of theprogram MULTI applied to an atomic model of the Al atom with 78 levelsof Al I and 13 levels of Al II, and compared to the observations. Results: With these new determinations, all the stars of the sample showa ratio Al/Fe close to the solar value: [Al/Fe] = -0.06±0.10 witha very small scatter. These results are compared to the models of thechemical evolution of the halo using different models of SN II and arecompatible with recent computations. The sodium-rich giants are notfound to be also aluminium-rich and thus, as expected, the convection inthese giants only brings to the surface the products of the Ne-Na cycle.Based on observations obtained with the ESO Very Large Telescope atParanal Observatory, Chile (Large Programme “First Stars”,ID 165.N-0276(A); P.I.: R. Cayrel).
| 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.
| First stars. VIII. Enrichment of the neutron-capture elements in the early Galaxy
Context: Extremely metal-poor (EMP) stars in the halo of the Galaxy aresensitive probes of the production of the first heavy elements and theefficiency of mixing in the early interstellar medium. The heaviestmeasurable elements in such stars are our main guides to understandingthe nature and astrophysical site(s) of early neutron-capturenucleosynthesis. Aims: Our aim is to measure accurate, homogeneousneutron-capture element abundances for the sample of 32 EMP giant starsstudied earlier in this series, including 22 stars with [Fe/H]< -3.0. Methods: Based on high-resolution, high S/N spectra from the ESOVLT/UVES, 1D, LTE model atmospheres, and synthetic spectrum fits, wedetermine abundances or upper limits for the 16 elements Sr, Y, Zr, Ba,La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb in all stars. Results: As found earlier, [Sr/Fe], [Y/Fe], [Zr/Fe] and [Ba/Fe] arebelow Solar in the EMP stars, with very large scatter. However, we finda tight anti-correlation of [Sr/Ba], [Y/Ba], and [Zr/Ba] with [Ba/H] for-4.5 <[Ba/H] < -2.5, also when subtracting the contribution of themain r-process as measured by [Ba/H]. Spectra of even higher S/N ratioare needed to confirm and extend these results below [Fe/H] ≃ -3.5.The huge, well-characterised scatter of the [n-capture/Fe] ratios in ourEMP stars is in stark contrast to the negligible dispersion in the [α/Fe] and [Fe-peak/Fe] ratios for the same stars found in Paper V. Conclusions: These results demonstrate that a second(“weak” or LEPP) r-process dominates the production of thelighter neutron-capture elements for [Ba/H] < -2.5. The combinationof very consistent [ α/Fe] and erratic [n-capture/Fe] ratiosindicates that inhomogeneous models for the early evolution of the haloare needed. Our accurate data provide strong constraints on futuremodels of the production and mixing of the heavy elements in the earlyGalaxy.Based on observations made with the ESO Very Large Telescope at ParanalObservatory, Chile (program ID 165.N-0276(A); P.I: R. Cayrel).
| 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.
| 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.
| Supernova Nucleosynthesis in Population III 13-50 Msolar Stars and Abundance Patterns of Extremely Metal-poor Stars
We perform hydrodynamic and nucleosynthesis calculations ofcore-collapse supernovae (SNe) and hypernovae (HNe) of Population (Pop)III stars. We provide new yields for the main-sequence mass ofMMS=13-50 Msolar and the explosion energy ofE=(1-40)×1051 ergs to apply to chemical evolutionstudies. Our HN yields based on the mixing-fallback model of explosionsreproduce the observed abundance patterns of extremely metal-poor (EMP)stars (-4<[Fe/H]<-3), while those of very metal-poor (VMP) stars(-3<[Fe/H]<-2) are reproduced by the normal SN yields integratedover the Salpeter initial mass function. Moreover, the observed trendsof abundance ratios [X/Fe] against [Fe/H] with small dispersions for theEMP stars can be reproduced as a sequence resulting from the variouscombination of MMS and E. This is because we adopt theempirical relation that a larger amount of Fe is ejected by more massiveHNe. Our results imply that the observed trends with small dispersionsdo not necessarily mean the rapid homogeneous mixing in the earlyGalactic halo at [Fe/H]<-3 but can be reproduced by the``inhomogeneous'' chemical evolution model. In addition, we examine howthe modifications of the distributions of the electron mole fractionYe and the density in the presupernova models improve theagreement with observations. In this connection, we discuss possiblecontributions of nucleosynthesis in the neutrino-driven wind and theaccretion disk.
| NLTE determination of the sodium abundance in a homogeneous sample of extremely metal-poor stars
Context: Abundance ratios in extremely metal-poor (EMP) stars are a goodindication of the chemical composition of the gas in the earliest phasesof the Galaxy evolution. It had been found from an LTE analysis that atlow metallicity, and in contrast with most of the other elements, thescatter of [Na/Fe] versus [Fe/H] was surprisingly large and that, ingiants, [Na/Fe] decreased with metallicity. Aims: Since it iswell-known that the formation of sodium lines is very sensitive tonon-LTE effects, to firmly establish the behaviour of the sodiumabundance in the early Galaxy, we have used high quality observations ofa sample of EMP stars obtained with UVES at the VLT, and we have takeninto account the non-LTE line formation of sodium. Methods: Theprofiles of the two resonant sodium D lines (only these sodium lines aredetectable in the spectra of EMP stars) have been computed in a sampleof 54 EMP giants and turn-off stars (33 of them with [Fe/H]<-3.0)with a modified version of the code MULTI, and compared to the observedspectra. Results: With these new determinations in the range {-4<[Fe/H]< -2.5}, both [Na/Fe] and [Na/Mg] are almost constant witha low scatter. In the turn-off stars and "unmixed" giants (located inthe low RGB): [Na/Fe] = -0.21 ± 0.13 or [Na/Mg] = -0.45 ±0.16. These values are in good agreement with the recent determinationsof [Na/Fe] and [Na/Mg] in nearby metal-poor stars. Moreover we confirmthat all the sodium-rich stars are "mixed" stars (i.e., giants locatedafter the bump, which have undergone an extra mixing). None of theturn-off stars is sodium-rich. As a consequence it is probable that thesodium enhancement observed in some mixed giants is the result of a deepmixing.
| 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.
| 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.
| First stars IX - Mixing in extremely metal-poor giants. Variation of the 12C/13C, [Na/Mg] and [Al/Mg] ratios
Context: .Extremely metal-poor (EMP) stars preserve a fossil record ofthe composition of the ISM when the Galaxy formed. It is crucial,however, to verify whether internal mixing has modified their surfacecomposition, especially in the giants where most elements can bestudied. Aims: .We aim to understand the CNO abundance variationsfound in some, but not all EMP field giants analysed earlier. Mixingbeyond the first dredge-up of standard models is required, and itsorigin needs clarification. Methods: .The 12C/^{13C}ratio is the most robust diagnostic of deep mixing, because it isinsensitive to the adopted stellar parameters and should be uniformlyhigh in near-primordial gas. We have measured 12C and ^{13C} abundances in 35 EMP giants (including 22 with {[Fe/H] <-3.0}) from high-quality VLT/UVES spectra analysed with LTE modelatmospheres. Correlations with other abundance data are used to studythe depth of mixing. Results: .The 12C/^{13C} ratio isfound to correlate with [C/Fe] (and Li/H), and clearly anti-correlatewith [N/Fe], as expected if the surface abundances are modified by CNOprocessed material from the interior. Evidence for such deep mixing isobserved in giants above {log L/Lȯ = 2.6}, brighter thanin less metal-poor stars, but matching the bump in the luminosityfunction in both cases. Three of the mixed stars are also Na- andAl-rich, another signature of deep mixing, but signatures of the ONcycle are not clearly seen in these stars. Conclusions: .Extramixing processes clearly occur in luminous RGB stars. They cannot beexplained by standard convection, nor in a simple way by rotatingmodels. The Na- and Al-rich giants could be AGB stars themselves, but aninhomogeneous early ISM or pollution from a binary companion remainpossible alternatives.
| Internal Kinematics of the Fornax Dwarf Spheroidal Galaxy
We present new radial velocity results for 176 stars in the Fornax dwarfspheroidal galaxy, of which at least 156 are probable Fornax members. Wecombine with previously published data to obtain a radial velocitysample with 206 stars, of which at least 176 are probable Fornaxmembers. We detect the hint of rotation about an axis near Fornax'smorphological minor axis, although the significance of the rotationsignal in the galactic rest frame is sensitive to the adopted value ofFornax's proper motion. Regardless, the observed stellar kinematics isdominated by random motions, and we do not find kinematic evidence oftidal disruption. The projected velocity dispersion profile of thebinned data set remains flat over the sampled region, which reaches amaximum angular radius of 65'. Single-component King models in whichmass follows light fail to reproduce the observed flatness of thevelocity dispersion profile. Two-component (luminous plus dark matter)models can reproduce the data, provided that the dark component extendssufficiently beyond the luminous component and the central dark matterdensity is of the same order as the central luminous density. Theserequirements suggest a more massive, darker Fornax than standardcore-fitting analyses have previously concluded, with M/LVover the sampled region reaching 10-40 times the M/LV of theluminous component. We also apply a nonparametric mass estimationtechnique, introduced in a companion paper. Although it is designed tooperate on data sets containing velocities for >1000 stars, theestimation yields preliminary results suggesting M/LV~15inside r<1.5 kpc.
| 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.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| Sulphur abundances in metal-poor stars
We investigate the debated "sulphur discrepancy" found among metal-poorstars of the Galactic halo with [Fe/H] < -2. This discrepancy stemsin part from the use of two different sets of sulphur lines, the veryweak triplet at 8694-95 Å and the stronger triplet lines at9212-9238 Å. For three representative cases of metal-poor dwarf,turnoff and subgiant stars, we argue that the abundances from theλλ8694-95 lines have been overestimated which has led to acontinually rising trend of [S/Fe] as metallicity decreases. Given thatthe near-IR region is subject to CCD fringing, these weak lines becomeexcessively difficult to measure accurately in the metallicity regime of[Fe/H] < -2. Based on homogeneously determined spectroscopic stellarparameters, we also present updated [S/Fe] ratios from theλλ9212-9238 lines which suggest a plateau-like behavioursimilar to that seen for other alpha elements.
| 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.
| First stars VI - Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements
We have investigated the poorly-understood origin of nitrogen in theearly Galaxy by determining N abundances from the NH band at 336 nm in35 extremely metal-poor halo giants, with carbon and oxygen abundancesfrom Cayrel et al. (\cite{CDS04}, A&A, 416, 1117), usinghigh-quality ESO VLT/UVES spectra (30 of our 35 stars are in the range-4.1 < [Fe/H] < -2.7 and 22 stars have [Fe/H] < -3.0). Nabundances derived both from the NH band and from the CN band at 389 nmfor 10 stars correlate well, but show a systematic difference of 0.4dex, which we attribute to uncertainties in the physical parameters ofthe NH band (line positions, gf values, dissociation energy, etc.).Because any dredge-up of CNO processed material to the surface maycomplicate the interpretation of CNO abundances in giants, we have alsomeasured the surface abundance of lithium in our stars as a diagnosticof such mixing. Our sample shows a clear dichotomy between two groups ofstars. The first group shows evidence of C to N conversion through CNcycling and strong Li dilution, a signature of mixing; these stars aregenerally more evolved and located on the upper Red Giant Branch (RGB)or Horizontal Branch (HB). The second group has [N/Fe] < 0.5, showsno evidence for C to N conversion, and Li is only moderately diluted;these stars belong to the lower RGB and we conclude that their C and Nabundances are very close to those of the gas from which they formed inthe early Galaxy, they are called ``unmixed stars''. The [O/Fe] and[(C+N)/Fe] ratios are the same in the two groups, confirming that thedifferences between them are caused by dredge-up of CN-processedmaterial in the first group, with negligible contributions from the O-Ncycle. The ``unmixed'' stars reflect the abundances in the early Galaxy:the [C/Fe] ratio is constant (about +0.2 dex) and the [C/Mg] ratio isclose to solar at low metallicity, favouring a high C production bymassive zero-metal supernovae. The [N/Fe] and [N/Mg] ratios scatterwidely. Their mean values in each metallicity bin decrease withincreasing metallicity, but this trend could be a statistical effect.The larger values of these ratios define a flat upper plateau ([N/Mg] =0.0, [N/Fe] = +0.1), which could reflect higher values within a widerange of yields of zero-metal SNe II. Alternatively, by analogy with theDLAs, the lower abundances ([N/Mg] = -1.1, [N/Fe] = -0.7) could reflectgenerally low yields from the first SNe II, the other stars being Nenhanced by winds of massive Asymptotic Giant Branch (AGB) stars. Sinceall the stars show clear [α/Fe] enhancements, they were formedbefore any significant enrichment of the Galactic gas by SNe Ia, andtheir composition should reflect the yields of the first SNe II.However, if massive AGB stars or AGB supernovae evolved more rapidlythan SNe Ia and contaminated the ISM, our stars would also reflect theyields of these AGB stars. At present it cannot be decided whetherprimary N is produced primarily in SNe II or in massive AGB stars, or inboth. The stellar N abundances and [N/O] ratios are compatible withthose found in Damped Lyman-α (DLA) systems. They extend thewell-known DLA ``plateau'' at [N/O] ≈ -0.8 to lower metallicities,albeit with more scatter; no star is found below the putative ``low[N/α] plateau'' at [N/O] ≈ -1.55 in DLAs.Based on observations obtained with the ESO VLT under ESO programme ID165.N-0276(A). This work has made use of the SIMBAD database.
| 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.
| Cu and Zn in the early Galaxy
We present Cu and Zn abundances for 38 FGK stars, mostly dwarfs,spanning a metallicity range between solar and [Fe/H] = -3. Theabundances were obtained using Kurucz's local thermal equilibrium (LTE)model atmospheres and the near-UV lines of Cu I 3273.95 Å and Zn I3302.58 Å observed at high spectral resolution. The trend of[Cu/Fe] versus [Fe/H] is almost solar for [Fe/H] > -1 and thendecreases to a plateau <[Cu/Fe]> = -0.98 at [Fe/H] < -2.5,whereas the [Zn/Fe] trend is essentially solar for [Fe/H] > -2 andthen slightly increases at lower metallicities to an average value of<[Zn/Fe]> = +0.18. We compare our results with previous work onthese elements, and briefly discuss them in terms of nucleosynthesisprocesses. Predictions of halo chemical evolution fairly reproduce thetrends, especially the [Cu/Fe] plateau at very low metallicities, but toa lesser extent the higher [Zn/Fe] ratios at low metallicities,indicating possibly missing yields.
| uvby-β photometry of high-velocity and metal-poor stars. X. Stars of very low metal abundance: Observations, reddenings, metallicities, classifications, distances, and relative ages
uvby(-β) photometry has been obtained for an additional 411 verymetal-poor stars selected from the HK survey, and used to derive basicparameters such as interstellar reddenings, metallicities, photometricclassifications, distances, and relative ages. Interstellar reddeningsadopted from the Schlegel et al. (\cite{schlegel}) maps agree well withthose from the intrinsic-color calibration of Schuster & Nissen(\cite{schuster89}). [Fe/H] values are obtained from the CaII K lineindex of the HK survey combined with the uvby and UBV photometry. Thec0,(b-y)0 diagram is seen to be very useful forclassifying these very metal-poor field stars into categories similar tothose derived from globular cluster color-magnitude diagrams; the HKsurvey has detected metal-poor candidates extending from the red-giantto the blue-horizontal branch, and from the horizontal branch tosubluminous stars. Distances derived from UBV photometry agreereasonably well with those from uvby, considering the paucity of goodcalibrating stars and the extrapolations required for the mostmetal-poor stars. These very metal-poor stars are compared to M 92 inthe c0,(b-y)0 diagram, and evidence is seen forfield stars 1-3 Gyrs younger than this globular cluster; uncertaintiesin the [Fe/H] scale for M 92 would only tend to increase this agedifference. Significant reddening uncertainties for M 92 are unlikelybut might decrease this difference. The significance of these youngervery metal-poor stars is discussed in the context of Galactic evolution,mentioning such possibilities as hierarchical star-formation/mass-infallof very metal-poor material and/or accretion events whereby thismaterial has been acquired from other (dwarf) galaxies with differentformation and chemical-enrichment histories.Based on observations collected at the H. L. Johnson 1.5 m telescope atthe Observatorio Astronómico Nacional at San Pedro Mártir,Baja California, México, and at the Danish 1.5 m telescope, LaSilla, Chile.Tables 1-9 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/527
| 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.
| First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy
In the framework of the ESO Large Programme ``First Stars'', veryhigh-quality spectra of some 70 very metal-poor dwarfs and giants wereobtained with the ESO VLT and UVES spectrograph. These stars are likelyto have descended from the first generation(s) of stars formed after theBig Bang, and their detailed composition provides constraints on issuessuch as the nature of the first supernovae, the efficiency of mixingprocesses in the early Galaxy, the formation and evolution of the haloof the Galaxy, and the possible sources of reionization of the Universe.This paper presents the abundance analysis of an homogeneous sample of35 giants selected from the HK survey of Beers et al. (\cite{BPS92},\cite{Be99}), emphasizing stars of extremely low metallicity: 30 of our35 stars are in the range -4.1 <[Fe/H]< -2.7, and 22 stars have[Fe/H] < -3.0. Our new VLT/UVES spectra, at a resolving power ofR45 000 and with signal-to-noise ratios of 100-200 per pixel overthe wavelength range 330-1000 nm, are greatly superior to those of theclassic studies of McWilliam et al. (\cite{MPS95}) and Ryan et al.(\cite{RNB96}).The immediate objective of the work is to determine precise,comprehensive, and homogeneous element abundances for this large sampleof the most metal-poor giants presently known. In the analysis wecombine the spectral line modeling code ``Turbospectrum'' with OSMARCSmodel atmospheres, which treat continuum scattering correctly and thusallow proper interpretation of the blue regions of the spectra, wherescattering becomes important relative to continuous absorption (λ< 400 nm). We obtain detailed information on the trends of elementalabundance ratios and the star-to-star scatter around those trends,enabling us to separate the relative contributions of cosmic scatter andobservational/analysis errors.Abundances of 17 elements from C to Zn have been measured in all stars,including K and Zn, which have not previously been detected in starswith [Fe/H] < -3.0. Among the key results, we discuss the oxygenabundance (from the forbidden [OI] line), the different and sometimescomplex trends of the abundance ratios with metallicity, the very tightrelationship between the abundances of certain elements (e.g., Fe andCr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within theerror bars, the trends of the abundance ratios with metallicity areconsistent with those found in earlier literature, but in many cases thescatter around the average trends is much smaller than found in earlierstudies, which were limited to lower-quality spectra. We find that thecosmic scatter in several element ratios may be as low as 0.05 dex.The evolution of the abundance trends and scatter with decliningmetallicity provides strong constraints on the yields of the firstsupernovae and their mixing into the early ISM. The abundance ratiosfound in our sample do not match the predicted yields frompair-instability hypernovae, but are consistent with element productionby supernovae with progenitor masses up to 100 Mȯ.Moreover, the composition of the ejecta that have enriched the matterBased on observations obtained in the frame of the ESO programme ID165.N-0276(A).Full Tables 3 and 8 are 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/416/1117 This work hasmade use of the SIMBAD database.
| 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.
| 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.
| Sodium Abundances in Stellar Atmospheres with Differing Metallicities
The non-LTE sodium abundances of 100 stars with metallicities-3<[Fe/H]<0.3 are determined using high-dispersion spectra withhigh signal-to-noise ratios. The sodium abundances [Na/Fe] obtained areclose to the solar abundance and display a smaller scatter than valuespublished previously. Giants (logg<3.8) with [Fe/H]<-1 do notdisplay overabundances of sodium, and their sodium abundances do notshow an anticorrelation with the oxygen abundance, in contrast toglobular-cluster giants. They likewise do not show sodium-abundancevariations with motion along the giant branch. No appreciable decreasein the sodium abundance was detected for dwarfs (logg>3.8) withmetallicities -2<[Fe/H]<-1. The observed relation between [Na/Fe]and [Fe/H] is in satisfactory agreement with the theoreticalcomputations of Samland, which take into account the metallicitydependence of the sodium yield and a number of other factors affectingthe distribution of elements in the Galaxy during the course of itsevolution.
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
