Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Reliability Checks on the Indo-US Stellar Spectral Library Using Artificial Neural Networks and Principal Component Analysis
The Indo-US coudé feed stellar spectral library (CFLIB) madeavailable to the astronomical community recently by Valdes et al. (2004,ApJS, 152, 251) contains spectra of 1273 stars in the spectral region3460 to 9464Å at a high resolution of 1Å (FWHM) and a widerange of spectral types. Cross-checking the reliability of this databaseis an important and desirable exercise since a number of stars in thisdatabase have no known spectral types and a considerable fraction ofstars has not so complete coverage in the full wavelength region of3460-9464Å resulting in gaps ranging from a few Å to severaltens of Å. We use an automated classification scheme based onArtificial Neural Networks (ANN) to classify all 1273 stars in thedatabase. In addition, principal component analysis (PCA) is carried outto reduce the dimensionality of the data set before the spectra areclassified by the ANN. Most importantly, we have successfullydemonstrated employment of a variation of the PCA technique to restorethe missing data in a sample of 300 stars out of the CFLIB.
| Neutron-Capture Elements in the Very Metal Poor Star HD 122563
We obtained high-resolution, high signal-to-noise ratio (S/N)spectroscopy for the very metal poor star HD 122563 with the SubaruTelescope High Dispersion Spectrograph. Previous studies have shown thatthis object has excesses of light neutron-capture elements, while itsabundances of heavy ones are very low. In our spectrum, which covers3070-4780 Å of this object, 19 neutron-capture elements have beendetected, including seven for the first time in this star (Nb, Mo, Ru,Pd, Ag, Pr, and Sm). Upper limits are given for five other elementsincluding Th. The abundance pattern shows a gradually decreasing trend,as a function of atomic number, from Sr to Yb, which is quite differentfrom those in stars with excesses of r-process elements. This abundancepattern of neutron-capture elements provides new strong constraints onthe models of nucleosynthesis responsible for the very metal poor starswith excesses of light neutron-capture elements but without enhancementof heavy ones.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Neutron-Capture Elements in the Metal-poor Globular Cluster M15
We report on observations of six giants in the globular cluster M15 (NGC7078) using the Subaru Telescope to measure neutron-capture elementalabundances. Our abundance analyses, based on high-quality blue spectra,confirm the star-to-star scatter in the abundances of heavyneutron-capture elements (e.g., Eu), and we found no significants-process contribution to them, as was found in previous studies. Wehave found that, for the first time, there are anticorrelations betweenthe abundance ratios of light to heavy neutron-capture elements ([Y/Eu]and [Zr/Eu]) and the heavy neutron-capture elements (e.g., Eu). Thisindicates that the light neutron-capture elements in these stars cannotbe explained by only a single r-process. Another process thatcontributed significantly to the light neutron-capture elements isrequired in M15. Our results suggest a complicated enrichment historyfor M15 and its progenitor.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Oxygen abundances in metal-poor subgiants as determined from [O I], O I and OH lines
The debate on the oxygen abundances of metal-poor stars has its originin contradictory results obtained using different abundance indicators.To achieve a better understanding of the problem we have acquired highquality spectra with the Ultraviolet and Visual Echelle Spectrograph atVLT, with a signal-to-noise of the order of 100 in the near ultravioletand 500 in the optical and near infrared wavelength range. Threedifferent oxygen abundance indicators, OH ultraviolet lines around 310.0nm, the [O i] line at 630.03 nm and the O i lines at 777.1-5 nm wereobserved in the spectra of 13 metal-poor subgiants with-3.0≤[Fe/H]≤-1.5. Oxygen abundances were obtained from theanalysis of these indicators which was carried out assuming localthermodynamic equilibrium and plane-parallel model atmospheres.Abundances derived from O i were corrected for departures from localthermodynamic equilibrium. Stellar parameters were computed usingT_eff-vs.-color calibrations based on the infrared flux method andBalmer line profiles, Hipparcos parallaxes and Fe II lines. [O/Fe]values derived from the forbidden line at 630.03 nm are consistent withan oxygen/iron ratio that varies linearly with [Fe/H] as[O/Fe]=-0.09(±0.08)[Fe/H]+0.36(±0.15). Values based on theO i triplet are on average 0.19±0.22 dex(s.d.) higher than thevalues based on the forbidden line while the agreement between OHultraviolet lines and the forbidden line is much better with a meandifference of the order of -0.09±0.25 dex(s.d.). In general, ourresults follow the same trend as previously published results with theexception of the ones based on OH ultraviolet lines. In that case ourresults lie below the values which gave rise to the oxygen abundancedebate for metal-poor stars.
| Improved Laboratory Transition Probabilities for Pt I and Application to the Platinum Abundances of BD +17°3248 and the Sun
Radiative lifetimes, accurate to +/-5%, have been measured for 58odd-parity levels of Pt I using laser-induced fluorescence. Thelifetimes were combined with branching fractions measured using gratingand Fourier transform spectrometry to determine transition probabilitiesfor 127 lines of Pt I. The new Pt lifetime measurements were found to bein good agreement with previous but less extensive measurements based onlaser-induced fluorescence. The new branching fraction measurements werefound to be in fair agreement with one earlier study. Absolute atomictransition probabilities from the new measurements were used todetermine the platinum abundance in the metal-poor Galactic halo star BD+17°3248. An attempt to refine the solar photospheric abundance ofplatinum was unsuccessful; the single Pt I line used in an earlierabundance determination was found to be even more severely blended thanexpected from earlier work.
| A New Stellar Library in the K Band
Stellar population synthesis models are crucial for the understanding ofthe large amount of data which is being gathered for galaxies at low andhigh redshift, and provide the only way to compare the ``real world''with the theoretical framework. The best models require extensiveempirical stellar spectral libraries, which at present are starting tobe quite complete in the optical range. However the situation isdifferent in the near-infrared, which observational windows have been,until recently, poorly exploited. This is specially due to the lack ofappropriate instrumentation. We present the preliminary results of anongoing observational program aimed to overcome this problem and toprovide a stellar library in the K band with the required coverage ofphysical stellar parameters: effective temperature, gravity, metallicityand non-solar abundance ratios. In particular, the CO feature at 2.3μ m is a very promising spectroscopic line-strength index that willhelp to face outstanding problems in galaxy formation and evolution. Theavailability of this library will be essential to interpret the stellarcontent of composite stellar populations with EMIR. In addition, thislibrary will be also useful for other purposes, like the study of highlyreddened objects, and the spectral classification of late type stars.
| 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.
| Spectroscopic Studies of Very Metal-poor Stars with the Subaru High Dispersion Spectrograph. III. Light Neutron-Capture Elements
Elemental abundance measurements have been obtained for a sample of 18very metal-poor stars using spectra obtained with the Subaru TelescopeHigh Dispersion Spectrograph. Seventeen stars, among which 16 are newlyanalyzed in the present work, were selected from candidate metal-poorstars identified in the HK survey of Beers and colleagues. Themetallicity range covered by our sample is -3.1<~[Fe/H]<~-2.4. Theabundances of carbon, α-elements, and iron-peak elementsdetermined for these stars confirm the trends found by previous work.One exception is the large overabundance of Mg, Al, and Sc found in BS16934-002, a giant with [Fe/H]=-2.8. Interestingly, this is the mostmetal-rich star (by about 1 dex in [Fe/H]) known with such largeoverabundances in these elements. Furthermore, BS 16934-002 does notshare the large overabundances of carbon that are associated with thetwo other, otherwise similar, extremely metal-poor stars CS 22949-037and CS 29498-043. By combining our new results with those of previousstudies, we investigate the distribution of neutron-capture elements invery metal-poor stars, focusing on the production of the lightneutron-capture elements (e.g., Sr, Y, and Zr). Large scatter is foundin the abundance ratios between the light and heavy neutron-captureelements (e.g., Sr/Ba, Y/Eu) for stars with low abundances of heavyneutron-capture elements. Most of these stars have extremely lowmetallicity ([Fe/H]<~-3). By contrast, the observed scatter in theseratios is much smaller in stars with excesses of heavy neutron-captureelements and with higher metallicity. These results can be naturallyexplained by assuming that two processes independently enriched theneutron-capture elements in the early Galaxy. One process increases bothlight and heavy neutron-capture elements and affects stars with[Fe/H]>~-3, while the other process contributes only to the lightneutron-capture elements and affects most stars with [Fe/H]>~-3.5.Interestingly, the Y/Zr ratio is similar in stars with high and lowabundances of heavy neutron-capture elements. These results provideconstraints on modeling of neutron-capture processes, in particular,those responsible for the nucleosynthesis of light neutron-captureelements at very low metallicity.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
| Hubble Space Telescope Observations of Heavy Elements in Metal-Poor Galactic Halo Stars
We present new abundance determinations of neutron-capture elements Ge,Zr, Os, Ir, and Pt in a sample of 11 metal-poor(-3.1<=[Fe/H]<=-1.6) Galactic halo giant stars, based on HubbleSpace Telescope UV and Keck I optical high-resolution spectroscopy. Thestellar sample is dominated by r-process-rich stars such as thewell-studied CS 22892-052 and BD +17°3248 but also includes ther-process-poor, bright giant HD 122563. Our results demonstrate thatabundances of the third r-process peak elements Os, Ir, and Pt in thesemetal-poor halo stars are very well correlated among themselves and withthe abundances of the canonical r-process element Eu (determined inother studies), thus arguing for a common origin or site for r-processnucleosynthesis of heavier (Z>56) elements. However, the large (andcorrelated) scatters of [Eu, Os, Ir, Pt/Fe] suggest that the heaviestneutron-capture r-process elements are not formed in all supernovae. Incontrast, the Ge abundances of all program stars track their Feabundances, very well. An explosive process on iron peak nuclei (e.g.,the α-rich freezeout in supernovae), rather than neutron capture,appears to have been the dominant synthesis mechanism for this elementat low metallicities: Ge abundances seem completely uncorrelated withEu. The correlation (with very small scatter) of Ge and Fe abundancessuggests that Ge must have been produced rather commonly in stars, evenat early times in the Galaxy, over a wide range of metallicity. The Zrabundances show much the same behavior as Ge with (perhaps) somewhatmore scatter, suggesting some variations in abundance with respect toFe. The Zr abundances also do not vary cleanly with Eu abundances,indicating a synthesis origin different than that of heavierneutron-capture elements. Detailed abundance distributions for CS22892-052 and BD +17°3248, combining the new elementaldeterminations for Os-Pt and recently published Nd and Ho measurements,show excellent agreement with the solar system r-process curve from theelements Ba to Pb. The lighter n-capture elements, including Ge, ingeneral fall below the same solar system r-process curve that matchesthe heavier elements.
| 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.
| The calcium isotopic anomaly in magnetic CP stars
Chemically peculiar stars in the magnetic sequence can show the sameisotopic anomaly in calcium previously discovered for mercury-manganesestars in the non-magnetic sequence. In extreme cases, the dominantisotope is the exotic 48Ca. Measurements of Ca II linesarising from 3d-4p transitions reveal the anomaly by showing shifts upto 0.2 Å for the extreme cases - too large to be measurementerrors. We report measurements of miscellaneous objects, including twometal-poor stars, two apparently normal F-stars, an Am-star, and theN-star U Ant. Demonstrable anomalies are apparent only for the Ap stars.The largest shifts are found in rapidly oscillating Ap stars and in oneweakly magnetic Ap star, HD 133792. We note the possible relevance ofthese shifts for the GAIA mission.Based on observations obtained at the European Southern Observatory, LaSilla and Paranal, Chile (ESO programme Nos. 65.L-0316, 68.D-0254 and266.D-5655).
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| 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.
| R-process nucleosynthesis in supernovae.
Not Available
| He I λ10830 Absorption in Metal-Poor Red Giants: Probing Fast Chromospheric Outflows
We present the results of a search for near-IR chromospheric He Iabsorption in 10 metal-poor red giants using high-resolution spectraobtained with NIRSPEC on Keck II. Four of these red giants are in thefield, and six are in the globular cluster M13. Two of the field starsand one of the cluster stars show evidence for He I absorption that isconsistent with chromospheric outflow velocities ranging from 30 to 140km s-1. If identified with a wind, the velocity found in theM13 red giant IV-15 (30 km s-1) is comparable to the escapevelocity of the cluster core, suggesting that such winds may beeffective in clearing out the interstellar medium of globular clusters.Absorption is confined to stars with Teff>~4600 K andMV fainter than -1.5. Since previous studies have foundHα and Ca II K2 emission line profiles indicative ofwinds in metal-poor giants brighter than this limit, the mechanismresponsible for He I absorption in metal-poor red giants apparentlyoperates less effectively in the cooler and brighter stars. On the otherhand, the He I line extends the detection of mass outflows further downthe giant branch of metal-poor stars than the Hα observations.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Cu and Zn in different stellar populations:. inferring their origin
We analyse recent high-resolution spectroscopic observations of Cu andZn for stars of different stellar populations and metallicities, usingthe best available stellar nucleosynthesis expectations. Theobservations include unevolved stars of the Galactic halo, thick-diskand thin-disk, bulge-like stars and stars of Omega Cen, globularclusters and Dwarf Spheroidal systems. Most cosmic Cu and half the Znare synthesised in massive stars during the hydrostatic He-burning andC-burning phases by the weak sr-process, which depends linearly onmetallicity. A minor primary contribution for Cu derives from explosivenucleosynthesis in SNe II. A large primary contribution to Zn (as64Zn) is ascribable to the alpha -rich freezout in nu -windsor to SNe II with large explosion energies (hypernovae). AGB stars andtype Ia supernovae do not contribute appreciably to either Cu or Zn.
| 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.
| The Rise of the s-Process in the Galaxy
From newly obtained high-resolution, high signal-to-noise ratio spectrathe abundances of the elements La and Eu have been determined over thestellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarfstars. Lanthanum is predominantly made by the s-process in the solarsystem, while Eu owes most of its solar system abundance to ther-process. The changing ratio of these elements in stars over a widemetallicity range traces the changing contributions of these twoprocesses to the Galactic abundance mix. Large s-process abundances canbe the result of mass transfer from very evolved stars, so to identifythese cases we also report carbon abundances in our metal-poor stars.Results indicate that the s-process may be active as early as[Fe/H]=-2.6, although we also find that some stars as metal-rich as[Fe/H]=-1 show no strong indication of s-process enrichment. There is asignificant spread in the level of s-process enrichment even at solarmetallicity.
| A New Contributor to Chemical Evolution in High-Redshift Galaxies
The recent discovery of a new population of stars exhibiting unusualelemental abundance patterns characterized by enhanced Ti-to-Ga elementsand low α- and n-capture elements suggests the contribution of anew class of supernovae, probably a kind of Type Ia supernovaeassociated with close binary evolution. The role of these supernovae inchemical evolution is negligible in normal galaxies that undergomoderate star formation such as our own. Thus, while the frequency ofoccurrence would be too low to detect in low-redshift galaxies, it mayrepresent a prominent population in high-redshift objects such as earlyepoch massive elliptical galaxies and QSOs. The chemical contributor ofthis proposed type of supernova in combination with recognizedsupernovae is shown to be compatible with the recent observationalfeatures in the distant universe, successfully reproducing the Type IIsupernova-like abundance pattern with enhancement of Ga and Ge in thegas of newborn massive galaxies and high iron abundances in QSOs even atredshifts of around 6.
| 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.
| The s-Process in Metal-Poor Stars: Abundances for 22 Neutron-Capture Elements in CS 31062-050
The CH star CS 31062-050 ([Fe/H]=-2.42) is one of the most useful starsyet discovered for evaluating the s-process in metal-poor stars. It isvery abundant in heavy elements (e.g., [La/Fe]=2.2), and its relativelycool temperature and low gravity mean that there are many lines ofinteresting elements present in the spectrum. We measured the abundancesof 22 elements with Z>=29, including the rarely measured Lu and Pd.We derive an upper limit on the Th abundance as well. The abundances inCS 31062-050 show a similar pattern to many other metal-poor CH stars:high [Pb/Fe] and [Pb/La] ratios, low [Y/La] ratios and high [Eu/La]values compared to the solar system s-process. However, the Th limit,with additional assumptions, is not consistent with the idea that theexcess Eu in CS 31062-050 is contributed by the r-process. In addition,the observed [Eu/Tb] cannot be explained by any ratio of solar-systems-process and r-process abundances. We therefore argue that theabundance pattern in CS 31062-050 is most likely the result of thes-process, and we discuss possible modifications that could explain thenon-solar-system pattern observed.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Galactic Evolution of Sr, Y, And Zr: A Multiplicity of Nucleosynthetic Processes
In this paper we follow the Galactic enrichment of three easily observedlight n-capture elements: Sr, Y, and Zr. Input stellar yields have beenfirst separated into their respective main and weak s-process componentsand r-process component. The s-process yields from asymptotic giantbranch (AGB) stars of low to intermediate mass are computed, exploring awide range of efficiencies of the major neutron source, 13C,and covering both disk and halo metallicities. AGB stars have been shownto reproduce the main s-component in the solar system, i.e., thes-process isotopic distribution of all heavy isotopes with atomic massnumber A>90, with a minor contribution to the light s-processisotopes up to A~90. The concurrent weak s-process, which accounts forthe major fraction of the light s-process isotopes in the solar systemand occurs in massive stars by the operation of the 22Neneutron source, is discussed in detail. Neither the main s- nor the weaks-components are shown to contribute significantly to theneutron-capture element abundances observed in unevolved halo stars.Knowing the s-process distribution at the epoch of the solar systemformation, we first employed the r-process residuals method to infer theisotopic distribution of the r-process. We assumed a primary r-processproduction in the Galaxy from moderately massive Type II supernovae thatbest reproduces the observational Galactic trend of metallicity versusEu, an almost pure r-process element. We present a detailed analysis ofa large published database of spectroscopic observations of Sr, Y, Zr,Ba, and Eu for Galactic stars at various metallicities, showing that theobserved trends versus metallicity can be understood in light of amultiplicity of stellar neutron-capture components. Spectroscopicobservations of the Sr, Y, and Zr to Ba and Eu abundance ratios versusmetallicity provide useful diagnostics of the types of neutron-captureprocesses forming Sr, Y, and Zr. In particular, the observed [Sr, Y,Zr/Ba, Eu] ratio is clearly not flat at low metallicities, as we wouldexpect if Ba, Eu and Sr, Y, Zr all had the same r-processnucleosynthetic origin. We discuss our chemical evolution predictions,taking into account the interplay between different processes to produceSr-Y-Zr. Making use of the very r-process-rich and very metal-poor starslike CS 22892-052 and CS 31082-001, we find hints and discuss thepossibility of a primary process in low-metallicity massive stars,different from the ``classical s-process'' and from the ``classicalr-process'' that we tentatively define LEPP (lighter element primaryprocess). This allows us to revise the estimates of the r-processcontributions to the solar Sr, Y, and Zr abundances, as well as of thecontribution to the s-only isotopes 86Sr, 87Sr,and 96Mo.
| Abundances of Extremely Metal-poor Star Candidates
We present chemical abundances for 110 stars identified inobjective-prism surveys as candidates for being very metal-poor. Theabundances are derived from high-S/N, intermediate-resolution spectraobtained with the Keck Observatory Echellette Spectrograph and Imager(ESI). An additional 25 stars with well-determined abundances rangingfrom [Fe/H]=-1.5 to -3.2 were observed and the results used to helpcalibrate our analysis and determine the accuracy of our abundancedeterminations. Abundances for the program stars were measured for Fe,Mg, Ca, Ti, Cr, and Ba with an accuracy of approximately 0.3 dex.Fifty-three of the stars in our sample have [Fe/H]<=-2, 22 have[Fe/H]<=-2.5, and 13 have [Fe/H]<=-2.9. Surprisingly,approximately one-third of the sample is relatively metal-rich, with[Fe/H]>-1.5. In addition to identifying a number of extremelymetal-poor stars, this study also shows that moderate-resolution spectraobtained with the Keck ESI yield relatively accurate abundances forstars as faint as V=14 with modest exposure time (~20 minutes). Thiscapability will prove useful if the so-far elusive stars at [Fe/H]<-4turn out to be mostly fainter than V=15.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby 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.
| 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.
| 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.
| 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.
| J - K DENIS photometry of a VLTI-selected sample of bright southern stars
We present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.Tables 1 and 2 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/413/1037
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Boote |
| Ascensione retta: | 14h02m31.80s |
| Declinazione: | +09°41'11.0" |
| Magnitudine apparente: | 6.2 |
| Distanza: | 265.957 parsec |
| Moto proprio RA: | -189.9 |
| Moto proprio Dec: | -70 |
| B-T magnitude: | 7.275 |
| V-T magnitude: | 6.304 |
Cataloghi e designazioni:
|
INHABITED SKY
