bluuebunny/crossref_metadata_2025_split huggingface dataset:
Enrichment of Thorium (Th) and Lead (Pb) in the early Galaxy
Authors: Aoki Wako , Honda Satoshi 
Published: March 2010

AbstractWe have been determining abundances of Th, Pb and other neutron-capture elements in metal-deficient cool giant stars to constrain the enrichment of heavy elements by the r- and s-processes. Our current sample covers the metallicity range between [Fe/H] = −2.5 and −1.0. (1) The abundance ratios of Pb/Fe and Pb/Eu of most of our stars are approximately constant, and no increase of these ratios with increasing metallicity is found. This result suggests that the Pb abundances of our sample are determined by the r-process with no or little contribution of the s-process. (2) The Th/Eu abundance ratios of our sample show no significant scatter, and the average is lower by 0.2 dex in the logarithmic scale than the solar-system value. This result indicates that the actinides production by the r-process does not show large dispersion, even though r-process models suggest high sensitivity of the actinides production to the nucleosynthesis environment.
The impact of metallicity on the formation of pre-collapsing minihalos
Authors: Aykutalp Aycin , Spaans Marco 
Published: March 2010

AbstractIn this study we consider the pre-enrichment of minihalos, and study the impact of metallicity on pre-collapsing minihalos by using the cosmological, N-body simulation code Enzo. The metallicities that we consider are assumed to be the result of pre-enrichment by earlier star formation. In the simulations of 10−3 and 10−1 Z⊙ we see a big difference for the collapse of the minihalo. In the high metallicity case the minihalo is more compact compared to the low metallicity case and we reach higher densities due to the efficient cooling. Also in the high metallicity case the gas cools down to lower temperatures and we see cold, dense gas which indicates a multi-phase ISM. This leads us to think that there is a transition region between metallicities of 10−3 and 10−1 Z⊙ which lowers the mass scale of the next generation of stars. Furthermore, because the gas cools more efficiently in the high metallicity case there is less pressure support against gravity and therefore we see higher velocities.
The importance of initial conditions and metallicity for the fragmentation of protogalactic gas
Authors: Jappsen Anne-Katharina , Glover Simon C. O. , Mac Low Mordecai-Mark , Klessen Ralf S. 
Published: March 2010

AbstractThe formation of the first stars out of metal-free gas appears to result in stars at least an order of magnitude more massive than in the present-day case. We here consider what controls the transition from a primordial to a modern initial mass function. We study the influence of low levels of metal enrichment and different initial conditions on the cooling and collapse of initially ionized gas in small protogalactic halos using three-dimensional, smoothed particle hydrodynamics simulations. We argue that fragmentation at moderate density depends on the initial conditions for star formation more than on the metal abundances present.
Silver Stars
Authors: Hansen Camilla Juul , Primas Francesca 
Published: March 2010

AbstractThe rapid neutron-capture process (r-process), which produces some of the heaviest elements, is not well understood. Obtaining accurate abundances of these heavy elements (Z > 38) is important, both in the context of the chemical evolution of the Galaxy and for understanding the site(s) and process(es) of formation of those elements. We have determined elemental abundances for several r-process elements, notably silver, from high resolution VLT/UVES spectra. Silver was chosen because it is predominantly a light r-process element (38 < Z < 50), and little is known about its formation and evolution in the Galaxy. Here, we present our preliminary results.
The first galactic stars and chemical enrichment in the halo
Authors: Bonifacio Piercarlo 
Published: March 2010

AbstractThe cosmic microwave background and the cosmic expansion can be interpreted as evidence that the Universe underwent an extremely hot and dense phase about 14 Gyr ago. The nucleosynthesis computations tell us that the Universe emerged from this state with a very simple chemical composition: H, 2H, 3He, 4He, and traces of 7Li. All other nuclei where synthesised at later times. Our stellar evolution models tell us that, if a low-mass star with this composition had been created (a “zero-metal” star) at that time, it would still be shining on the Main Sequence today. Over the last 40 years there have been many efforts to detect such primordial stars but none has so-far been found. The lowest metallicity stars known have a metal content, Z, which is of the order of 10−4Z⊙. These are also the lowest metallicity objects known in the Universe. This seems to support the theories of star formation which predict that only high mass stars could form with a primordial composition and require a minimum metallicity to allow the formation of low-mass stars. Yet, since absence of evidence is not evidence of absence, we cannot exclude the existence of such low-mass zero-metal stars, at present. If we have not found the first Galactic stars, as a by product of our searches we have found their direct descendants, stars of extremely low metallicity (Z ≤ 10−3Z⊙). The chemical composition of such stars contains indirect information on the nature of the stars responsible for the nucleosynthesis of the metals. Such a fossil record allows us a glimpse of the Galaxy at a look-back time equivalent to redshift z = 10, or larger. The last ten years have been full of exciting discoveries in this field, which I will try to review in this contribution.
An overall picture of EMP stars using the stellar abundances for galactic archaeology (SAGA) database
Authors: Suda Takuma , Yamada Shimako , Katsuta Yutaka , Ishizuka Chikako , Komiya Yutaka , Nishimura Takanori , Aoki Wako , Fujimoto Masayuki Y. 
Published: March 2010

AbstractWe explore the general characteristics of extremely metal-poor (EMP) stars in the Galaxy using the Stellar Abundances for Galactic Archaeology (SAGA) database (Suda et al. 2008, PASJ, 60, 1159). The overall trend of EMP stars suggests that there are at least two types of extra mixing to change the surface abundances of EMP stars. One is to deplete lithium abundance during the early phase of giant branch and another is to decrease C/N ratio by one order of magnitude during the red giant branch or AGB phase. On the other hand, these mixing processes are different from those suggested in the Galactic globular clusters because of the different relations between O, Na, Mg, and Al abundances.
HST-STIS abundances in the uranium-rich very metal-poor star CS 31082-001
Authors: Barbuy B. , Spite M. , Hill V. , Primas F. , Plez B. , Cayrel R. , Sneden C. , Spite F. , Beers T. C. , Andersen J. , Nordström B. , Bonifacio P. , François P. , Molaro P. , Siqueira-Mello C. 
Published: March 2010

AbstractThe abundance derivation of heavy r-elements may provide a better understanding of the r-process, and the determination of several reference r-elements should allow a better determination of the star's age. The spatial ultraviolet (UV) region presents a large number of lines of heavy elements, and in some cases such as Bi, Pt, Au, detectable lines are only available in the UV. The extreme “r-process star” CS 31082-001 ([Fe/H] = −2.9) was observed in the spatial UV in order to determine abundances of the heavy elements, using STIS on board HST.
Near-IR Spectroscopy of CEMP Stars with SOAR/OSIRIS
Authors: Kennedy Catherine R. , Sivarani Thirupathi , Beers Timothy C. , Rossi Silvia , Placco Vinicius M. , Johnson J. , Masseron T. 
Published: March 2010

AbstractWe report on medium-resolution near-IR spectroscopy of a sample of over 60 Carbon-Enhanced Metal-Poor (CEMP) stars observed with SOAR/OSIRIS, selected from the HK survey of Beers and colleagues and the Hamburg/ESO Survey of Christlieb and colleagues. Oxygen abundances from the molecular CO lines as well as rough estimates of 12C/13C ratios are estimated from the near-IR spectra of these stars. Near-IR model spectra with varying oxygen abundances, in combination with previously determined parameters from optical spectra are used for the estimation of abundances for this sample. As both oxygen abundances and 12C/13C ratios are tracers of nucleosynthesis, we hope to gain information about Galactic nucleosynthesis through the analysis of this sample.
A Search for Unrecognized Carbon-Enhanced Metal-Poor Stars
Authors: Placco Vinicius M. , Kennedy Catherine R. , Rossi Silvia , Beers Timothy C. , Christlieb Norbert , Sivarani Thirupathi 
Published: March 2010

AbstractWe have developed a new procedure to search for Carbon-Enhanced Metal-Poor (CEMP) stars from the Hamburg/ESO (HES) prism-survey plates. This method uses an extended line index for the CH G-band, which we demonstrate to have superior behavior when compared to the narrower G-band index formerly employed for these spectra.A first subsample, biased towards brighter stars (B<15.5), has been extracted from the scanned HES plates. After visual inspection (to eliminate spectra compromised by plate defects, overlapping spectra, etc., and to carry out rough spectral classifications), a list of 669 previously unidentified candidate CEMP stars was compiled. Follow-up spectroscopy for a pilot sample of 132 candidates was obtained on the SOAR 4.1m telescope. Our results show that most of the stars observed lie in the targetted metallicity range, and possess prominent carbon absorption features at 4300Å. The success rate for the identification of new CEMP stars is ~50% for [Fe/H]< −2.0. For stars with [Fe/H]< −2.5, the ratio increases to 100%.
A view of the Galactic halo using beryllium as a time scale
Authors: Smiljanic Rodolfo , Pasquini L. , Bonifacio P. , Galli D. , Barbuy B. , Gratton R. , Randich S. 
Published: March 2010

AbstractBeryllium stellar abundances were suggested to be a good tracer of time in the early Galaxy. In an investigation of its use as a cosmochronometer, using a large sample of local halo and thick-disk dwarfs, evidence was found that in a log(Be/H) vs. [α/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined.
The Cosmic Chemical Evolution as seen by the Brightest Events in the Universe
Authors: Savaglio Sandra 
Published: March 2010

AbstractGamma-ray bursts (GRBs) are the brightest events in the universe. They have been used in the last five years to study the cosmic chemical evolution, from the local universe to the first stars. The sample size is still relatively small when compared to field galaxy surveys. However, GRBs show a universe that is surprising. At z > 2, the cold interstellar medium in galaxies is chemically evolved, with a mean metallicity of about 1/10 solar. At lower redshift (z < 1), metallicities of the ionized gas are relatively low, on average 1/6 solar. Not only is there no evidence of redshift evolution in the interval 0 < z < 6.3, but also the dispersion in the ~30 objects is large. This suggests that the metallicity of host galaxies is not the physical quantity triggering GRB events. From the investigation of other galaxy parameters, it emerges that active star-formation might be a stronger requirement to produce a GRB. Several recent striking results strongly support the idea that GRB studies open a new view on our understanding of galaxy formation and evolution, back to the very primordial universe at z ~ 8.
Possibility of measuring the amount of intergalactic metals with 14N VII HFS line
Authors: Docenko Dmitrijs , Sunyaev Rashid A. 
Published: March 2010

AbstractWe discuss possibility of observations of the warm-hot intergalactic medium using the hyperfine structure line of highly charged nitrogen ion 14N VII (rest wavelength λ = 5.652 mm). Observations of this line will allow to separate bulk and turbulent motions in the observed target and will broaden the information about the gas ionization state, chemical and isotopic composition.Wavelength of this line is well-suited for ground-based observation of objects at z ≈ 0.15 − 0.6 when it is redshifted to the widely-used 6.5 − 9 mm spectral band, and, for example, for z ≥ 1.3, when the line can be observed in 1.3 cm band and at lower frequencies.
Quasar Metal Abundances & Host Galaxy Evolution
Authors: Hamann Fred , Simon Leah E. 
Published: March 2010

AbstractHigh-redshift quasars provide a unique glimpse into the early evolution of massive galaxies. The physical processes that trigger major bursts of star formation in quasar host galaxies (mergers and interactions) probably also funnel gas into the central regions to grow the super-massive black holes (SMBHs) and ignite the luminous quasar phenomenon. The globally dense environments where this occurs were probably also among the first to collapse and manufacture stars in significant numbers after the big bang. Measurements of the elemental abundances near quasars place important constraints on the nature, timing and extent of this star formation. A variety of studies using independent emission and absorption line diagnostics have shown that quasar environments have gas-phase metallicities that are typically a few times solar at all observed redshifts. These results are consistent with galaxy evolution scenarios in which large amounts of star formation (e.g., in the central regions) precede the visibly bright quasar phase. An observed trend for higher metallicities in more luminmous quasars (powered by more massive SMBHs) is probably tied to the well-known mass–metallicity relation among ordinary galaxies. This correlation and the absence of a trend with redshift indicate that mass is a more important parameter in the evolution than the time elapsed since the big bang.
Lookback time evolution of metals: discarding the closed box model
Authors: Rodrigues M. , Hammer F. , Flores H. , Puech M. 
Published: March 2010

AbstractWe have gathered a representative sample of 88 intermediate mass galaxies at z~0.6 and have provided robust estimates of their gas phase metallicity based on the strong line method R23. We have found that these galaxies have undergone a strong evolution of their metal content during the last 8 Gyrs. We confirmed the shift about ~0.3dex to lower abundance of the M-Z relation at z~0.6 found by Liang et al. 2006. This result shows that the evolution of the gas phase is still active down to z=0.4 and that the close box model is not a valid scenario for local spiral progenitors.
Tracing Metallicity in High Redshift Quasars
Authors: Simon Leah E. , Hamann Fred 
Published: March 2010

AbstractWe present two ongoing studies of gas phase abundances around high redshift quasars. First, we examine broad emission line (BEL) metallicities for 29 quasars with 2.3 < z < 4.6 and far-infrared (far-IR) luminosities (LFIR) from 1013.4 to ≤ 1012.2 L⊙, corresponding to star formation rates (SFRs) of 6740 to ≤ 1360 M⊙ yr−1. Quasar samples sorted by LFIR might represent an evolutionary sequence if SFRs in quasar hosts generally diminish across quasar lifetimes. We create three composite spectra from rest-frame ultra-violet Sloan Digital Sky Survey spectra with increasing far-IR luminosity. We measure the N V(λ1240)/C IV(λ1550) and Si IV(λ1397)+O IV](λ1402)/C IV(λ1550) emission line flux ratios for each composite and find uniformly high (~5-10 times solar) metallicities for the three composites, and no evidence for changes in metal enrichment with changes in ongoing SFR. Second, we present preliminary results from the largest ever survey of high resolution associated absorption line (AAL) region metallicities and physical properties in a sample of high redshift (z > 3) quasars. This includes five quasars with previously known AALs at z > 4 and two well measured z ~3 quasars with unusually rich absorption spectra. We determine well-constrained metallicities of about twice solar for five AAL systems. We find a range of lower limits for AAL metallicities in the z > 4 quasars from 1/100ths solar to 3 times solar. Overall, these results for typically super-solar gas-phase metallicities near quasars are consistent with evolutionary schemes where the major episodes of star formation in the host galaxies occur before the visibly luminous quasar phase. High SFRs (comparable to ULIRGs) in the host galaxies are not clearly linked to younger or chemically less mature quasar environments.
Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE
Authors: Mashonkina Lyudmila , Gehren Thomas , Shi Jianrong , Korn Andreas , Grupp Frank 
Published: March 2010

AbstractNon-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and β Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.
Problems in abundance determination from UV spectra of hot supergiants
Authors: Deković M. Sarta , Kotnik-Karuza D. , Jurkić T. , Prester D. Dominis 
Published: March 2010

AbstractWe present measurements of equivalent widths of the UV, presumably photospheric lines: C III 1247 Å, N III 1748 Å, N III 1752 Å, N IV 1718 Å and He II 1640 Å in high-resolution IUE spectra of 24 galactic OB supergiants. Equivalent widths measured from the observed spectra have been compared with their counterparts in the Tlusty NLTE synthetic spectra. We discuss possibilities of static plan-parallel model to reproduce observed UV spectra of hot massive stars and possible reasons why observations differ from the model so much.
The Determination of the Abundances of the Fe Group Elements in Early B Stars from High Resolution FUV Spectra
Authors: Peters Geraldine J. , Adelman Saul J. , Hubeny Ivan , Lanz Thierry 
Published: March 2010

AbstractWe present selected results from an investigation that is currently underway to determine the abundances of the Fe group elements in early B stars and assess the extent to which contemporary NLTE and LTE models represent their atmospheres. High resolution UV and optical spectra of B stars that display ultrasharp lines are compared with computations from TLUSTY/SYNSPEC and SYNTHE. Some results from our analysis of the abundance standard ι Her (B3V) are presented here.
Accurate Fundamental Stellar Parameters
Authors: Bruntt Hans 
Published: March 2010

AbstractWe combine results from interferometry, asteroseismology and spectroscopic analyses to determine accurate fundamental parameters (mass, radius and effective temperature) of 10 bright solar-type stars covering the H-R diagram from spectral type F5 to K1. Using “direct” techniques that are only weakly model-dependent we determine the mass, radius and effective temperature. We demonstrate that model-dependent or “indirect” methods can be reliably used even for relatively faint single stars for which direct methods are not applicable. This is important for the characterization of the targets of the CoRoT and Kepler space missions.
Abundance patterns and the chemical enrichment of nearby dwarf galaxies
Authors: Hill Vanessa 
Published: March 2010

AbstractAs the least massive galaxies we know, dwarf spheroidal galaxies (dSph) allow to probe chemical enrichement on the smallest scales, and perhaps in its simplest expression. Particularly interesting are the issues concerning the efficency with which metals are retained or lost in these shallow potential wells (supernovae feedback), and the effect of this on star formation itself. Another fundamental issue concerns the earliest epochs of star formation: are first stars formed in similar ways and proportions in all halos ? Finally, as the smallest galaxies know, dSph have been suggested to be the surviving cousins of galaxy building blocs that (in λ-CDM) assemble to make larger galaxies. This parenthood would not necessarily hold at all late times, when survivors have lived their own differentiated life, but is expected at least at the earliest epochs.I review here the chemical abundances of individual stars in the nearest dwarf spheroidal galaxies, that have become available in increasing numbers (sample size and galaxies probed) in the last decade. Special emphasis is given to: a) recent results obtain with FLAMES on VLT, highlighting the power of detailed chemical abundance patterns of large samples of stars to unravel the various evolutionnary paths followed by dSph; b) the oldest and most metal-poor populations in dSph.
Complexity in small-scale dwarf spheroidal galaxies
Authors: Koch Andreas , Adén Daniel , Grebel Eva K. , Feltzing Sofia 
Published: March 2010

AbstractOur knowledge about the chemical evolution of the more luminous dwarf spheroidal (dSph) galaxies is constantly growing. However, little is known about the enrichment of the ultrafaint systems recently discovered in large numbers in large Sky Surveys. Low-resolution spectroscopy and photometric data indicate that these galaxies are predominantly metal-poor. On the other hand, the most recent high-resolution abundance analyses indicate that some of these galaxies experienced highly inhomogenous chemical enrichment, where star formation proceeds locally on the smallest scales. Furthermore, these galaxy-contenders appear to contain very metal-poor stars with [Fe/H]< −3 dex and could be the sites of the first stars. Here, we consider the presently available chemical abundance information of the (ultra-) faint Milky Way satellite dSphs. In this context, some of the most peculiar element and inhomogeneous enrichment patterns will be discussed and related to the question of to what extent the faintest dSph candidates and outer halo globular clusters could have contributed to the metal-poor Galactic halo.