bluuebunny/crossref_metadata_2025_split huggingface dataset:
(Un)true deuterium abundance in the Galactic disk
Authors: Prodanović Tijana , Steigman Gary , Fields Brian D. 
Published: April 2010

AbstractDeuterium has a special place in cosmology, nuclear astrophysics, and galactic chemical evolution, because of its unique property that it is only created in the big bang nucleosynthesis while all other processes result in its net destruction. For this reason, among other things, deuterium abundance measurements in the interstellar medium (ISM) allow us to determine the fraction of interstellar gas that has been cycled through stars, and set constraints and learn about different Galactic chemical evolution (GCE) models. However, recent indications that deuterium might be preferentially depleted onto dust grains complicate our understanding about the meaning of measured ISM deuterium abundances. For this reason, recent estimates by Linsky et al. (2006) have yielded a lower bound to the “true”, undepleted, ISM deuterium abundance that is very close to the primordial abundance, indicating a small deuterium astration factor contrary to the demands of many GCE models. To avoid any prejudice about deuterium dust depletion along different lines of sight that are used to determine the “true” D abundance, we propose a model-independent, statistical Bayesian method to address this issue and determine in a model-independent manner the undepleted ISM D abundance. We find the best estimate for the gas-phase ISM deuterium abundance to be (D/H)ISM ≥ (2.0 ± 0.1) × 10−5. Presented are the results of Prodanović et al. (2009).
Measurements of 3He in Galactic HII regions and planetary nebulae
Authors: Bania T. M. , Rood R. T. , Balser D. S. 
Published: April 2010

AbstractThe cosmic abundance of the 3He isotope has important implications for many fields of astrophysics. We are using the 8.665 GHz hyperfine transition of 3He+ to determine the 3He/H abundance in Milky Way H ii regions and planetary nebulae. Here we review the 30 year history of our 3He program, report on its current status, and describe our future plans.
4He abundances: Optical versus radio recombination line measurements
Authors: Balser Dana S. , Rood Robert T. , Bania T. M. 
Published: April 2010

AbstractAccurate measurements of the 4He/H abundance ratio are important in constraining Big Bang nucleosynthesis, models of stellar and Galactic evolution, and H ii region physics. We discuss observations of radio recombination lines using the Green Bank Telescope toward a small sample of H ii regions and planetary nebulae. We report 4He/H abundance ratio differences as high as 15–20% between optical and ratio data that are difficult to reconcile. Using the H ii regions S206 and M17 we determine 4He production in the Galaxy to be dY/dZ = 1.71 ± 0.86.
The primordial abundance of 4He from a large sample of low-metallicity H ii regions
Authors: Izotov Yuri I. 
Published: April 2010

AbstractWe determine the primordial helium mass fraction Yp using 1700 spectra of low-metallicity extragalactic H ii regions. This sample is selected from the Data Release 7 of the Sloan Digital Sky Survey, from European Southern Observatory archival data and from our own observations. We have considered known systematic effects which may affect the 4He abundance determination. They include collisional and fluorescent enhancements of He i recombination lines, underlying He i and hydrogen stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the H ii region. Monte Carlo methods are used to solve simultaneously the above systematic effects. We find a primordial helium mass fraction Yp = 0.2512 ± 0.0006(stat.) ± 0.0020 (syst.). This value is higher than the value given by Standard Big Bang Nucleosynthesis (SBBN) theory. If confirmed, it would imply slight deviations from SBBN.
Uncertainties in nebular helium abundances
Authors: Skillman Evan D. 
Published: April 2010

AbstractEfforts to determine the primordial helium abundance via observations of metal poor HII regions have been limited by significant uncertainties. Because of a degeneracy between the solutions for density and temperature, the precision of the helium abundance determinations is limited. Spectra from the literature are used to show the effects of new atomic data and to demonstrate the challenges of determining precise He abundances. Several suggestions are made for meeting these challenges.
Revisiting the helium abundance in globular clusters with multiple main sequences
Authors: Casagrande Luca , Portinari Laura , Flynn Chris 
Published: April 2010

AbstractFor nearby K dwarfs, the broadening of the observed Main Sequence at low metallicities is much narrower than expected from isochrones with the standard helium–to–metal enrichment ratio ΔY/ΔZ~2. A much higher value, of order 10, is formally needed to reproduce the observed broadening, but it returns helium abundances in awkward contrast with Big Bang Nucleosynthesis. This steep enrichment ratio resembles, on a milder scale, the very high ΔY/ΔZ estimated from the multiple Main Sequences observed in some metal-poor Globular Clusters. We argue that a revision of low Main Sequence stellar models, suggested from nearby stars, could help to reduce the overwhelmingly high ΔY/ΔZ deduced so far for those clusters. Under the most favourable assumptions, the estimated helium content for the enriched populations may decrease from Y ≃ 0.4 to as low as Y ≃ 0.3, with intermediate values being plausible.
Helium-rich stars in globular clusters: constraints for self-enrichment by massive stars
Authors: Decressin Thibaut , Meynet G. , Charbonnel C. 
Published: April 2010

AbstractGlobular clusters exhibit peculiar chemical patterns where Fe and heavy elements are constant inside a given cluster while light elements (Li to Al) show strong star-to-star variations. This pattern can be explained by self-pollution of the intracluster gas by the slow winds of fast rotating massive stars. Besides, several main sequences have been observed in several globular clusters which can be understood only with different stellar populations with distinct He content. Here we explore how these He abundances can constrain the self-enrichment in globular clusters.
Primordial helium abundance of the SMC: a view from intermediate mass stars
Authors: Costa Roberto D. D. , Maciel Walter J. , Idiart Thais E. P. 
Published: April 2010

AbstractHelium abundances were derived for a sample of planetary nebulae of the Small Magellanic Cloud. These abundances were corrected from the chemical evolution of the galaxy as well as from stellar nucleosynthesis, and the primordial helium abundance then was estimated for the sample. Results indicate that the resulting average value for the sample is consistent with the expected values for primordial helium from SBBN and other values derived from HII regions in different stellar systems, even varying the enrichment ratio within its uncertainty range.
Lithium and proton-capture elements in globular clusters: the case of 47 Tucanae
Authors: D'Orazi Valentina , Lucatello Sara , Gratton Raffaele , Bragaglia Angela , Carretta Eugenio 
Published: April 2010

AbstractThe determination of lithium (Li) abundances in Globular Clusters (GCs), along with proton-capture elements (Na, O, Mg, Al), offer a key tool to address the pollution scenario and its mechanisms, the dilution process acting within each star and the first phases in the lifetime of GCs. We present our results on Na, O and Li abundance determination in a large sample of dwarf stars in the GC 47 Tucanae (NGC 104). While we found a clear Na-O anti-correlation, in perfect agreement with giant members by Carretta et al. (2009a, b), Li abundance appears neither positively correlated with oxygen, nor anti-correlated with sodium. Our finding unveils an intrinsic scatter in Li content, independent of intra-cluster pollution by a first generation of more massive, faster evolving stars.
Chemical composition of stellar populations in ω Centauri
Authors: Marino A. F. , Piotto G. , Gratton R. , Milone A. P. , Zoccali M. , Bedin L. R. , Villanova S. , Bellini A. 
Published: April 2010

AbstractWe derive abundances of Fe, Na, O, α and s-elements from GIRAFFE@VLT spectra for more than 200 red giant stars in the Milky Way satellite ω Centauri. Our preliminary results are that: (i) we confirm that ω Centauri exhibits large star-to-star metallicity variation (~1.4 dex); (ii) the metallicity distribution reveals the presence of at least five stellar populations with different [Fe/H]; (iii) a distinct Na-O anticorrelation is clearly observed for the metal-poor and metal-intermediate stellar populations while apparently the anticorrelation disappears for the most metal rich populations. Interestingly the Na level grows with iron.
The light elements in the light of 3D and non-LTE effects
Authors: Asplund Martin , Lind Karin 
Published: April 2010

AbstractIn this review we discuss possible systematic errors inherent in classical 1D LTE abundance analyses of late-type stars for the light elements (here: H, He, Li, Be and B). The advent of realistic 3D hydrodynamical model atmospheres and the availability of non-LTE line formation codes place the stellar analyses on a much firmer footing and indeed drastically modify the astrophysical interpretations in many cases, especially at low metallicities. For the Teff-sensitive hydrogen lines both stellar granulation and non-LTE are likely important but the combination of the two has not yet been fully explored. A fortuitous near-cancellation of significant but opposite 3D and non-LTE effects leaves the derived 7Li abundances largely unaffected but new atomic collisional data should be taken into account. We also discuss the impact on 3D non-LTE line formation on the estimated lithium isotopic abundances in halo stars in light of recent claims that convective line asymmetries can mimic the presence of 6Li. While Be only have relatively minor non-LTE abundance corrections, B is sensitive even if the latest calculations imply smaller non-LTE effects than previously thought.
Li isotopes in metal-poor halo dwarfs: a more and more complicated story
Authors: Spite Monique , Spite François 
Published: April 2010

AbstractThe nuclei of the lithium isotopes are fragile, easily destroyed, so that, at variance with most of the other elements, they cannot be formed in stars through steady hydrostatic nucleosynthesis.The7Li isotope is synthesized during primordial nucleosynthesis in the first minutes after the Big Bang and later by cosmic rays, by novae and in pulsations of AGB stars (possibly also by the ν process).6Li is mainly formed by cosmic rays. The oldest (most metal-deficient) warm galactic stars should retain the signature of these processes if, (as it had been often expected) lithium is not depleted in these stars. The existence of a “plateau” of the abundance of7Li (and of its slope) in the warm metal-poor stars is discussed. At very low metallicity ([Fe/H] < −2.7dex) the star to star scatter increases significantly towards low Li abundances. The highest value of the lithium abundance in the early stellar matter of the Galaxy (logϵ(Li) = A(7Li) = 2.2 dex) is much lower than the the value (logϵ(Li) = 2.72) predicted by the standard Big Bang nucleosynthesis, according to the specifications found by the satellite WMAP. After gathering a homogeneous stellar sample, and analysing its behaviour, possible explanations of the disagreement between Big Bang and stellar abundances are discussed (including early astration and diffusion). On the other hand, possibilities of lower productions of7Li in the standard and/or non-standard Big Bang nucleosyntheses are briefly evoked.A surprisingly high value (A(6Li)=0.8 dex) of the abundance of the6Li isotope has been found in a few warm metal-poor stars. Such a high abundance of6Li independent of the mean metallicity in the early Galaxy cannot be easily explained. But are we really observing6Li?
Observational signatures for depletion in the Spite plateau: solving the cosmological Li discrepancy?
Authors: Meléndez Jorge , Casagrande Luca , Ramírez Iván , Asplund Martin , Schuster William J. 
Published: April 2010

AbstractWe present Li abundances for 73 stars in the metallicity range −3.5 < [Fe/H] < −1.0 using improved IRFM temperatures (Casagrande et al. 2010) with precise E(B-V) values obtained mostly from interstellar NaI D lines, and high-quality equivalent widths (σEW ~ 3%). At all metallicities we uncover a fine-structure in the Li abundances of Spite plateau stars, which we trace to Li depletion that depends on both metallicity and mass. Models including atomic diffusion and turbulent mixing seem to reproduce the observed Li depletion assuming a primordial Li abundance ALi = 2.64 dex (MARCS models) or 2.72 (Kurucz overshooting models), in good agreement with current predictions (ALi = 2.72) from standard BBN. We are currently expanding our sample to have a better coverage of different evolutionary stages at the high and low metallicity ends, in order to verify our findings.
Convection and 6Li in the atmospheres of metal-poor halo stars
Authors: Steffen Matthias , Cayrel R. , Bonifacio P. , Ludwig H.-G. , Caffau E. 
Published: April 2010

AbstractBased on 3D hydrodynamical model atmospheres computed with the CO5BOLD code and 3D non-LTE (NLTE) line formation calculations, we study the effect of the convection-induced line asymmetry on the derived 6Li abundance for a range in effective temperature, gravity, and metallicity covering the stars of the Asplund et al. (2006) sample. When the asymmetry effect is taken into account for this sample of stars, the resulting 6Li/7Li ratios are reduced by about 1.5% on average with respect to the isotopic ratios determined by Asplund et al. (2006). This purely theoretical correction diminishes the number of significant 6Li detections from 9 to 4 (2σ criterion), or from 5 to 2 (3σ criterion). In view of this result the existence of a 6Li plateau appears questionable. A careful reanalysis of individual objects by fitting the observed lithium 6707 Å doublet both with 3D NLTE and 1D LTE synthetic line profiles confirms that the inferred 6Li abundance is systematically lower when using 3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with unquestionable 6Li detection do exist even if analyzed in 3D-NLTE, the most prominent example being HD 84937.
Boron abundances in diffuse interstellar clouds
Authors: Ritchey Adam M. , Federman S. R. , Sheffer Y. , Lambert D. L. 
Published: April 2010

AbstractWe present a comprehensive survey of B abundances in diffuse interstellar clouds from HST/STIS observations along 56 Galactic sight lines. Our sample is the result of a complete search of archival STIS data for the B II λ1362 resonance line, with each detection confirmed by the presence of absorption from other dominant ions at the same velocity. The data probe a range of astrophysical environments including both high-density regions of massive star formation as well as low-density paths through the Galactic halo, allowing us to clearly define the trend of B depletion onto interstellar grains as a function of gas density. Many extended sight lines exhibit complex absorption profiles that trace both local gas and gas associated with either the Sagittarius-Carina or Perseus spiral arm. Our analysis indicates a higher B/O ratio in the inner Sagittarius-Carina spiral arm than in the vicinity of the Sun, which may suggest that B production in the current epoch is dominated by a secondary process. The average gas-phase B abundance in the warm diffuse ISM [log ϵ(B) = 2.38±0.10] is consistent with the abundances determined for a variety of Galactic disk stars, but is depleted by 60% relative to the solar system value. Our survey also reveals sight lines with enhanced B abundances that potentially trace recent production of 11B either by cosmic-ray or neutrino-induced spallation. Such sight lines will be key to discerning the relative importance of the two production routes for 11B synthesis.
Lithium in globular clusters
Authors: Korn Andreas J. 
Published: April 2010

AbstractI review recent progress in mapping out and understanding the behaviour of stellar surface abundances of lithium as evidenced by spectroscopic studies of nearby globular clusters (GCs). It will become clear that these observations necessitate revisions to the canonical picture of stellar and globular-cluster evolution: stars evolve with additional non-convective mixing processes and GCs are not simple stellar populations. In spite of these complications, GCs are excellent test beds for chemical-abundance studies. Spectroscopic observations of GC stars of different evolutionary stages reveal systematic trends of surface abundances likely caused by atomic diffusion and mixing. Correcting for their combined effect on surface lithium, a stellar solution to the cosmological lithium discrepancy is likely, if not probable. However, a definitive answer can only be given once we know the effective temperatures of warm subdwarfs and subgiants to high accuracy and understand the processes which give rise to the mixing needed to moderate atomic diffusion.
Observational signatures of lithium depletion in the metal-poor globular cluster NGC6397
Authors: Lind Karin , Primas Francesca , Charbonnel Corinne , Grundahl Frank , Asplund Martin 
Published: April 2010

AbstractThe “stellar” solution to the cosmological lithium problem proposes that surface depletion of lithium in low-mass, metal-poor stars can reconcile the lower abundances found for Galactic halo stars with the primordial prediction. Globular clusters are ideal environments for studies of the surface evolution of lithium, with large number statistics possible to obtain for main sequence stars as well as giants. We discuss the Li abundances measured for >450 stars in the globular cluster NGC 6397, focusing on the evidence for lithium depletion and especially highlighting how the inferred abundances and interpretations are affected by early cluster self-enrichment and systematic uncertainties in the effective temperature determination.
Lithium in a metal-poor external galaxy: ω Centauri
Authors: Bonifacio P. , Monaco L. , Sbordone L. , Villanova S. , Pancino E. 
Published: April 2010

Abstractω Centauri is a massive stellar system which is currently going through the Galactic Halo. Its compact aspect and spheroidal shape have for a long time led to it being classified as a Globular Cluster. However the fact that its stars cover a wide metallicity range (−0.6 < [Fe/H] < −2.1), points to this object as an external galaxy, satellite of the Milky Way. Lithium among warm metal-poor stars shows a roughly constant abundance, the “Spite Plateau”. This has been interpreted as evidence for a primordial origin of the lithium nucleus, at the time of nucleosynthesis. After the physical conditions under which nucleosynthesis occurred, have been constrained by the observations of the fluctuations of the Cosmic Microwave Background, we are facing a “cosmological lithium problem”, namely the primordial lithium was a factor of three to four higher than what observed in the Spite plateau. Several avenues may be taken to solve this conundrum, either relying on fundamental physics or on stellar physics, however the realm of possibilities may be considerably narrowed by observing stellar populations in different galaxies, which have experienced different evolutionary histories. Some of the proposed “solutions” may be clearly ruled out, depending on the observation of lithium in the metal-poor populations of external galaxies. ω Centauri is the only external galaxy amenable to such an investigation in the era of 8m telescopes. We have pushed to its limits FLAMES at the ESO 8.2m telescope to obtain high resolution spectra of the Li i doublet in 91 Turn-Off and Sub-Giant stars at V ~ 18 in ω Centauri. We present our preliminary results on this data which suggest that the Li content in ω Centauri warm stars is comparable to that observed in Galactic Halo field stars of similar metallicities and temperatures. This may effectively rule out a whole class of models which invoke a severe Li depletion through processing of material in an early generation of massive stars.
Observations of Lithium in red giant stars
Authors: Smith Verne V. 
Published: April 2010

AbstractConnections between observations of the lithium abundance in various types of red giants and stellar evolution are discussed here. The emphasis is on three main topics; 1) the depletion of Li as stars ascend the red giant branch for the first time, 2) the synthesis of 7Li in luminous and massive asymptotic giant branch stars via the mechanism of hot-bottom burning, and 3) the possible multiple sources of excess Li abundances found in a tiny fraction of various types of G and K giants.
Mass loss and luminosities of S and C AGB stars with and without Li
Authors: Guandalini Roald , Palmerini Sara , Busso Maurizio , Maiorca Enrico , Uttenthaler Stefan 
Published: April 2010

AbstractWe present the preliminary results of an analysis performed on two samples of thermally pulsing Asymptotic Giant Branch stars from our Galaxy, the first made of carbon-rich sources and the second of S-type stars. We have estimated their absolute luminosities and updated rates of the stellar winds through methods based on their infrared spectrophotometry and on updated estimates of their variability and distance.We then focus on those sources in our database showing Li in their spectra looking for correlations between the Li abundance and the other physical parameters, in the aim of establishing observational criteria for understanding the conditions for the occurrence of the deep mixing phenomena to which the production of Li is currently attributed.
Observations of light elements in massive stars
Authors: Kaufer A. 
Published: April 2010

AbstractObservations of light elements in hot massive stars are limited to few transitions of boron in the satellite-ultraviolet; lithium and beryllium are not observable at all. But because of its high sensitivity to the effects of rotational mixing, boron abundance determinations in massive stars have excelled as the definite test for evolutionary models with rotation. In this paper the observational evidence for rotational mixing in massive stars is reviewed and alternative interpretations are discussed.
Survey for Li-rich K giants
Authors: Kumar Y. Bharat , Reddy Bacham E. 
Published: April 2010

AbstractWe present results from an ongoing survey of searching Li-rich K giants among low mass giants along the Red Giant Branch (RGB). A sample of 2500 stars with accurate astrometry have been selected from Hipparcos catalogue covering both the RGB luminosity bump and the red clump regions on the HR diagram. Lithium abundances have been determined for half of the sample from low resolution spectra using line depth ratio method. Results confirm the rarity of Li-rich K giants, just under 1%, in the solar neighbourhood. This study increased the total number of known Li-rich K giants by a factor of two. The analysis of high resolution spectra of candidate Li-rich K giants showed that the K giant HD 77361 is highly enriched in lithium (log ϵ(Li) = 3.82) and at the same time has anomalously low carbon isotopic ratio (12C/13C = 4.3). The results put important constraints on the theoretical modelling of the stellar structure and the mixing process, particularly, of the K giants.
Ultra-lithium-deficient halo stars
Authors: Elliott Lisa M. , Ryan Sean G. 
Published: April 2010

AbstractWhile most warm halo dwarfs show lithium abundances at the level of the Spite Plateau, a small number (~5%) have undetectable lithium lines. The existence of these stars has long raised questions when interpreting the plateau abundances: are they an extreme example of a depletion mechanism that has affected the plateau stars, or do they have an entirely different history? We provide an overview of what is currently known about the lithium-poor halo stars and discuss a possible origin for the lithium deficiency in this unique group of stars.