98,247 research outputs found
A FIRST HIPPARCOS CONTRIBUTION TO THE LITHIUM PROBLEM F. Crifo, F. Spite, M. Spite
No full textA small but significant amount of the 6 Li isotope has been detected in the old metal-poor star HD 84937. Following the standard model of stellar evolution, the presence of this fragile element in the star is not expected if the star is a dwarf. The classical parallax indicates that the star is a dwarf, challenging either the standard stellar model or the standard primordial nucleosynthesis model. The Hipparcos parallax solves the problem, indicating that the star is a subgiant, the existence of 6 Li is then in agreement with the standard model of stellar evolution. Key words: Population II, lithium abundance; primordial nucleosynthesis. 1. INTRODUCTION The problem of the light elements, and especially lithium, has been often presented (e.g. Reeves 1994, Spite 1995, Balachandran 1995, Spite 1996, Spite et al. 1996, see also Thornburn 1996 for a slightly different point of view. The lithium observed in the atmosphere of the old metal-poor stars, has been formed only by the Big Bang ..
Spite and the scale of competition in Pseudomonas aeruginosa
No full textThis work was funded by the European Research Council and the Leverhulme Trust (A.B.), the Natural Environment Research Council (R.F.I. and A.B.), and Balliol College and the Royal Society (A.G.).Scale of competition has been shown to be an important factor in shaping the evolution of social interactions. Although many theoretical and experimental studies have examined its effect on altruistic cooperation, relatively little research effort has been focused on spiteful behaviors-actions that harm both the actor and the recipient. In this study, we expand on existing theory by investigating the importance of the global frequency of spiteful alleles, and we determine experimentally how the scale of competition affects selection for spite in the bacterial pathogen Pseudomonas aeruginosa under high and intermediate spatial relatedness. Consistent with our theoretical results, we found in our experiments that spiteful genotypes are more favored under local (rather than global) competition and intermediate (rather than high) spatial relatedness, conditions that have been shown to select against indiscriminate altruism.Peer reviewe
Comparative Statics of Altruism and Spite
No full textThe equilibrium outcome of a strategic interaction between two or more people may depend on the weight they place on each other’s payoff. A positive, negative or zero weight represents altruism, spite or complete selfishness, respectively. Paradoxically, the real, material payoff in equilibrium for a group of altruists may be lower than for selfish or spiteful groups. However, this can only be so if the equilibria involved are unstable. If they are stable, the total (equivalently, average) payoff can only increase or remain unchanged with an increasing degree of altruism.Altruism, spite, comparative statics, strategic games, stability of equilibrium
The evolution of the milky way from its earliest phases: constraints on stellar nucleosynthesis
No full textWe computed the evolution of the abundances of O, Mg, Si, Ca, K, Ti, Sc, Ni, Mn, Co, Fe and Zn in the Milky Way. We made use of the most widely adopted nucleosynthesis calculations and compared the model results with observational data with the aim of imposing constraints upon stellar yields. To best fit the data in the solar neighborhood, when adopting the Woosley & Weaver (1995, ApJS, 101, 181) yields for massive stars and the Iwamoto et al. (\cite{Iwamoto99}, ApJS, 125, 439) ones for type Ia SNe, it is required that: i) the Mg yields should be increased in stars with masses from 11 to 20 Msun and decreased in masses larger than 20 Msun. The Mg yield should be also increased in SNe Ia. ii) The Si yields should be slightly increased in stars above 40 Msun, whereas those of Ti should be increased between 11 and 20 Msun and above 30 Msun. iii) The Cr and Mn yields should be increased in stars with masses in the range 11-20 Msun; iv) the Co yields in SNe Ia should be larger and smaller in stars in the range 11-20 Msun; v) the Ni yield from type Ia SNe should be decreased; vi) the Zn yield from type Ia SNe should be increased. vii) The yields of O (metallicity dependent SN models), Ca, Fe, Ni, and Zn (the solar abundance case) in massive stars from Woosley & Weaver (\cite{Woosley95}) are the best to fit the abundance patterns of these elements since they do not need any changes. We also adopted the yields by Nomoto et al. (\cite{Nomoto97}, Nucl. Phys. A, 621, 467) and Limongi & Chieffi (\cite{Limongi03}, ApJ, 592, 404) for massive stars and discuss the corrections required in these yields in order to fit the observations. Finally, the small spread in the [el/Fe] ratios in the metallicity range from [Fe/H] = -4.0 up to -3.0 dex (Cayrel et al. \cite{Cayrel03}, A&A, 416, 1117) is a clear sign that the halo of the Milky Way was well mixed even in the earliest phases of its evolution
The Spite Dilemma Revisited: Comparison between Chinese and Japanese
No full textThis paper studies Chinese choice behavior in the provision of public goods via the voluntary contribution mechanism. The laboratory experiment conducted in China adopts the same design as the one used in Saijo and Nakamura (1995), i.e. either cooperating (full contribution) or free riding (no contribution) is predicted as the unique Nash equilibrium with a high (larger than one) or low (smaller than one) marginal return of contribution. Comparing the results of Chinese subjects with their Japanese counterparts, we find significant differences between these two countries in terms of their choice behavior, despite the similarities in their cultures and the proximity in geographical positions. Japanese subjects are more likely to act spitefully, and, in contrast, Chinese subjects are more likely to perform cooperatively. In addition, concerning the deviations from the Nash equilibria with different marginal returns, the statistical results indicate that Chinese subjects behave more consistent with the theoretical prediction in the high marginal return case, while Japanese choice behavior seems less different from the theoretical expectation in the low marginal return case.Voluntary contribution mechanism, Spite dilemma, Chinese, Japanese
High-speed stars: Galactic hitchhikers
No full textContext. The search for stars born in the very early stages of the Milky Way star formation history is of paramount importance in the study of the early Universe since their chemistry carries irreplaceable information on the conditions in which early star formation and galaxy buildup took place. The search for these objects has generally taken the form of expensive surveys for faint extremely metal-poor stars, the most obvious but not the only candidates to a very early formation. Aims. Thanks to Gaia DR2 radial velocities and proper motions, we identified 72 bright cool stars displaying heliocentric transverse velocities in excess of 500 km s-1. These objects are most likely members of extreme outer-halo populations, either formed in the early Milky Way build-up or accreted from since-destroyed self-gravitating stellar systems. Methods. We analysed low-resolution FORS spectra of the 72 stars in the sample and derived the abundances of a few elements. Despite the large uncertainties on the radial velocity determination, we derived reliable orbital parameters for these objects. Results. The stars analysed are mainly slightly metal poor, with a few very metal-poor stars. Their chemical composition is much more homogeneous than expected. All the stars have very eccentric halo orbits, some extending well beyond the expected dimension of the Milky Way. Conclusions. These stars can be the result of a disrupted small galaxy or they could have been globular cluster members. Age estimates suggest that some of them are evolved blue stragglers, now on the subgiant or asymptotic giant branches
The GALAH survey: Accreted stars also inhabit the Spite plateau
No full textThe European Space Agency (ESA) Gaia mission has enabled the remarkable discovery that a large fraction of the stars near the solar neighbourhood are debris from a single in-falling system, the so-called Gaia-Sausage-Enceladus (GSE). This discovery provides astronomers for the first time with a large cohort of easily observable, unevolved stars that formed in a single extragalactic environment. Here we use these stars to investigate the 'Spite plateau' - the near-constant lithium abundance observed in unevolved metal-poor stars across a wide range of metallicities (-3 < [Fe/H] < -1). Our aim is to test whether individual galaxies could have different Spite plateaus - e.g. the interstellar medium could be more depleted in lithium in a lower galactic mass system due to it having a smaller reservoir of gas. We identified 93 GSE dwarf stars observed and analysed by the GALactic Archaeology with HERMES (GALAH) survey as part of its Data Release 3 (DR3). Orbital actions were used to select samples of GSE stars, and comparison samples of halo and disc stars. We find that the GSE stars show the same lithium abundance as other likely accreted stars and in situ Milky Way stars. Formation environment leaves no imprint on lithium abundances. This result fits within the growing consensus that the Spite plateau, and more generally the 'cosmological lithium problem' - the observed discrepancy between the amount of lithium in warm, metal-poor dwarf stars in our Galaxy, and the amount of lithium predicted to have been produced by big bang nucleosynthesis - is the result of lithium depletion processes within stars
Variations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
The evolution of Lithium: Implications of a universal Spite plateau
No full textThe cosmological 7Li problem consists in explaining why the primordial Li abundance, as predicted by the standard Big Bang nucleosynthesis theory with constraints from WMAP and Planck, is a factor of 3 larger than the Li abundance measured in the stars of the Spite plateau defined by old, warm dwarf stars of the Milky Way halo. Several explanations have been proposed to explain this difference, including various Li depletion processes as well as non-standard Big Bang nucleosynthesis, but the main question remains unanswered. In this paper, we present detailed chemical evolution models for dwarf spheroidal and ultra faint galaxies, compute the galactic evolution of 7Li abundance in these objects, and compare it with observations of similar objects. In our models, Li is mainly produced by novae and cosmic rays, and to a minor extent, by low and intermediate mass stars. We adopt the yield combination that best fits the Li abundances in the Milky Way stars. It is evident that the observations of dwarf objects define a Spite plateau, identical to that observed in the Milky Way, thus suggesting that the Spite plateau could be a universal feature and its meaning should be discussed. The predictions of our models for dwarf galaxies are obtained by assuming as Li primordial abundance either the one detected in the atmospheres of the oldest halo stars (Spite plateau; A(Li) ~ 2.2 dex), or the one from cosmological observations (WMAP; A(Li) ~ 2.66 dex). Finally, we discuss the implications of the universality of the Spite plateau results
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