Heterogeneous distribution of Al-26 at the birth of the Solar System

It is believed that Al-26, a short-lived (t1/2 = 0.73 Ma) and now extinct radionuclide, was uniformly distributed in the nascent Solar System with the initial Al-26/Al-27 ratio of ~5.2\times10-5, suggesting its external stellar origin. However, the stellar source of Al-26 and the manner in which it was injected into the solar system remain controversial: the Al-26 could have been produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star and injected either into the protosolar molecular cloud or protoplanetary disk. Corundum (Al2O3) is thermodynamically predicted to be the first condensate from a cooling gas of solar composition. Here we show that micron-sized corundum condensates from O-16-rich gas (Big Delta O-17 ~ -25%) of solar composition recorded heterogeneous distribution of Al-26 at the birth of the solar system: the inferred initial Al-26/Al-27 ratio ranges from ~6.5x10-5 to <2x10-6; ~50% of the corundum grains measured are Al-26-poor. Other Al-26-poor, O-16-rich refractory objects include grossite (CaAl4O7)- and hibonite(CaAl12O19)-rich calcium-aluminum-rich inclusions (CAIs) in CH chondrites, platy hibonite crystals in CM chondrites, and FUN (fractionation and unidentified nuclear isotopic anomalies) CAIs in CV, CO, and CR chondrites. Considering the apparently early and short duration (<0.3 Ma) of condensation of refractory O-16-rich solids in the solar system, we infer that Al-26 was injected into the collapsing protosolar molecular cloud and later homogenized in the protoplanetary disk. The apparent lack of correlation between Al-26 abundance and O-isotope compositions of corundum grains put important constraints on the stellar source of Al-26 in the solar system.Comment: Astrophysical Journal Letters 733, L3

Trace element geochemistry of ordinary chondrite chondrules: the type I/type II chondrule dichotomy

We report trace element concentrations of silicate phases in chondrules from LL3 ordinary chondrites Bishunpur and Semarkona. Results are similar to previously reported data for carbonaceous chondrites, with rare earth element (REE) concentrations increasing in the sequence olivine < pyroxene < mesostasis, and heavy REE (HREE) being enriched by 1-2 orders of magnitude (CI-normalized) relative to light REE (LREE) in ferromagnesian silicates, although no single olivine with very large LREE/HREE fractionation has been found. On average, olivine in type II chondrules is poorer in refractory lithophile incompatible elements (such as REE) than its type I counterpart by a factor of ~2. This suggests that olivine in type I and II chondrules formed by batch and fractional crystallization, respectively, implying that type II chondrules formed under faster cooling rates (> ~ 10 K/h) than type I chondrules. Appreciable Na concentrations (3-221 ppm) are measured in olivine from both chondrule types; type II chondrules seem to have behaved as closed systems, which may require chondrule formation in the vicinity of protoplanets or planetesimals. At any rate, higher solid concentrations in type II chondrule forming regions may explain the higher oxygen fugacities they record compared to type I chondrules. Type I and type II chondrules formed in different environments and the correlation between high solid concentrations and/or oxygen fugacities with rapid cooling rates is a key constraint that chondrule formation models must account for.Comment: 46 pages, 7 figure

Quelle puissance ?

Sur le thème de la puissance divine, cet article propose une réflexion en deux temps. Le premier à partir des données bibliques et d’une analyse de concepts, conteste la notion de toute-puissance divine. Le deuxième temps, après avoir distingué entre puissance et pouvoir, tente de dégager les caractéristiques de la puissance divine.This article on the theme of divine power is divided into two sections. Based on biblical research and concept analysis, the first section challenges the notion of an all-powerful God. The second section, having drawn a distinction between strength and power, attempts to identify the characteristics of divine strength

Science et christianisme

Quelles relations établir entre la science et religion ? On a proposé deux grandes réponses à cette question : la première cherche par des moyens divers à les unifier, la seconde veut, au contraire, les séparer radicalement. Le théologien luthérien français Gérard Siegwalt propose une troisième voie : les connecter, tout en maintenant leur différence et en écartant des interférences indues. Scientifiquement, les affirmations de la foi ne sont ni démontrées ni démontrables, mais sont crédibles.What relationships can be established between science and religion ? Two important answers to this question have been proposed : the first seeks, by various means, the unity of the two ; the second, on the contrary, radically separates them. The French Lutheran theologian Gérard Siegwalt proposes a third way : to connect them, while maintaining their differences and avoiding any undue interference. Scientifically speaking, the affirmations of faith are neither demonstrated nor demonstrable ; they are credible

Philosophie de la religion et méthode de corrélation chez Paul Tillich

On se demande dans cet article ce qu’est devenue la philosophie de la religion chez le Tillich américain, où il en est fort peu mention. On rappelle alors l’histoire de cette discipline. Elle devient indépendante de la théologie au cours des dix-huitième et dix-neuvième siècles, où elle finit par s’identifier à la « religion naturelle ». Elle devient ainsi la cible des virulentes critiques de la théologie néo-orthodoxe inspirée par Karl Barth. Dans sa Théologie systématique, Tillich entend surmonter ce conflit par sa « méthode de corrélation ». Les deux grands problèmes de la philosophie de la religion sont par là pris en compte. Le problème plus théorique des catégories du religieux est repris dans la corrélation de la substance (religieuse) et de la forme (culturelle), tandis que celui, plus existentiel, de la présence et de la perception de l’ultime est traité avec la corrélation de la question et de la réponse.Does a theology genuinely based on the Gospel preclude any attempt to reach God outside of his historical revelation, i.e., does it exclude any philosophy of religion or natural theology ? This is the position held in the 20th century by the neo-orthodox movement inspired by Barth. Tillich prefers the correlative approach. To actually take place, human experience and divine revelation have to meet. They are only meaningful and real in relationship to one another. It is neither possible to eliminate one of these components, nor can one confine them to two distinct realms. They must be dealt with together in their mutual tension. The themes of substance and form, as well as those of question and answer, thus give structure to the interrelation between two poles. The correlative method integrates philosophy of religion and theology of revelation in a joint process