Accretion in Protoplanetary Disks by Collisional Fusion

The formation of a solar system is believed to have followed a multi-stage process around a protostar. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag; there is a "bottleneck" at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Thus, successful planetary accretion requires rapid planetesimal growth to km scale. A commonly accepted picture is that for collisional velocities $V_c$ above a certain threshold collisional velocity, ${V_{th}} \sim$ 0.1-10 cm s$^{-1}$, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all collisions the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt explicitly with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter--phase diagrams, amorphs and polymorphs--has been neglected. Here it is demonstrated that novel aspects of surface phase transitions provide a physical basis for efficient sticking through collisional melting or amphorph-/polymorphization and fusion to extend the collisional velocity range of primary accretion to $\Delta V_c \sim$ 1-100 m s$^{-1}$, which bound both turbulent RMS speeds and the velocity differences between boulder sized and small grains $\sim$ 1-50 m s$^{-1}$. Thus, as inspiraling meter sized bodies collide with smaller particles in this high velocity collisional fusion regime they grow rapidly to km scales and hence settle into stable Keplerian orbits in $\sim$ 10$^5$ years before photoevaporative wind clears the disk of source material.Comment: 11 pages, 7 figures, 1 tabl

The origin of short-lived radionuclides and the astrophysical environment of solar system formation

Based on early solar system abundances of short-lived radionuclides (SRs), such as $^{26}$Al (T$_{1/2} = 0.74$ Myr) and $^{60}$Fe (T$_{1/2} = 1.5$ Myr), it is often asserted that the Sun was born in a large stellar cluster, where a massive star contaminated the protoplanetary disk with freshly nucleosynthesized isotopes from its supernova (SN) explosion. To account for the inferred initial solar system abundances of short-lived radionuclides, this supernova had to be close ($\sim$ 0.3 pc) to the young ($\leqslant$ 1 Myr) protoplanetary disk. Here we show that massive star evolution timescales are too long, compared to typical timescales of star formation in embedded clusters, for them to explode as supernovae within the lifetimes of nearby disks. This is especially true in an Orion Nebular Cluster (ONC)-type of setting, where the most massive star will explode as a supernova $\sim$ 5 Myr after the onset of star formation, when nearby disks will have already suffered substantial photoevaporation and/or formed large planetesimals. We quantify the probability for {\it any} protoplanetary disk to receive SRs from a nearby supernova at the level observed in the early solar system. Key constraints on our estimate are: (1) SRs have to be injected into a newly formed ($\leqslant$ 1 Myr) disk, (2) the disk has to survive UV photoevaporation, and (3) the protoplanetary disk must be situated in an enrichment zone permitting SR injection at the solar system level without disk disruption. The probability of protoplanetary disk contamination by a supernova ejecta is, in the most favorable case, 3 $\times$ 10$^{-3}$

Size of the group IVA iron meteorite core: Constraints from the age and composition of Muonionalusta

The group IVA fractionally crystallized iron meteorites display a diverse range of metallographic cooling rates. These have been attributed to their formation in a metallic core, approximately 150 km in radius, that cooled to crystallization in the absence of any appreciable insulating mantle. Here we build upon this formation model by incorporating several new constraints. These include (i) a recent U-Pb radiometric closure age of <2.5 Myr after solar system formation for the group IVA iron Muonionalusta, (ii) new measurements and modeling of highly siderophile element compositions for a suite of IVAs, and (iii) consideration of the thermal effects of heating by the decay of the short-lived radionuclide 60Fe. Our model for the thermal evolution of the IVA core suggests that it was approximately 50 - 110 km in radius after being collisionally exposed. This range is due to uncertainties in the initial abundance of live 60Fe incorporated into the IVA core. Our models define a relationship between cooling rate and closure age, which is used to make several predictions that can be tested with future measurements. In general, our results show that diverse cooling rates and early U-Pb closure ages can only coexist on mantle-free bodies and that energy released by the decay of 60Fe reduces the core size necessary to produce diverse metallographic cooling rates. The influence of 60Fe on cooling rates has largely been neglected in previous core formation models; accounting for this heat source can affect size estimates for other iron meteorite cores that cooled to crystallization in the presence of live 60Fe. Candidates for such a scenario of early, mantle-free formation include the iron IIAB, IIIAB and IVB groups.Comment: 30 pages, 3 figures, accepted to Earth and Planetary Science Letter

Silicon isotope variations in the Earth and meteorites

A fluorhydric acid-free sample preparation method derived from Georg et al. [1] has been used to measure the natural variations of silicon isotope compositions in terrestrial (including 12 geological standard materials) and meteoritic bulk-rock samples. All measurements were done using a Neptune MC-ICPMS in medium resolution mode (m/Δm = 7000, peak-edge definition). Magnesium was used as internal standard for mass-bias drift correction. The δ30Si values are expressed relative to the NBS-28 silica standard. IRMM-17 reference material yields a δ30Si of -1.4‰ ± 0.05‰ (2SD, n=11) in agreement with previous data [2-3]. Long-term reproducibilities were obtained for BHVO-2 (δ30Si = -0.27‰ ± 0.08‰ (2SD, n=30)) and a in-house Si standard (δ30Si = -0.01‰ ± 0.07‰ (2SD, n=20)) on a 7 months time scale. Total variation of δ30Si in natural samples ranges from - 0.5‰ to -0.1‰. Comparison with δ29Si values shows that this isotopic fractionation is mass-dependent. A 0.2‰ isotopic variation occurs among terrestrial samples suggesting an enrichment in the heavier silicon isotopes as a function of magma differentiation, as initially hinted by Douthitt [4]. Terrestrial samples mean value (δ30SiEarth= -0.23‰) is heavier by about 0.24‰ in δ30Si compared to chondrites. This may be explained by silicon isotope fractionation during planetary accretion and/or differentiation

Husserl’s phenomenology and ethics of values

La philosophie de Husserl est guidée par la volonté d'atteindre une connaissance objective du monde. La description de l'expérience de la conscience qu'elle propose porte en effet d'abord sur l'expérience des vécus par lesquels nous prenons connaissance du monde et sur les différences qu'il existe entre une connaissance authentique et une simple présomption, une croyance ou une illusion. Mais cette priorité de la connaissance objective dans l'ordre de la méthode ne doit pas occulter l'antériorité, dans l'ordre réel, de l'expérience non-conceptuelle et inobjective des biens. En effet, le monde se donne à nous d'abord comme un monde de biens et comme la manifestation sensible de certaines valeurs. Nous voyons de beaux objets, nous trouvons l'espace utile ou pratique, nous sommes attirés par un bruit, gênés par une lumière, etc. Comment concilier ces deux exigences, celle de la méthode et celle de l'expérience ? Cette question porte sur les conditions de possibilité de la phénoménologie elle-même, puisque cette méthode, pour intégrer le nouveau genre de problèmes que posent les valeurs, dans l'ordre esthétique ou éthique, doit s'amender en permanence. Les valeurs touchent en effet aux limites de la description des actes objectivants, car elles relèvent d'une dimension qui n'est pas assimilable à celle de la nature objective des choses. Les valeurs éthiques, plus précisément, touchent aux limites de l'exigence phénoménologique de l'unité de la raison et du strict parallélisme entre les différents types d'actes correspondant aux divers registres de l'expérience (théorique, axiologique et pratique). Ainsi, l'injustice ou la vertu ne se donnent pas à la manière dont se donnent les choses dans la perception sensible et ne sauraient faire l'objet d'une « constitution intentionnelle » depuis les prestations subjectives. Le sens d'une valeur ne dépasse-t-il pas toujours et par principe les possibilités constitutives du sujet ? En d'autres termes, peut-on penser les valeurs éthiques comme des « analoga » des propriétés empiriques des objets, dont il faudrait rendre compte et qu'il faudrait parvenir à connaître de manière objective, ou bien n'y a-t-il pas là un ordre qui échappe à la description, voire au rêve husserlien d'une science de tous les phénomènes et d'une description pure de tous les types d'expérience ? Pour répondre à cette question, nous procédons en deux temps. Le premier s'interroge sur les conditions de possibilité d'une théorie des valeurs élaborée au fil de la méthode phénoménologique, c'est-à-dire au moyen d'un double réquisit – analogique et transcendantal. Le second montre comment la compréhension du sens des valeurs éthiques implique des glissements et des réélaborations de cette méthode qui ont pour fonction de révéler, à la racine des actes d'évaluation et de volition qui se rapportent aux valeurs, la vie et l'œuvre du sujet éthique.Husserl's philosophy is an attempt to obtain objective knowledge on the world, which offers a description of the cognitive consciousness, in order to distinguish a presumptive or an illusory belief and an authentic knowledge. However, such a methodological precedence of the objective knowledge should not hide the actual priority of a non-objective and pre-predicative experience of the world. Indeed, the world first appears to us as a practical and valuable world. We can see beautiful objects, useful tools, pleasant landscapes. One is drawn by a sound or disturbed by a light. How are these two points of view to be reconciled? May we conciliate the methodological priority of theoretical consciousness and the empirical precedence of values and goods? This question deals with the possibility of phenomenology itself, because this philosophical method has to transform itself in order to take into account the idiosyncratic kinds of problems that raise the experience of values. Values reach the limits of the “objectifying consciousness's” possibilities. Because they do not belong to the realm of nature, values are not one of the empirical properties of things. More precisely, ethical values query the need of unity in Reason and the need of a strict parallelism between different intentional acts. May injustice or virtue appear as do so colours or shapes in the sensible world? Can these values be intentionally constituted? Does the meaning of values ever exceed the constitutive capabilities in the subject? In order to answer these questions, two fields shall be developed. The first one questions the possibility of a value theory, which is brought by the phenomenological method, that is to say, through a demand of analogy and a demand of transcendental constitution. The second one indicates how the understanding of ethical values implies some shifting and new elaborations in the methodology itself, in order to show, behind the rationality of emotional and volitional acts, the life and work of the ethical subject

La fonction de l’analogie dans la fondation de l’éthique chez Husserl

International audienceOne of Husserl’s leading ideas in the field of practical and evaluative Reason is the idea of an analogy between ethics and logic. This method is required in order to constitute ethics as a pure science. But how has this analogy to be understood and justified? Does it mean the control of theoretical reason on the acts of willing and feeling? Is it to say that ethics and logic are similar, or are sharing some essential features? In what sense may we assert that an analogy is not a pure identity? This paper tries to provide some answers to those questions, through an examination of the meaning and the function of this central concept of analogy, which is firstly introduced in Husserl’s writings on mathematics (Philosophie der Arithmetik) and logic (Logische Untersuchungen

Etude des météorites à l'aide du système isotopique Hf-W (contraintes sur les évènements du système solaire primitif)

Les évènements précoces du système solaire tels que l'accrétion et la différenciation des planétésimaux peuvent être datés en utilisant des isotopes radioactifs naturels présents dans les météorites. Nous nous intéréssons ici aux processus de différenciation planétaire à l'aide du système 182Hf - 182W, basé sur la radioactivité éteinte de 182Hf (T1/2 = 9 Ma). L'hafniumétant fortement lithophile et le tungstène modérément sidérophile, ces deux éléments fractionnent au cours des processus de ségrégation du métal et de fusion partielle des silicates. Une nouvelle méthode de mesure isotopique du tungstène par NTIMS - 100 fois plus sensible que les méthodes existantes a été mise au point, permettant d'analyser avec une grande précision de faibles quantités de matériaux pauvres en tungstène. L'isochrone Hf-W de roches totales obtenue pour les eucrites indique qu'elles se sont formées 11,1 Ma après les chondrites ordinaires. Par ailleurs, la différenciation du corps parent des eucrites et la fusion partielle du manteau qui a donné naissance aux liquides basaltiques sont des processus simultanés d'après les isotopes du tungstène. Cependant, aucune météorite de fer connue ni aucune pallasite ne correspond à la phase métallique complémentaire des eucrites au moment de leur différenciation, tandis que le métal de quelques mésosidérites présente la signature isotopique voulue. La composition isotopique en W est très variable d'un échantillon à l'autre parmi les phases métalliques des mésosidérites et les pallasites. Ceci semble dû à des processus de diffusion (échange isotopique) entre les silicates et le métal. Pour les mésosidérites, d'autres causes telles que la présence d'inclusion contenant de l'hafnium ou des métaux provenant de différents réservoirs peuvent aussi être envisagés. Une utilisation différente des systèmes Hf-W et Mn-Cr - comme traceurs de sources - permet également de déterminer la nature chondrique de l'impacteur qui a heurté la Terre il y a 65 Ma, au moment de la limite Crétacé-TertiairePARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF