Journal Club//2010 2e trimestre
Journal Club on 13/4/2010
(Ce texte n'est pas disponible en français, nous nous en excusons.)
The Chondrites: primitive meteorites as windows into the early Solar System
By Emmanuel Jacquet (LMCM, Museum National d'Histoire Naturelle)
Room D18 (2nd floor) at ENS 24 rue Lhomond, 13:30 to 14:00
Meteorites are our only witnesses of processes in the early solar system, some 4.57 billions years ago, and among them, chondrites are the most primitive. They provide a wealth of details otherwise unobtainable from astronomical observation of current protoplanetary disks and thus must be considered when matching disk theory with observations. Chondrites contain the oldest solids of the solar system, the refractory inclusions, and ubiquitous igneous spherules called chondrules. Their formation mechanism, certainly an important energetic process in the accretion disk, remains a mystery even after two centuries of study. In this talk, I will present what chondrites are, describe their most noteworthy components, and sketch some aspects of their diversity. In any case, no attempt at systematic completeness will be made. No background in geology and meteoritics is required. And last but not least, some hand samples of genuine meteorites will be shown!
Dernière mise à jour 16-03-2010 10:59 / Henrik Latter
Journal Club on 06/05/2010
From the initial core mass function to the formation of a hydrostatic protostellar core: the role of magnetic fields, dust, and radiative transfer
By Matt Kunz (Oxford)
Room D18 (2nd floor) at ENS 24 rue Lhomond, 13:30 to 14:00
Magnetic fields are an importunate necessity in the formulation of a theory of star formation. Not only do they play a crucial role in the fragmentation of molecular clouds and the subsequent contraction of prestellar cores, but they also significantly affect (and are affected by) the dynamics of dust grains and hence the emitted radiation. In this talk, I will present results from two recent advances in our understanding of the fragmentation of magnetically-supported molecular clouds and the dynamical contraction of magnetically-supercritical cores that form within them. First, Kunz & Mouschovias (2009b) have developed a relatively simple analytical prescription for the initial core mass function due to ambipolar-diffusion--initiated fragmentation. The resulting CMF is very similar to the IMF and is in excellent agreement with existing observations. Second, I will present results of the first radiative, 6-fluid, nonideal MHD star formation simulations to accurately track the evolution of a protostellar fragment over eleven orders of magnitude in density, from the early ambipolar-diffusion--initiated fragmentation phase, through the magnetically-supercritical dynamical collapse phase and the magnetic decoupling stage, all the way to the nonisothermal phase, including the formation and evolution of the hydrostatic core (Kunz & Mouschovias 2010). The ability of ambipolar diffusion and/or Ohmic dissipation to resolve the magnetic flux problem of star formation will be addressed. Emphasis will be placed on quantitative comparisons with both current and ongoing observations.
Dernière mise à jour 04-05-2010 12:22 / Henrik Latter
Journal Club on 19/05/2010
(Ce texte n'est pas disponible en français, nous nous en excusons.)
Magnetic-field amplification by supernova-driven interstellar turbulence
By Oliver Gressel (Queen Mary)
Room D18 (2nd floor) at ENS 24 rue Lhomond, 13:30 to 14:00
Supernovae are the predominant drivers of interstellar turbulence. Yet their effect on magnetic field amplification in galaxies is still poorly understood. By modeling the complex interstellar plasma via direct MHD simulations, we find an exponential growth of the large-scale field on timescales of 100 Myr. This self-consistent numerical verification of a "fast dynamo" is highly beneficial in explaining strong fields in young galaxies.
Since resolved global simulations of entire galaxies are still prohibitive, we attempt a hybrid strategy: from our local boxes, we first infer closure parameters for mean-field models via the kinematic test field method and then run 1D dynamo models. The growth rates and topology of the mean fields in the DNS are well reproduced. This lends support for further 3D modeling based on the obtained scaling relations.
Dernière mise à jour 11-05-2010 18:16 / Henrik Latter
Journal Club on 08/06/2010
(Ce texte n'est pas disponible en français, nous nous en excusons.)
3D vortices in accretion discs: a path to planet formation?
By Héloïse Meheut (APC- Paris VII)
Room D18 (2nd floor) at ENS 24 rue Lhomond, 13:30 to 14:00
Rossby waves were initially studied in planetary atmosphere physics for their role in Earth cyclones, but more recently they have been used in astrophysics in the study of accretion discs. It was shown that these waves can destabilize the entire disc and form vortices. This could happen both in the vicinity of black holes or in protoplanetary disks and in this last case the existence of vortices had been proposed to solve the time scale problem for the growth of grains to planetesimals. After a presentation of the mechanism of this instability, I will focus on how the Rossby vortices can be an important element in a planet formation scenario by trapping the particles in their centres. Until now only 2D planar studies of the instability had been realized leaving this scenario hypothetical for a real 3D disc. I will present a full 3D numerical study and how it has modified our understanding of the grain trapping inside the vortex.
Dernière mise à jour 08-04-2010 15:39 / Henrik Latter
Journal Club on 21/06/2010
(Ce texte n'est pas disponible en français, nous nous en excusons.)
The Feedback Effects of Protostellar Outflows on High Mass and Low Mass Star Formation
By Richard Klein (Berkeley)
Room D18 (2nd floor) at ENS 24 rue Lhomond, 13:30 to 14:00
The formation of massive stars remains one of the most significant unsolved problems in astrophysics, with implications for the production of heavy elements in the universe and the structure and evolution of galaxies. High mass star formation poses a major theoretical challenge: How is it possible to sustain a sufficiently high mass accretion rate into a protostellar core despite the radiation pressure and the dynamical effects of protostellar outflows on the accreting envelope. I present a series of our recent high resolution radiation-hydrodynamic adaptive mesh refinement simulations including for the first time the feedback effects of protostellar outflows on the formation of massive stars by comparing the effects of protostellar outflow feedback with our earlier work that did not include outflows. I show that feedback from protostellar outflows creates highly evacuated optically thin cavities in the surrounding core, drives Kelvin Helmholtz instabilities in the core and allows the efficient escape of radiation through the development of an anisotropic radiation field. I show that with the additional mechanism of protostellar outflows, radiation pressure again cannot halt accretion thereby allowing massive stars to form. I present predictions for massive star formation including outflows with upcoming ALMA submillimeter observations. Finally, with high resolution radiation-hydrodynamic AMR simulations I discuss the effects of protostellar outflow feedback on low mass star formation in a turbulent molecular cloud. I compare the distribution of stellar masses, accretion rates, multiplicity and temperatures in simulations with and without protostellar outflow feedback.
Dernière mise à jour 08-06-2010 11:10 / Henrik Latter
Archives Journal Club
| 2014 | 3e trimestre | 1 | résultat |
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