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Sarazin Y.,French Atomic Energy Commission | Strugarek A.,French Atomic Energy Commission | Strugarek A.,CEA Saclay Nuclear Research Center | Dif-Pradalier G.,Center for Astrophysics and Space Science | And 6 more authors.
Journal of Physics: Conference Series | Year: 2010

Ion Temperature Gradient driven turbulence is investigated with the global full-f gyrokinetic code GYSELA for different magnetic equilibria. Reversed shear and monotonous q profile cases do not exhibit dramatic changes nor in the dynamics nor in the level of turbulence, leading to similar mean profiles. Especially, no transport barrier is observed in the vicinity of s = 0 in the general case, although the radial extent of the gap without resonant modes is larger than the typical turbulence correlation length. Conversely, a transport barrier is found to develop in the gap region if non resonant modes are artificially suppressed from the simulation. Such simulations tend to reconcile previously published contradictory results, while extending the analysis to more realistic flux-driven gyrokinetic regimes. © 2010 IOP Publishing Ltd.

Sanchez-Lavega A.,University of the Basque Country | Wesley A.,Acquerra Pty. Ltd. | Orton G.,Jet Propulsion Laboratory | Hueso R.,University of the Basque Country | And 12 more authors.
Astrophysical Journal Letters | Year: 2010

On 2009 July 19, we observed a single, large impact on Jupiter at a planetocentric latitude of 55°S. This and the Shoemaker-Levy 9 (SL9) impacts on Jupiter in 1994 are the only planetary-scale impacts ever observed. The 2009 impact had an entry trajectory in the opposite direction and with a lower incidence angle than that of SL9. Comparison of the initial aerosol cloud debris properties, spanning 4800km east-west and 2500km north-south, with those produced by the SL9 fragments and dynamical calculations of pre-impact orbit indicates that the impactor was most probably an icy body with a size of 0.5-1km. The collision rate of events of this magnitude may be five to ten times more frequent than previously thought. The search for unpredicted impacts, such as the current one, could be best performed in 890nm and K (2.03-2.36 μm) filters in strong gaseous absorption, where the high-altitude aerosols are more reflective than Jupiter's primary clouds. © 2010 The American Astronomical Society. All rights reserved.

Norman M.L.,Center for Astrophysics and Space Science | Norman M.L.,University of California at San Diego
Proceedings of the International Astronomical Union | Year: 2011

This contribution contains the introductory remarks that I presented at IAU Symposium 270 on Computational Star Formation held in Barcelona, Spain, May 31June 4, 2010. I discuss the historical development of numerical MHD methods in astrophysics from a personal perspective. The recent advent of robust, higher-order accurate MHD algorithms and adaptive mesh refinement numerical simulations promises to greatly improve our understanding of the role of magnetic fields in star formation. © 2011 International Astronomical Union.

Lu J.,Huazhong University of Science and Technology | Zou Y.-C.,Huazhong University of Science and Technology | Lei W.-H.,Huazhong University of Science and Technology | Lei W.-H.,University of Nevada, Las Vegas | And 6 more authors.
Astrophysical Journal | Year: 2012

The bulk Lorentz factor of the gamma-ray burst (GRB) ejecta (Γ0) is a key parameter to understanding GRB physics. Liang et al. have discovered a correlation between Γ0 and isotropic γ-ray energy: Γ0E 0.25 γ, iso, 52. By including more GRBs with updated data and more methods to derive Γ0, we confirm this correlation and obtain Γ0 ≃ 91E 0.29 γ, iso, 52. Evaluating the mean isotropic γ-ray luminosities L γ, iso of the GRBs in the same sample, we discover an even tighter correlation Γ0 ≃ 249L 0.30 γ, iso, 52. We propose an interpretation to this later correlation. Invoking a neutrino-cooled hyperaccretion disk around a stellar mass black hole as the central engine of GRBs, we derive jet luminosity powered by neutrino annihilation and baryon loading from a neutrino-driven wind. Applying beaming correction, we finally derive Γ0L 0.22 γ, iso, which is consistent with the data. This suggests that the central engine of long GRBs is likely a stellar mass black hole surrounded by a hyper-accreting disk. © 2012. The American Astronomical Society. All rights reserved.

Lu H.-J.,Guangxi University | Lu H.-J.,Center for Astrophysics and Space Science | Liang E.-W.,Guangxi University | Liang E.-W.,University of Nevada, Las Vegas | And 3 more authors.
Astrophysical Journal | Year: 2010

Recent Swift observations suggest that the traditional long versus short gamma-ray burst (GRB) classification scheme does not always associate GRBs to the two physically motivated model types, i.e., Type II (massive star origin) versus Type I (compact star origin). We propose a new phenomenological classification method of GRBs by introducing a new parameter ε = E γ,iso,52/Ep,z,25/3, where E γ,iso is the isotropic gamma-ray energy (in units of 10 52 erg) and Ep,z is the cosmic rest-frame spectral peak energy (in units of 100 keV). For those short GRBs with "extended emission," both quantities are defined for the short/hard spike only.With the current complete sample of GRBs with redshift and Ep measurements, the ε parameter shows a clear bimodal distribution with a separation at ε ∼ 0.03. The high-ε region encloses the typical long GRBs with high luminosity, some high-z "rest-frame-short" GRBs (such as GRB 090423 and GRB 080913), aswell as some high-z shortGRBs (such as GRB 090426).All these GRBs have been claimed to be of Type II origin based on other observational properties in the literature.All theGRBs that are argued to be of Type I origin are found to be clustered in the low-ε region. They can be separated from some nearby low-luminosity long GRBs (in 3σ) by an additional T90 criterion, i.e., T90,z ≲ 5 s in the Swift/BAT band.We suggest that this newclassification scheme can bettermatch the physically motivated Type II/I classification scheme. © 2010. The American Astronomical Society. All rights reserved.

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