2,282 research outputs found

    Critical Surface for Explosions of Rotational Core-Collapse Supernovae

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    The effect of rotation on the explosion of core-collapse supernovae is investigated systematically in three-dimensional simulations. In order to obtain the critical conditions for explosion as a function of mass accretion rate, neutrino luminosity, and specific angular momentum, rigidly rotating matter was injected from the outer boundary with an angular momentum, which is increased every 500 ms. It is found that there is a critical value of the specific angular momentum, above which the standing shock wave revives, for a given combination of mass accretion rate and neutrino luminosity, i.e. an explosion can occur by rotation even if the neutrino luminosity is lower than the critical value for a given mass accretion rate in non-rotational models. The coupling of rotation and hydrodynamical instabilities plays an important role to characterize the dynamics of shock revival for the range of specific angular momentum that are supposed to be realistic. Contrary to expectations from past studies, the most rapidly expanding direction of the shock wave is not aligned with the rotation axis. Being perpendicular to the rotation axis on average, it can be oriented in various directions. Its dispersion is small when the spiral mode of the standing accretion shock instability (SASI) governs the dynamics, while it is large when neutrino-driven convection is dominant. As a result of the comparison between 2D and 3D rotational models, it is found that m=!0 modes of neutrino-driven convection or SASI are important for shock revival around the critical surface.Comment: First revised version, submitted to ApJ, 14 pages, 13 figures, 2 table

    Co-Researching by African and Japanese: The Way We Started and the Way Forward

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    ETS-5, ETS-6, and COMETS projects in Japan

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    Three satellite communication projects now in progress in Japan are described. The first is a project to establish a telecommunication network for tele-education, TV conference, and tele-medicine in the Asia-Pacific region by using the Japan's Engineering Test Satellite-5 (ETS-5). The second is a project of the ETS-6 satellite, to be launched in 1993, for inter-satellite communication, mobile and fixed communication, and millimeter wave personal communication experiments. The third is a project of the Communications and Broadcasting Engineering Test Satellite (COMETS), to be launched in 1997, for advanced mobile satellite communication, inter-satellite link, and advanced broadcasting experiments at higher frequencies

    The importance of Nuclear Weapons Free Zones

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    Some say that it is nothing but an illusion to think of a world totally without nuclear weapons. This might be true. Nonetheless, although the total abolition of nuclear weapons seems, at present, to be quite difficult, promoting Nuclear Weapons Free Zones (NWFZs) from a regional standpoint might encourage further large-scale non-proliferation processes. NWFZs are important elements to be brought into the debate when dealing with the non-proliferation of nuclear weapons on a global scale. Also for regional security reasons, NWFZs are crucial. In particular, there is a need for an NWFZ in Northeast Asia

    The Future of Online Privacy: A Proposal for International Legislation

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    Simultaneous Measurement of Torsional Oscillator and NMR of Very Dilute 3He in Solid 4He

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    We have investigated the NMR properties of dilute 3He impurities in solid 4He contained in a torsional oscillator (TO) by the simultaneous measurement of the NMR and the torsional oscillator response of the so-called supersolid 4He. From measurements on samples with one hundred to a few hundred ppm of 3He, we have found three different states of 3He. The first is the homogeneously distributed isolated 3He atom in a solid matrix of 4He. The second is the 3He cluster in a homogeneous 4He matrix, which appears below the phase separation temperature of a solid mixture. The third is the 3He cluster in some nonuniform part of a 4He crystal. We find that 3He atoms contained in the third component remain in a nearby location even above the phase separation temperature. Based on the fact that even a ppm of 3He affects the supersolid response in a TO below and above the phase separation temperature, we propose that the nonuniform part of a crystal that holds the third type of 3He and thus has a higher local concentration of 3He plays an important role in the supersolid phenomenon in a TO.Comment: 5 pages, 5 figures, Accepted for publication in Physical Review

    Propagation research in Japan

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    L-band propagation measurements for land-mobile, maritime, and aeronautical satellite communications have been carried out by using the Japanese Engineering Test Satellite-Five (ETS-5) which was launched in Aug. 1987. This paper presents propagation characteristics for each of the mobile satellite communication channels
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