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The time evolution of barred structures is examined under the influence of the external forces exerted by a spherical halo and by prolate halos. In particular, galaxy disks are placed in the plane including the major axis of prolate halos, whose configuration is often found in cosmological simulations. N-body disks in fixed external halo fields are simulated, so that bars are formed via dynamical instability. In the subsequent evolution, the bars in prolate halos dissolve gradually with time, while the bar pattern in a spherical halo remains almost unchanged to the end of the simulation. The decay times of the bars suggest that they can be destroyed in a time smaller than a Hubble time. Our results indicate that this dissolution process could occur in real barred galaxies, if they are surrounded by massive dark prolate halos, and the configuration adopted here is not unusual from the viewpoint of galaxy formation. For a prolate halo model, an additional simulation that is restricted to two-dimensional in-plane motions has also ended up with similar bar dissolution. This means that the vertical motions of disk stars do not play an essential role in the bar dissolution demonstrated here. 相似文献
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The collapse of rotating clouds is investigated using three-dimensional self-gravitating hydrodynamical simulations. We take
into account the detailed non-equilibrium chemical reactions for primordial gas that consists of pure hydrogen. The parameters
of the collapse and the condition of the fragmentation are compared with those of isothermal clouds. It is shown that the
geometrical flatness of the central region of the disc is a good indicator for predicting whether the clouds fragment or not.
If the flatness is greater than the critical value, ∼ 4π, a cloud fragments into filaments and blobs. On the other hand, if
the flatness is smaller than the critical value, fragmentation is not expected before the central core formation even if the
cooling is efficient and the total mass becomes much greater than the local Jeans mass at the centre. The critical mass is
found to be 3 × 106 M⊙ for a typical initial condition. If the initial cloud mass is smaller than this critical value, fragmentation before the
central core formation is not expected. For a typically estimated first collapsing cosmological baryonic object, M ≲ 106 M⊙, central core formation is expected and will have a significant effect on the later evolution of the whole system.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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Kazuyuki Omukai Shu-ichiro Inutsuka 《Monthly notices of the Royal Astronomical Society》2002,332(1):59-64
We investigate analytically the formation of an H ii region in the accreting envelope of a newborn star. Special care is taken to examine the role of ionizing radiation force. This effect modifies velocity and density distributions, and thereby affects the expansion of the H ii region. As a result, the upper limit of the stellar mass imposed by the growth of an H ii region around a forming star is increased by a larger factor than the previous estimate. In particular, for a star forming out of metal-free gas, this mechanism does not impose a firm upper limit on its mass. 相似文献
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Christiane Helling R. Giles Harrison Farideh Honary Declan A. Diver Karen Aplin Ian Dobbs-Dixon Ute Ebert Shu-ichiro Inutsuka Francisco J. Gordillo-Vazquez Stuart Littlefair 《Surveys in Geophysics》2016,37(4):705-756
Detailed observations of the solar system planets reveal a wide variety of local atmospheric conditions. Astronomical observations have revealed a variety of extrasolar planets none of which resembles any of the solar system planets in full. Instead, the most massive amongst the extrasolar planets, the gas giants, appear very similar to the class of (young) brown dwarfs which are amongst the oldest objects in the Universe. Despite this diversity, solar system planets, extrasolar planets and brown dwarfs have broadly similar global temperatures between 300 and 2500 K. In consequence, clouds of different chemical species form in their atmospheres. While the details of these clouds differ, the fundamental physical processes are the same. Further to this, all these objects were observed to produce radio and X-ray emissions. While both kinds of radiation are well studied on Earth and to a lesser extent on the solar system planets, the occurrence of emissions that potentially originate from accelerated electrons on brown dwarfs, extrasolar planets and protoplanetary disks is not well understood yet. This paper offers an interdisciplinary view on electrification processes and their feedback on their hosting environment in meteorology, volcanology, planetology and research on extrasolar planets and planet formation. 相似文献
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Masahiro N. Machida Kazuyuki Omukai Tomoaki Matsumoto Shu-ichiro Inutsuka 《Monthly notices of the Royal Astronomical Society》2009,399(3):1255-1263
The fragmentation process in collapsing clouds with various metallicities is studied using three-dimensional nested-grid hydrodynamics. Initial clouds are specified by three parameters: cloud metallicity, initial rotation energy and initial cloud shape. For different combinations of these parameters, we calculate 480 models in total and study cloud evolution, fragmentation conditions, orbital separation and binary frequency. For the cloud to fragment during collapse, the initial angular momentum must be higher than a threshold value, which decreases with decreasing metallicity. Although the exact fragmentation conditions depend also on the initial cloud shape, this dependence is only modest. Our results indicate a higher binary frequency in lower metallicity gas. In particular, with the same median rotation parameter as in the solar neighbourhood, a majority of stars are born as members of binary/multiple systems for <10−4 Z⊙ . With initial mass <0.1 M⊙ , if fragments are ejected in embryo from the host clouds by multibody interaction, they evolve to substellar-mass objects. This provides a formation channel for low-mass stars in zero- or low-metallicity environments. 相似文献
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Based on a comparison of similar colour indices we investigate the effects of luminosity and metallicity that influence the Strömgren and Vilnius photometric quantities for different spectral types. We notice significant differences for similar indices, apparently due to small deviations of central wavelengths and band-widths of the filters. For the G-K type stars of moderate luminosity we interpret the difference introduced by v and X pass-bands as an log g effect, reflecting the presence of H&;K Ca II stellar lines in these spectral types. We discuss a possible influence of the H&;K Ca II lines on the luminosities derived from the Vilnius classification diagrams. 相似文献
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