4,410 research outputs found

    The Dust Mantle of Comet 9P/Tempel 1: Dynamical Constraints on Physical Properties

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    The trajectories of dust particles ejected from a comet are affected by solar radiation pressure as a function of their ratios of radiation pressure cross section to mass. Therefore, a study on the orbital evolution of the particles caused by the radiation pressure reveals the physical properties of dust on the surface of the comet nucleus. In the course of NASA's Deep Impact mission, the ejecta plume evolved under the influence of the radiation pressure. From the evolution and shape of the plume, we have succeeded in obtaining β0.4\beta \approx 0.4, where β\beta is the ratio of the radiation pressure to the solar gravity. Taking into account β0.4\beta \approx 0.4 as well as the observational constraints of a high color temperature and a small silicate-feature strength, dust particles ejected from the surface of comet 9P/Tempel 1 are likely compact dust aggregates of sizes 20μ\approx 20\,\mum (mass 108\sim 10^{-8}\,g). This is comparable to the major dust on the surface of comet 1P/Halley (10μ\sim 10\mum) inferred from in-situ measurements and theoretical considerations. Since such dust aggregates with β0.4\beta \approx 0.4 must have survived on the surface against jets due to ice sublimation, the temperature of ice in the nucleus must be kept below 145\,K, which is much lower than equilibrium temperature determined by solar irradiation and thermal emission. These facts indicate that 9P/Tempel 1 has a dust mantle composed of 20μ20\,\mum-sized dust aggregates with low thermal conductivities 1ergcm1K1s1\sim 1 \, {\rm erg\, cm}^{-1} \, {\rm K}^{-1}\,{\rm s}^{-1}.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and Astrophysic

    Virological aspects of Epstein-Barr virus infections.

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    Epstein-Barr virus (EBV) is usually maintained in an asymptomatic and latent form by the host immune system, and primarily by EBV-specific cytotoxic T cells (CTLs). However, EBV has been linked to several refractory diseases such as EBV-associated hemophagocytic syndrome(EBV-AHS) and chronic active EBV infection (CAEBV). In these ectopic diseases, EBV infects T/NK cells, causing severe immunodeficiency with a very high EBV load. In recent years, the laboratory procedure to assess these types of EBV infections has been improved. In particular, real-time polymerase chain reaction (PCR) has been used to quantify the EBV load, and the MHC: peptide tetramer assay has been used to quantitate EBV-specific CTLs; these tests have been employed for the management of the illnesses associated with EBV infection. Here, we have reviewed the recent progress in the clinical application of these assays. The pathogenesis of EBV-infected T/NK cells, and the host immune response to infection, including the roles carried out by innate immunity and inflammatory cytokines, are likely to be revealed in the future.</p

    Sublimation Temperature of Circumstellar Dust Particles and Its Importance for Dust Ring Formation

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    Dust particles in orbit around a star drift toward the central star by the Poynting-Robertson effect and pile up by sublimation. We analytically derive the pile-up magnitude, adopting a simple model for optical cross sections. As a result, we find that the sublimation temperature of drifting dust particles plays the most important role in the pile-up rather than their optical property does. Dust particles with high sublimation temperature form a significant dust ring, which could be found in the vicinity of the sun through in-situ spacecraft measurements. While the existence of such a ring in a debris disk could not be identified in the spectral energy distribution (SED), the size of a dust-free zone shapes the SED. Since we analytically obtain the location and temperature of sublimation, these analytical formulae are useful to find such sublimation evidences.Comment: 9 pages, 5 figures, to be published in Earth Planets Spac

    On doubly transitive permutation groups of degree n and order 2p(n-1)n

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