The dynamical evolution and star formation history of the Small Magellanic Cloud: effects of interactions with the Galaxy and the Large Magellanic Cloud     

Akira M. Yoshizawa  Masafumi Noguchi 《Monthly notices of the Royal Astronomical Society》2003,339(4):1135-1154

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Real-time difference imaging analysis of MOA Galactic bulge observations during 2000     

I.A. Bond  F. Abe  R.J. Dodd  J.B. Hearnshaw  M. Honda  J. Jugaku  P.M. Kilmartin  A. Marles  K. Masuda  Y. Matsubara  Y. Muraki  T. Nakamura  G. Nankivell  S. Noda  C. Noguchi  K. Ohnishi  N.J. Rattenbury  M. Reid  To. Saito  H. Sato  M. Sekiguchi  J. Skuljan  D.J. Sullivan  T. Sumi  M. Takeuti  Y. Watase  S. Wilkinson  R. Yamada  T. Yanagisawa  P.C.M. Yock 《Monthly notices of the Royal Astronomical Society》2001,327(3):868-880

We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg² were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.  相似文献   

Markarian 313: An intermediate object between a seyfert and a starburst galaxy     

Hideo Maehara  Takeshi Noguchi 《Astrophysics and Space Science》1988,143(2):339-348

An ultraviolet-excess galaxy Markarian 313 (NGC 7465), which consists of a multiple system with NGC 7463 and NGC 7464, is studied using the low- and high-resolution optical spectrum. Emission lines of H, H, [NII], and [OIII] have conspicuous blueward asymmetrical wings or blue slanted profiles in the spectrum of the nuclear region of the galaxy. The width of these emission lines is as broad as 600 km s^–1 at the zero-intensity level, and the velocity difference between the narrow and broad components is estimated at around 80 km s^–1 from the two-component Gaussian profile fitting. This fact could be an evidence of a large-scale dynamical motion in or surrounding the nuclear region of the galaxy, implying that it bears an intermediate characteristic between a Seyfert and a starburst galaxy.  相似文献   

A new photographic method for mapping lunar eclipse shadow     

Nakamura  Tsuko  Hirayama  Tomohiro  Noguchi  Motokazu 《Earth, Moon, and Planets》1986,35(1):55-71

A new photographic method suitable for digital processing of a two-dimensional density distribution of lunar eclipse shadow has been developed and it has been applied to the observation of the eclipse of December 30, 1982. The principle of this method is to extract the terrestrial shadow by utilizing immediate post- or pre-eclipse full Moon image as a reference template. The uncertainties of the obtained shadow maps in density and position are mostly 0.03–0.05 and 10–15 respectively. Iso-density contours of the penumbra have revealed unknown systematic deviations from a geometrical (concentric) shadow model in terms of their directions of center and radii of curvature. A density map of the umbra has shown very irregular shapes of isophotoes and density variation in the outer umbra has been proved to be consistent with the optical depth measurement of the stratosphere performed in the same period of time with lidars and aircrafts. Geographic consideration of the shadow terminator on the Earth suggests that the remnant aerosols above the northern Pacific and northern Europe flown up by a Mexican volcano 8 months ago prior to this eclipse will be mainly responsible for the formation of such variation of density in the outer penumbra.On leave from Tokyo Astronomical Observatory as Visiting Scientist (from 28 Aug. 1984 to 27 Aug. 1985).  相似文献   

The role of tidal vortices in material transport around straits     

Yoshio Takasugi  Akira Hoshika  Hideaki Noguchi  Terumi Tanimoto 《Journal of Oceanography》1994,50(1):65-80

The strong tidal current (tidal jet) in straits generates tidal vortices with a scale of several kilometers. The role of the vortices in material transport was investigated in the Neko Seto Sea, located in the western part of the Seto Inland Sea of Japan. A clockwise vortex with a diameter of about 0.8 km was observed in Nigata Bay (lying between two straits, the Neko Seto Strait and the Meneko Seto Strait). It was concluded that the clockwise vortex was the tidal vortex which was generated by the tidal jet in the Meneko Seto Strait. The vortex moved into the bay with the tide, but tended to stay on the sand bank in the bay. It was confirmed by current measurement with an ADCP and turbidity measurement that the secondary convergent flow was generated in the bottom layer of the vortex. This secondary flow seemed to contribute to the formation of the sand bank. It was suggested that tidal vortices may play an important role in the sediment transport and formation of topography in and around straits.  相似文献   

Chemical gradient at diffusive interfaces in magma chambers     

Takehiro Koyaguchi 《Contributions to Mineralogy and Petrology》1989,103(2):143-152

The results of high pressure experiments on diffusion and Soret separation in natural silicate melts show that the diffusive behaviour between natural silicic and mafic magmas can be approximately modelled as if the system were a binary mixture of SiO₂ and other components such as MgO+FeO+CaO. Steady state compositional profiles across a diffusive interface between silicic and mafic magma layers are calculated on the basis of phenomenological relationships for the fluxes of chemical species and heat in the binary mixtures, using the experimental data of diffusion and Soret coefficients in natural silicate melts. The compositional profiles show a curvature with a minimum SiO₂ value within the interface due to the Soret effect and temperature dependence of diffusion coefficient. The compositional gradient at the lower half of the diffusive interface is similar to that resulting from the Soret separation of a mafic melt regardless of the composition of the silicic magmas. These results suggest that picritic magma can be formed in the interfacial region between the mafic and silicic magma layers. The compositional gradient explains chemical variation of mafic to picritic inclusions in a mixed andesite of the Abu Volcano Group, Japan.  相似文献   

Life-time of a stratified magma chamber recorded in ultramafic xenoliths from Ichinomegata volcano,northeastern Japan     

Takehiro Koyaguchi 《Bulletin of Volcanology》1986,48(6):313-323

Variations in major-element chemistry and modal compositions of the mantle xenolith-bearing calc-alkalic ejecta from Ichinomegata volcano are inferred to be due to mixing of three magmatic end members: Basalt I (SiO₂ 51 wt% , MgO 8.5 wt%), Basalt II (SiO₂ 54 wt%, MgO 5 wt%), and Dacite (SiO₂ 65 wt%, MgO₂ wt%). Ultramafic xenoliths are found in mafic mixtures of Dacite-Basalt I and Dacite-Basalt II. The thermal histories of the xenoliths in both mixtures are compared with each other. Chemical compositions of olivine and orthopyroxene in xenoliths suggest that xenoliths in Basalt I were equilibrated at about 800 °C, while those in Basalt II were also equilibrated originally at about 800 °C but were subsequently annealed at about 1000 °C for more than 10^2–3 years prior to the eruption.The chemical composition of Basalt I indicates that it can coexist with upper mantle peridotite and it is an appropriate candidate for a carrier of ultramafic xenoliths from the upper mantle. On the other hand, Basalt II is fractionated and it cannot be directly derived from the upper mantle. Two pulses of xenolith-bearing basalt injection into a dacite magma chamber are inferred to have occurred. The first injection did not lead to eruption and subsequently formed a dacite/basalt stratified magma chamber. In the lower layer, the basalt was slightly differentiated to become Basalt II and ultramafic xenoliths carried by the first pulse were annealed at the bottom of the layer. The duration of the annealing of the xenoliths implies a minimum life-time of the Dacite-Basalt II stratification in the magma chamber beneath Ichinomegata of 10^2–3 years. The second injection of the xenolith-bearing basalt (Basalt I) was immediately followed by eruption, and all the magmas were effused with mixing in a conduit. Consequently, the ultramafic xenoliths carried by the second pulse are not annealed.  相似文献   

Revision of magnitudes of large shallow earthquakes, 1897–1912     

Katsuyuki Abe  Shin&#x;ichi Noguchi 《Physics of the Earth and Planetary Interiors》1983,33(1):1-11

In previous research, trace amplitudes of surface wave maxima recorded by undamped Milne seismographs were used to determine the surface-wave magnitudes M_s of large shallow earthquakes which occurred prior to 1912. For this purpose, the effective gain of these instruments was calibrated by using the surface-wave magnitudes M_s(GR) which were calculated from the unpublished worksheets for Seismicity of the Earth of Gutenberg and Richter. In this paper, the real quality of M_s(GR) is critically re-evaluated by using independent sets of data. It is found that M_s(GR) for the period 1904–1909 is considerably overestimated. The average excess from the real M_s is 0.5 units for 1904–1906, 0.4 for 1907, 0.3 for 1908–1909 and 0.0 for 1910–1912. This overestimation is so systematic and large that the previous results are all redetermined. The average effective gain of Milne instruments is revised to be 21.9; previously, the gain depended on M_s. This revision results in systematic reduction in the previously assigned magnitudes. The revised values of M_s for 264 shallow earthquakes, with M_s=6.8 and over in the period 1897–1912 inclusive, are listed. The present revision is large enough to preclude the possibility of the high activity of large shallow earthquakes around the turn of the century. The present results have a direct effect on all the magnitude catalogues of shallow earthquakes which occurred prior to 1909.  相似文献   

An experimental study on the effects of phenocrysts on convection in magmas     

Takehiro Koyaguchi  Mark A. Hallworth  Herbert E. Huppert 《Journal of Volcanology and Geothermal Research》1993,55(1-2)

The effect of phenocrysts on convection in magma chambers is investigated experimentally using small heavy particles in convecting fluids. The particles are initially uniformly distributed in a fluid which is either heated from below or cooled from above. The system is allowed to evolve, and temperature and particle concentration profiles are measured as functions of time. When the concentration of particles is sufficiently small, convection is basically unaffected by their presence. When the concentration is above a critical value, however, the convective motion is considerably altered. The effect of particles on the subsequent fluid behaviour is different in the cases of heating from below and cooling from above. In the former case, there are strong convective motions confined to a sedimentary layer of decreasing thickness beneath a clear layer which displays rather weak convective motions. With time, the destabilizing increase of temperature in the lower layer overcomes the stabilizing contribution to the bulk density due to the particles and the layer overturns quite suddenly. In the situation of cooling from above, a critical condition separates a case of continual overturn from a case of no overturn at all, with the sedimentary layer falling unimpeded to the bottom. Theoretical analysis suggests that the critical value is determined primarily by the ratio of the contribution to the bulk density of the suspension due to particles to the change in fluid density due to the thermal effect. The size distribution of the particles can also modify the fluid behaviour. Applying our general results to geological situations, we suggest that the presence of relatively small concentrations of phenocrysts can critically influence the mode of convection in magmas.  相似文献   

Thermal alteration of hydrated minerals during hypervelocity capture to silica aerogel at the flyby speed of Stardust     

Takaaki Noguchi  Tomoki Nakamura  Kyoko Okudaira  Hajime Yano  Seiji Sugita  Mark J. Burchell 《Meteoritics & planetary science》2007,42(3):357-372

Abstract— Outside the Earth's atmosphere, silica aerogel is one of the best materials to capture finegrained extraterrestrial particles in impacts at hypervelocities. Because silica aerogel is a superior insulator, captured grains are inevitably influenced by frictional heat. Therefore, we performed laboratory simulations of hypervelocity capture by using light‐gas guns to impact into aerogels finegrained powders of serpentine, cronstedtite, and Murchison CM2 meteorite. The samples were shot at >6 km s^?1 similar to the flyby speed at comet P/Wild‐2 in the Stardust mission. We investigated mineralogical changes of each captured particle by using synchrotron radiation X‐ray diffraction (SR‐XRD), transmission electron microscope (TEM), and field emission scanning electron microscope (FE‐SEM). SR‐XRD of each grain showed that the majority of the bulk grains keep their original mineralogy. In particular, SR‐XRD and TEM investigations clearly exemplified the presence of tochilinite whose decomposition temperature is about 300 °C in the interior of the captured Murchison powder. However, TEM study of these grains also revealed that all the samples experienced melting and vesiculation on the surface. The cronstedtite and the Murchison meteorite powder show remarkable fracturing, disaggregation, melting, and vesiculation. Steep thermal gradients, about 2500 °C/μm were estimated near the surface of the grains (<2 μm thick) by TEM observation. Our data suggests that the interior of >4 μm across residual grains containing abundant materials that inhibit temperature rise would have not experienced >300 °C at the center.  相似文献