Swift observations of GW Lib: a unique insight into a rare outburst     

K. Byckling  J. P. Osborne  P. J. Wheatley  G. A. Wynn  A. Beardmore  V. Braito  K. Mukai  R. G. West 《Monthly notices of the Royal Astronomical Society》2009,399(3):1576-1586

The second known outburst of the WZ Sge type dwarf nova GW Lib was observed in 2007 April. We have obtained unique multiwavelength data of this outburst which lasted ∼26 days. The American Association of Variable Star Observers ( AAVSO ) recorded the outburst in the optical, which was also monitored by Wide Angle Search for Planets , with a peak V magnitude of ∼8. The outburst was followed in the ultraviolet and X-ray wavelengths by the Swift ultraviolet/optical and X-ray telescopes. The X-ray flux at optical maximum was found to be three orders of magnitude above the pre-outburst quiescent level, whereas X-rays are normally suppressed during dwarf nova outbursts. A distinct supersoft X-ray component was also detected at optical maximum, which probably arises from an optically thick boundary layer. Follow-up Swift observations taken 1 and 2 years after the outburst show that the post-outburst quiescent X-ray flux remains an order of magnitude higher than the pre-outburst flux. The long interoutburst time-scale of GW Lib with no observed normal outbursts support the idea that the inner disc in GW Lib is evacuated or the disc viscosity is very low.  相似文献   

Models for the Origin of the Quadrantids     

Tomoyasu Tanigawa  Shinsuke Abe  Masateru Ishiguro  Tadashi Mukai 《Earth, Moon, and Planets》2010,106(1):55-65

We present two scenarios for production of the Quadrantid stream based on two different models for its origin: the extinct model in which 2003EH1 was an active comet that released the dust particles during past 5000 years, stopping its activity abruptly in AD 1488; and the split model; in which a catastrophic disruption of an asteroid at AD 1488 released a large number of dust particles in a single event. We calculate the orbital evolution of test particles released in both cases and derive the resulting size distribution of surviving meteoroids in the current Quadrantid stream in the form of s ^−α ds, where s denotes the radius of a meteoroid. We find α = 3.1 in the extinct model and 2.0 in the split model. In addition, the radius of the surviving meteoroids is derived as s >10 μm in the both models. We propose, based on our estimation of the infrared color ratio for the Quadrantid stream derived from both models, that infrared observations of the Quadrantid stream may determine which origin model is more reasonable.  相似文献   

Infrared photometry of Venus: Variation during 10 years?     

Tetsuya Nagata  Shuji Sato  Tadashi Mukai  Sonoyo Mukai 《Icarus》1985,63(2):217-221

The infrared flux of Venus has been observed with a narrowband filter (λ = 3.6 μm, Δλ = 0.08 μm) from 1982 through 1984, covering a range of the phase angle α from 27 to 94°. Normalized values of flux at the Venus-Earth distance of 1 AU were (4.0–5.4) × 10^?17W/cm²/cm^?1 and the α dependence of the data is rather weak. Furthermore, when the evening terminator of Venus was seen, lower values of flux were obtained in contrast with higher values at the morning terminator. The α dependence is quite different from that of J.V. Martonchik and R. Beer (1975, J. Atmos. Sci.32, 1151–1156). Since we cannot find any significant problem in the two observational methods, the difference might suggest an intrinsic time variation of haze particles during these 10 years in the upper haze layer of the Venus cloud.  相似文献   

A Numerical Simulation of the Seasonal Cycle of Temperature, Salinity and Velocity Fields in Tokyo Bay     

Fumio Horiguchi  Joji Yamamoto  Kisaburo Nakata 《Marine pollution bulletin》2001,43(7-12):145-153

To understand when oxygen-depleted waters occur, how they develop and when they dissipate in inner Tokyo Bay, realistic simulations were attempted with fine spatial and temporal resolution by applying realistic time dependent external forcing. A 3D hydrodynamic model was driven by time-dependent external forcing factors/parameters such as solar radiation, air temperature, relative humidity, wind velocity, and fluvial discharge, under the open boundary conditions of 1995. A simulated time series of salinity and temperature agreed fairly well with observed data, except in summer. The model failed to reproduce the development of the surface mixed layer in summer. Several sensitivity analyses on the external forcing parameters such as wind velocity and vertical diffusivity were conducted to reproduce the mixed layer. However, changing these parameter values did not improve the model results.  相似文献   

Radiation Pressure and the Poynting-Robertson Effect for Fluffy Dust Particles     

Hiroshi KimuraHajime Okamoto  Tadashi Mukai 《Icarus》2002,157(2):349-361

A correct understanding of the dynamical effect of solar radiation exerted on fluffy dust particles can be achieved with assistance of a light scattering theory as well as the equation of motion. We reformulate the equation of motion so that the radiation pressure and the Poynting-Robertson effect on fluffy grains are given in both radial and nonradial directions from the center of the Sun. This allows numerical estimates of these radiation forces on fluffy dust aggregates in the framework of the discrete dipole approximation, in which the first term of the scattering coefficients in Mie theory determines the polarizability of homogeneous spheres forming the aggregates.The nonsphericity in shape turns out to play a key role in the dynamical evolution of dust particles, while its consequence depends on the rotation rate and axis of the grains. Unless a fluffy dust particle rapidly revolves on its randomly oriented axis, the nonradial radiation forces may prevent, apart from the orbital eccentricity and semimajor axis, the orbital inclination of the particle from being preserved in orbit around the Sun. However, a change in the inclination is most probably controlled by the Lorentz force as a consequence of the interaction between electric charges on the grains and the solar magnetic field. Although rapidly and randomly rotating grains spiral into the Sun under the Poynting-Robertson effect in spite of their shapes and structures, fluffy grains drift inward on time scales longer at submicrometer sizes and shorter at much larger sizes than spherical grains of the same sizes. Numerical calculations reveal that the dynamical lifetimes of fluffy particles are determined by the material composition of the grains rather than by their morphological structures and sizes. The Poynting-Robertson effect alone is nevertheless insufficient for giving a satisfactory estimate of lifetimes for fluffy dust grains since their large ratios of cross section to mass would reduce the lifetimes by enhancing the collisional probabilities. We also show that the radiation pressure on a dust particle varies with the orbital velocity of the particle but that this effect is negligibly small for dust grains in the Solar System.  相似文献   

Geological and microbial anomalies in the extinct submarine volcano, Shiribeshi Seamount, in the eastern margin of the Japan Sea     

MARI  OGAWA  AKIRA  TAKEUCHI  MUTSUO  HATTORI  MASAHARU  OKANO  MISUMI  AOKI  MAKIKO  IMAMURA  TOYOKI  KSUBOI  TOMONORI  TSUBOI  DAISUKE  TANIMOTO  TAKESHI  NAGANUMA 《Island Arc》2002,11(4):274-286

Abstract   The manned submersible Shinkai 2000 investigated yellow patches on the near-summit slope of Shiribeshi Seamount in the Japan Sea. Yellowish patches are often associated with seepage, and the possibility of seepage at Shiribeshi Seamount was tested by the following four lines of observation: (i) high subsurface temperature was measured at a ring-like patch, although no increase in subsurface temperature was observed at other patches; (ii) high gamma ray (γ-ray) intensity from the thorium series was recorded in the patch zone; (iii) the yellowish deposit was composed of calcite, quartz and amorphous iron compound, as seen at the yellowish patches in other seeps and volcanoes; (iv) lipid phosphate, a measure of microbial abundance, in sediments of the ring-like patch was determined, and the recorded microbial abundance was higher inside the patch than outside it. The four lines of observation are explained consistently by postulating that the seepage of warm fluid contained Fe and γ-ray sources. A hydrothermal origin of the yellow patches is not ruled out for the extinct but young (0.9 Ma) arc volcano.  相似文献   

Contamination by Persistent Organochlorines in Small Cetaceans from Hong Kong Coastal Waters     

Tu Binh Minh  Mafumi Watanabe  Haruhiko Nakata  Shinsuke Tanabe  Thomas A. Jefferson 《Marine pollution bulletin》1999,39(1-12):383-392

Blubber samples of 2 cetacean species, the Indo-Pacific hump-backed dolphin (Sousa chinensis) (n=11) and finless porpoise (Neophocaena phocaenoides) (n=9), collected from Hong Kong coastal waters were analysed for persistent organochlorines. Mean concentration in the blubber of DDTs was ranked first (46 μg/g wet wt.), followed by PCBs (24 μg/g wet wt.), HCHs (0.76 μg/g wet wt.), chlordane compounds (0.38 μg/g wet wt.) and HCB (0.07 μg/g wet wt.). The present cetacean species may potentially be faced with high risk due to the elevated level of DDTs and PCBs. Relatively high concentrations of PCBs and DDTs can be attributed to the continuous environmental input of these compounds in the Far East region including Hong Kong. Highly chlorinated PCBs such as IUPAC Nos. 138, 153, 180 were the prominent congeners, accounting for 39% of the total PCBs. Mean concentrations of 6 non- and mono-ortho coplanar congeners were 2.1 and 2.8 μg/g wet wt. in the hump-backed dolphin and finless porpoise, respectively. Among highly toxic coplanar PCBs, mono-ortho congener IUPAC No. 118 and non-ortho congener IUPAC No. 126 were estimated to have the greatest toxicity contribution. Tris(4-chlorophenyl) methane (TCPMe) and tris(4-chlorophenyl) methanol (TCPMeOH), which have been the most recently identified microcontaminants, were also detected with the highest concentration of 290 and 300 ng/g (wet wt. basis), respectively. The concentrations of TCPMe relative to TCPMeOH in cetaceans from Hong Kong coastal waters were significantly higher than those found in various seals collected from other parts of the world, suggesting the different metabolic system of these two compounds between seals and cetaceans. Correlations between the concentrations of tris-chlorophenyl compounds with other persistent organochlorines such as HCHs, CHLs, DDTs and PCBs were significant, suggesting their accumulation nature similar to these organochlorines.  相似文献   

Probing the evolution of early-type galaxies using multicolour number counts and redshift distributions     

Fumiaki Nakata  Kazuhiro Shimasaku  Mamoru Doi  Nobunari Kashikawa  Wataru Kawasaki  Yutaka Komiyama  Sadanori Okamura  Maki Sekiguchi  Masafumi Yagi  Naoki Yasuda 《Monthly notices of the Royal Astronomical Society》1999,309(4):L25-L29

We investigate pure luminosity evolution models for early-type (elliptical and S0) galaxies (i.e. no number density change or morphological transition), and examine whether these models are consistent with observed number counts in the B , I and K bands, and redshift distributions of two samples of faint galaxies selected in the I and K bands. The models are characterized by the star formation time-scale τ _SF and the time t _gw when the galactic wind starts to blow, in addition to several other conventional parameters. We find that the single-burst model ( τ _SF=0.1 Gyr and t _gw=0.353 Gyr), which is known to reproduce the photometric properties of early-type galaxies in clusters, is inconsistent with the redshift distributions of early-type galaxies in the field environment, owing to overpredictions of the number of galaxies at z ≳1.4 even with strong extinction which is at work until t _gw. In order for dust extinction to be more effective, we treat τ _SF and t _gw as free parameters, and find that models with τ _SF≳0.5 Gyr and t _gw>1.0 Gyr can be made consistent with both the observed redshift distributions and the number counts, if we introduce strong extinction [ E ( B − V )≥1 as a peak value]. These results suggest that early-type galaxies in the field environment do not have the same evolutionary history as described by the single-burst model.  相似文献   

Upwelling of oxygen-depleted water (Sumishio) in Omura Bay,Japan     

Tetsuya Takahashi  Hideaki Nakata  Keiji Hirano  Kazumi Matsuoka  Mitsunori Iwataki  Hitoshi Yamaguchi  Tomoyuki Kasuya 《Journal of Oceanography》2009,65(1):113-120

Wind-induced circulation and the distribution of hypoxia corresponding to the upwelling of oxygen-depleted water (called “Aoshio” in Japan and “Sumishio” locally in Omura Bay) in Omura Bay, Japan, was examined field observations and by three-dimensional modeling. During the calm weather in summer, well-mixed strait water, in rich oxygen at the mouth of the bay intruded into the middle layer of the bay, southward and northward along the west and east coast, respectively, forming basin-scale cyclonic circulation. A stagnant water mass was formed below the center of this cyclonic circulation, and it became hypoxic water. As a result of the prevailing strong southeast (northeast) wind, the bottom hypoxia moved in a southeasterly (northeasterly) direction. This induces the upwelling of hypoxic water, accompanied by mass mortality of marine organisms.  相似文献   

Temporal changes in benthic communities of seagrass beds impacted by a tsunami in the Andaman Sea,Thailand     

Napakhwan Whanpetch  Masahiro Nakaoka  Hiroshi Mukai  Takao Suzuki  Satoshi Nojima  Takashi Kawai  Chittima Aryuthaka 《Estuarine, Coastal and Shelf Science》2010

Seagrass beds are susceptible to various natural and human-induced disturbances. Disturbances affect not only seagrasses, but also the abundance and diversity of associated macrofaunal communities. The Andaman Sea coast of Thailand was heavily affected by the tsunami of December 26, 2004. To examine its impact on seagrass macrofaunal communities, we compared the abundance, diversity and taxa composition by quantitative samplings in 2001 (before the tsunami) and in 2005 and 2006 (after the tsunami). Macrobenthic animals and sediments were collected from vegetated and nonvegetated areas of two sites that had received different levels of tsunami disturbances. A large decline in abundance and diversity was observed in the nonvegetated areas after the tsunami, whereas an increase was observed in the vegetated areas. The magnitude of decline and subsequent recovery of abundance and diversity of macrobenthic animals in nonvegetated areas were similar between the two sites, suggesting that their temporal changes were not solely related to the magnitude of the tsunami disturbance. Similarity of the benthic animals differed greatly between 2001 and 2005–2006 at the nonvegetated areas, whereas it varied less among the 3 years at the seagrass-vegetated areas. This study demonstrated that the presence of seagrass vegetation alters the patterns of temporal variation in macrofaunal assemblages and subsequent recovery processes following a tsunami.  相似文献