排序方式： 共有53条查询结果，搜索用时 187 毫秒
21.
Icenius I. de Zwart Susie J. Meade Andrew J. Pratt 《Geochimica et cosmochimica acta》2004,68(20):4093-4098
Iron(II) minerals have been found to catalyse the formation of pyrophosphate from activated phosphate compounds and inorganic phosphate at near neutral pH in water. Iron(II) phosphate catalyses the formation of pyrophosphate, from either acetyl phosphate or phosphoenolpyruvate as the activated phosphoryl donor, in yields of up to 25% and 14% respectively. Under similar conditions, these minerals also retard the hydrolysis of pyrophosphate. The catalysis of pyrophosphate formation is tolerant of sulfide ions: pyrophosphate being produced from acetyl phosphate in 12% yield in the presence of equimolar amounts of sulfide. These reactions mimic the extant biosynthesis of ATP from acetyl phosphate or phosphoenolpyruvate; they counter the concern that hydrolysis of polyphosphates would out-compete their accumulation under prebiotic conditions; and hence suggest a possible prebiotic route to polyphosphates under conditions that may have occurred on the early earth. 相似文献
22.
M. G. Mennen H. J. M. A. Zwart J. E. M. Hogenkamp J. W. Erisman 《Boundary-Layer Meteorology》1996,81(3-4):353-371
Flow distortion errors on wind and friction velocity induced by a box simulating the housing of a gas analyzer used in dry
deposition eddy correlation measurements were determined in a field experiment. ‘Undisturbed’ and ‘disturbed’ wind and friction
velocities, measured with two dry deposition monitoring systems run simultaneously, were compared, one to the other. In the
‘disturbed’ case the box was mounted below the 3-component probe of the sonic anemometer of one of these systems, while in
the ‘undisturbed’ case the box was removed. When the probe was located on the upstream side of the box, the results showed
satisfactory agreement with theoretical estimates using Wyngaard’s potential-flow approach and a spherical model for the box.
This model can be applied to obtain first-order corrections for flow distortion errors induced by cubic-like (or spherical)
obstacles such as a gas analyzer housing used in dry deposition research systems, or to determine the optimal location of
this housing relative to the sonic probe in such systems. When the probe was located halfway downstream and halfway to the
side of the box, the experimental flow distortion errors did not exceed those for the upstream case. This implies that to
keep flow distortion errors in dry deposition systems as small as possible the sonic probe can be placed upstream but also
to the side of the gas analyzer housing. The results of our experiments also confirmed that correcting for flow distortion
with the commonly used tilt equations yields underestimated values. 相似文献
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24.
R. Schneider V. Ferrari S. Matarrese S. F. Portegies Zwart † 《Monthly notices of the Royal Astronomical Society》2001,324(4):797-810
We consider gravitational waves emitted by various populations of compact binaries at cosmological distances. We use population synthesis models to characterize the properties of double neutron stars, double black holes and double white dwarf binaries, and white dwarf–neutron star, white dwarf–black hole and black hole–neutron star systems.
We use the observationally determined cosmic star formation history to reconstruct the redshift distribution of these sources and their merging rate evolution.
The gravitational signals emitted by each source during its early spiralling in phase add randomly to produce a stochastic background in the low-frequency band with spectral strain amplitude between ~10−18 and ~5×10−17 Hz−1/2 at frequencies in the interval ~5×10−6 –5×10−5 Hz.
The overall signal, which at frequencies above 10−4 Hz is largely dominated by double white dwarf systems, might be detectable with LISA in the frequency range 1–10 mHz and acts like a confusion-limited noise component, which might limit the LISA sensitivity at frequencies above 1 mHz. 相似文献
We use the observationally determined cosmic star formation history to reconstruct the redshift distribution of these sources and their merging rate evolution.
The gravitational signals emitted by each source during its early spiralling in phase add randomly to produce a stochastic background in the low-frequency band with spectral strain amplitude between ~10
The overall signal, which at frequencies above 10
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26.
Simon F. Portegies Zwart † Lev R. Yungelson † 《Monthly notices of the Royal Astronomical Society》1999,309(1):26-30
We discuss the formation of pulsars with massive companions in eccentric orbits. We demonstrate that the probability for a non-recycled radio pulsar to have a white dwarf as a companion is comparable to that of having an old neutron star as a companion. Special emphasis is given to PSR B1820−11 and PSR B2303+46. Based on population synthesis calculations we argue that PSR B1820−11 and PSR B2303+46 could very well be accompanied by white dwarfs with mass ≳1.1 M⊙ . For PSR B1820−11, however, we cannot exclude the possibility that its companion is a main-sequence star with a mass between ∼0.7 M⊙ and ∼5 M⊙ . 相似文献
27.
Simon F. Portegies Zwart † Tomonori Totani 《Monthly notices of the Royal Astronomical Society》2001,328(3):951-957
We present an internal shock model with external characteristics for explaining the complicated light curves of gamma-ray bursts. Shocks produce gamma-rays in the interaction between a precessing beam of relativistic particles and the interstellar medium. Each time the particle beam passes the same line of sight with the observer the interstellar medium is pushed outward. Subsequent interactions between the medium and the beam are delayed by the extra distance to be travelled for the particles before the shock can form. This results in a natural retardation and leads to an intrinsic asymmetry in the light curves produced for gamma-ray bursts. In addition, we account for the cooling of the electron–proton plasma in the shocked region, which gives rise to an exponential decay in the gamma-ray flux. The combination of these effects and the precessing jet of ultrarelativistic particles produces light curves that can be directly compared with observed gamma-ray burst light curves. We illustrate the model by fitting a number of observed gamma-ray bursts that are difficult to explain with only a precessing jet. We develop a genetic algorithm to fit several observed gamma-ray bursts with remarkable accuracy. We find that for different bursts the observed fluence, assuming isotropic emission, easily varies over four orders of magnitude from the energy generated intrinsically. 相似文献
28.
A. M. Dziewonski G. Ekstrm J. H. Woodhouse G. Zwart 《Physics of the Earth and Planetary Interiors》1990,60(1-4):243-253
Centroid-moment tensor solutions are presented for 226 earthquakes that occurred during the second quarter of 1989. The solutions are obtained using corrections for aspherical Earth structure. 相似文献
29.
Niels C. Munksgaard Costijn Zwart Jordahna Haig Lucas A. Cernusak Michael I. Bird 《水文研究》2020,34(1):111-124
High-frequency stable isotope data are useful for validating atmospheric moisture circulation models and provide improved understanding of the mechanisms controlling isotopic compositions in tropical rainfall. Here, we present a near-continuous 6-month record of O- and H-isotope compositions in both water vapour and daily rainfall from Northeast Australia measured by laser spectroscopy. The data set spans both wet and dry seasons to help address a significant data and knowledge gap in the southern hemisphere tropics. We interpret the isotopic records for water vapour and rainfall in the context of contemporaneous meteorological observations. Surface air moisture provided near-continuous tracking of the links between isotopic variations and meteorological events on local to regional spatial scales. Power spectrum analysis of the isotopic variation showed a range of significant periodicities, from hourly to monthly scales, and cross-wavelet analysis identified significant regions of common power for hourly averaged water vapour isotopic composition and relative humidity, wind direction, and solar radiation. Relative humidity had the greatest subdiurnal influence on isotopic composition. On longer timescales (weeks to months), isotope variability was strongly correlated with both wind direction and relative humidity. The high-frequency records showed diurnal isotopic variations in O- and H-isotope compositions due to local dew formation and, for deuterium excess, as a result of evapotranspiration. Several significant negative isotope anomalies on a daily scale were associated with the activity of regional mesoscale convective systems and the occurrence of two tropical cyclones. Calculated air parcel back trajectories identified the predominant moisture transport paths from the Southwest Pacific Ocean, whereas moisture transport from northerly directions occurred mainly during the wet season monsoonal airflow. Water vapour isotope compositions reflected the same meteorological events as recorded in rainfall isotopes but provided much more detailed and continuous information on atmospheric moisture cycling than the intermittent isotopic record provided by rainfall. Improved global coverage of stable isotope data for atmospheric water vapour is likely to improve simulations of future changes to climate drivers of the hydrological cycle. 相似文献
30.
Simon F. Portegies Zwart Jasinta Dewi Tom Maccarone 《Astrophysics and Space Science》2005,300(1-3):247-253
The evolution of young (≲ 10 Myr) star clusters with a density exceeding about 105 star pc−3 are strongly affected by physical stellar collisions during their early lifetime. In such environments the same star may
participate in several tens to hundreds of collisions ultimately leading to the collapse of the star to a black hole of intermediate
mass. At later time, the black hole may acquire a companion star by tidal capture or by dynamical – three-body – capture.
When the captured star evolves it starts to fill its Roche-lobe and transfers mass to its accompanying black hole. This then
leads to a bright phase of X-ray emission, which lasts for the remaining main-sequence lifetime of the donor. If the star
captured by the intermediate mass black hole is relatively low mass ≲ 2 M⊙) the binary will also be visible as a bright source in gravitational waves. Based on empirical models we argue that, for
as long as the donor remains on the main sequence, the source will be ultraluminous Lx >rsim 1040 ergs-1 for about a week every few month. When the donor star is more massive >15 M⊙, or evolved off the main sequence the bright time is longer, but the total accretion phase lasts much shorter. 相似文献