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61.
We have analysed short-exposure high-resolution images obtained on the William Herschel Telescope (WHT), Big Telescope Azimuthal (BTA) and CanadaFranceHawaii Telescope (CFHT) in order to investigate mechanical vibrations that are capable of reducing the high angular resolution allowed by speckle-interferometric methods. After filtering the photon noise we have computed power spectra of the image centres of gravity. In these spectra we have found vibration features that differ from one telescope to another and that vary in particular with the zenith angle. In the case of the WHT we discuss these results in terms of possible causes and in terms of degradation of the transfer function. We present some means to improve the data. 相似文献
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64.
J'erôme P'etri 《Astrophysics and Space Science》2006,302(1-4):117-139
This is the second of a series of papers aimed to look for an explanation on the generation of high frequency quasi-periodic
oscillations (QPOs) in accretion disks around neutron star, black hole, and white dwarf binaries. The model is inspired by
the general idea of a resonance mechanism in the accretion disk oscillations as was already pointed out by Abramowicz and
Klu’zniak (2001). In a first paper (P'etri, 2005a, paper I), we showed that a rotating misaligned magnetic field of a neutron
star gives rise to some resonances close to the inner edge of the accretion disk. In this second paper, we suggest that this
process does also exist for an asymmetry in the gravitational potential of the compact object. We prove that the same physics
applies, at least in the linear stage of the response to the disturbance in the system. This kind of asymmetry is well suited
for neutron stars or white dwarfs possessing an inhomogeneous interior allowing for a deviation from a perfectly spherically
symmetric gravitational field. After a discussion on the magnitude of this deformation applied to neutron stars, we show by
a linear analysis that the disk initially in a cylindrically symmetric stationary state is subject to {three kinds of resonances:
a corotation resonance, a Lindblad resonance due to a driven force and a parametric resonance}. In a second part, we focus
on the linear response of a thin accretion disk in the 2D limit. {Waves are launched at the aforementioned resonance positions
and propagate in some permitted regions inside the disk, according to the dispersion relation obtained by a WKB analysis}.
In a last part, these results are confirmed and extended via non linear hydrodynamical numerical simulations performed with
a pseudo-spectral code solving Euler's equations in a 2D cylindrical coordinate frame. {We found that for a weak potential
perturbation, the Lindblad resonance is the only effective mechanism producing a significant density fluctuation}. In a last
step, we replaced the Newtonian potential by the so called logarithmically modified pseudo-Newtonian potential in order to
take into account some general-relativistic effects like the innermost stable circular orbit (ISCO). The latter potential
is better suited to describe the close vicinity of a neutron star or a black hole. However, from a qualitative point of view,
the resonance conditions remain the same. The highest kHz QPOs are then interpreted as the orbital frequency of the disk at
locations where the response to the resonances are maximal. It is also found that strong gravity is not required to excite
the resonances. 相似文献
65.
Measurements of salinity perturbations in a partially mixed estuary have been used to evaluate the usefulness of an inductive salinometer and to determine some of the characteristics of the salinity perturbations. The salinometer performed satisfactorily under most conditions. Although internal wave like effects were present, the turbulence fluctuations were dominant. The salinity fluctuations and the turbulent fluxes and were found to behave in a manner similar to the density fluctuations in a thermally stratified atmospheric boundary layer and a laboratory open channel flow. A quadrant analysis suggested that the contribution of each quadrant to the turbulent flux changed with Ri. The turbulence parameters ν and cγ were found to decrease and increase respectively as Ri increases. 相似文献
66.
1 INTRODUCTIONThe South China Sea (SCS) is a semi-enclosedmarginal sea in western North Pacific Ocean withvery complex topography and is the important pas-sage connecting the Pacific and Indian Oceans. Ithas great impact to the global climate and a greatinterest of many oceanography researchers. Twodominant surface hydrographic and circulation fea-tures in the northern SCS are a strong fresh waterexpansion and a warm and high-salinity seawaterintrusion such as the SCS Diluted Water… 相似文献
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WEIHai-quan JINBo-lu LIUYong-shun 《东北亚地学研究》2004,7(1):28-35
A number of different lahars have been recognized from a systematic survey of a mapping project. The high setting temporamre feature of the deposits indicates a relationship between the lahar and the Millennium eruption event of Tianchi Volcano. The lahars caused a dramatic disaster. Recognize of the huge avalanche scars and deposits around Tianchi Volcano imply another highly destructive hazard. Three types of different texture of the avalanche deposits have been recognized. There was often magma mixing processes during the Millennium eruption of Tianchi Volcano, indicating a mixing and co-eruption regime of the eruption. 相似文献
69.
The UV spectra of mixture solutions consisting of tyrosine,tryptophane,phenylalanine,cystine,histidineand 3,4-dihydroxyl phenylalanine have been measured.The numbers,identities and concentrations of theamino acids in the mixtures have been determined successfully using target factor analysis.The effectsof the wavelength range and the selected sampling interval on the results are discussed.Twenty-fivesynthetic mixture samples have been analysed successfully.The average recoveries are 98·9 for Tyr,96·5for Trp,105·6 for Phe,98·1 for Cys,98·9 for His and 106·4 for Di-phe.The results obtained are ingood agreement with those obtained by the Kalman filter method. 相似文献
70.