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101.
102.
S. G. Turyshev B-G. Andersson 《Monthly notices of the Royal Astronomical Society》2003,341(2):577-582
We have studied the science rationale, goals and requirements for a mission aimed at using the gravitational lensing from the Sun as a way of achieving high angular resolution and high signal amplification. We find that such a mission concept is compromised by several practical problems. Most severe are the effects due to the plasma in the solar atmosphere which cause refraction and scattering of the propagating rays. These effects either limit the frequencies that can be observed to those above ∼1 THz, or they move the optical point outwards beyond the vacuum value of ≥550 au. (Thus for observing frequency of 300 GHz the optical point is moved outwards to ∼ 680 au.) Density fluctuations in the inner solar atmosphere will further cause random pathlength differences for different rays. The corrections for the radiation from the Sun itself will also be a major challenge at any wavelength used, but could be mitigated with coronographic techniques. Given reasonable constraints on the spacecraft (particularly in terms of size and propulsion), source selection as well as severe navigational constraints further add to the difficulties for a potential mission. Nevertheless, unbiased surveys of small-scale structure on the sky at short wavelengths might be the most promising application of such a mission. 相似文献
103.
The great continental ice sheets of the Pleistocene represented a significant topographic obstacle to the westerly winds
in northern midlatitudes. This work explores how consequent changes in the atmospheric stationary wave pattern might have
affected the shape and growth of the ice sheets themselves. A one dimensional (1-D) model is developed which permits an examination
of the types and magnitudes of the feedbacks that might be expected. When plausible temperature perturbations are introduced
at the ice-sheet margin which are proportional to the stationary wave amplitude, the equilibrium shape of the ice sheet is
significantly altered, and depends on the sign of the perturbation. The proposed feedback also affects the response of the
ice sheet to time-varying climate forcing. The results suggest that the evolution of a continental-scale ice sheet with a
land-based margin may be significantly determined by the changes it induces in the atmospheric circulation.
Received: 1 October 1999 / Accepted: 17 July 2000 相似文献
104.
An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (XCO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al2O3–SiO2–H2O–CO2, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower XCO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids. 相似文献
105.
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
106.
We present the results of a year-long monitoring campaign on J1819+3845.We interpret the results of this WSRT campaign to
infer critical source parameters such as source lifetime and structure on tens of microarcseconds. The long lifetime of the
source at such high brightness temperatures requires continuous energy injection or exotic emission processes. We have previously
interpreted the extreme scintillation ofJ1819+3845 as due to a relatively nearby (∼ 20 pc) scattering screen. We show this
screen has a velocity w.r.t. the LSR of ∼ 25 kms-1, as measured by the changing scintillation properties throughout the year: the `velocity parallax'.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
107.
Summary
Silica-undersaturated phlogopite schists from the Cackleberry Metamorphics, Arunta Inlier, central Australia, preserve relatively
low-temperature sapphirine-bearing parageneses that developed during low-pressure upper amphibolite facies metamorphism. Peak
metamorphic phlogopite–cordierite–sapphirine assemblages are interpreted to have formed during the same event recorded in
nearby metapelites, at c.3 kbar and 650–700 °C. Initial cooling of the terrain resulted in the breakdown of sapphirine to corundum–chlorite–phlogopite
and corundum–spinel–chlorite assemblages. Further retrogression at greenschist facies conditions resulted in the replacement
of sapphirine by diaspore–chlorite intergrowths. The reaction textures are consistent with a near-isobaric heating-cooling
path at low-pressure, and provide evidence for the stability of sapphirine at c.700 °C at low pressures in rocks of an appropriate Mg- and Fe3+-rich bulk composition.
Received August 15, 2001 accepted December 27, 2001 相似文献
108.
P. B. Tissera R. Domínguez-Tenreiro C. Scannapieco A. Sáiz 《Monthly notices of the Royal Astronomical Society》2002,333(2):327-338
We use cosmological smooth particle hydrodynamical (SPH) simulations to study the effects of mergers in the star formation history of galactic objects in hierarchical clustering scenarios. We find that during some merger events, gaseous discs can experience two starbursts: the first one during the orbital decay phase, owing to gas inflows driven as the satellite approaches, and the second one when the two baryonic clumps collide. A trend for these first induced starbursts to be more efficient at transforming the gas into stars is also found. We detect that systems that do not experience early gas inflows have well-formed stellar bulges and more concentrated potential wells, which seem to be responsible for preventing further gas inward transport triggered by tidal forces. The potential wells concentrate owing to the accumulation of baryons in the central regions and of dark matter as the result of the pulling in by baryons. The coupled evolution of the dark matter and baryons would lead to an evolutionary sequence during which systems with shallower total potential wells suffer early gas inflows during the orbital decay phase that help to feed their central mass concentration, pulling in dark matter and contributing to build up more stable systems. Within this scenario, starbursts triggered by early gas inflows are more likely to occur at early stages of evolution of the systems and to be an important contributor to the formation of stellar bulges. Our results constitute the first proof that bulges can form as the product of collapse, collisions and secular evolution in a cosmological framework, and they are consistent with a rejuvenation of the stellar population in bulges at intermediate z with, at least, 50 per cent of the stars (in SCDM) being formed at high z . 相似文献
109.
110.
A. Greco G. Zimbardo P. Veltri A.L. Taktakishvili L.M. Zelenyi 《Astrophysics and Space Science》2001,277(1-2):35-38
We study the ion dynamics in a magnetic field reversal with a constant electric field and with a model of three dimensional
magnetic turbulence. By computing the mean square displacements in the plane of the current sheet we find superdiffusive and
superballistic transport regimes. Since velocity increases with the length of the free path, we have accelerated Lévyflights.
The possibility to generate power law velocity distribution functions is pointed out, as well as the long memory effects and
non local properties of ion transport.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献