A three-dimensional large-scale cloud model: testing the role of radiative heating and ice phase processes     

J. L. LEE  K. N. LIOU  S. C. OU 《地球，A辑:动力气象学与海洋学》1992,44(3):197-216

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Star cluster ecology – V. Dissection of an open star cluster: spectroscopy     

Simon F. Portegies Zwart †  Piet Hut  Stephen L. W. McMillan  Junichiro Makino 《Monthly notices of the Royal Astronomical Society》2004,351(2):473-486

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MAXIPOL: a balloon-borne experiment for measuring the polarization anisotropy of the cosmic microwave background radiation     

B. R. Johnson  M. E. Abroe  P. Ade  J. Bock  J. Borrill  J. S. Collins  P. Ferreira  S. Hanany  A. H. Jaffe  T. Jones  A. T. Lee  L. Levinson  T. Matsumura  B. Rabii  T. Renbarger  P. L. Richards  G. F. Smoot  R. Stompor  H. T. Tran  C. D. Winant 《New Astronomy Reviews》2003,47(11-12):1067

We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10^′ to 2°. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary dataset was collected during a 26 h turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress.  相似文献   

Microphysical modeling of ethane ice clouds in titan’s atmosphere     

Erika L. Barth  Owen B. Toon 《Icarus》2003,162(1):94-113

A time-dependent microphysical model is used to study the evolution of ethane ice clouds in Titan’s atmosphere. The model simulates nucleation, condensational growth, evaporation, coagulation, and transport of particles. For a critical saturation of 1.15 (a lower limit, determined by laboratory experiments), we find that ethane clouds can be sustained between altitudes of 8 and 50 km. Growth due to coalescence is inefficient, limiting the peak in the size distribution (by number) to 10 μm. These clouds vary with a period of about 20 days. This periodicity disappears for higher critical saturation values where clouds remain subvisible. Rainout of ethane due to methane cloud formation raises the altitude of the ethane cloud bottom to near the tropopause and may eliminate ethane clouds entirely if methane cloud formation occurs up to 30 km. However, clouds formed above the troposphere from other gases in Titan’s atmosphere could be sustained even with rainout up to 30 km. Although the optical depth of ethane clouds above 20 km is typically low, short-lived clouds with optical depths of order 0.1-1 can be created sporadically by dynamically driven atmospheric cooling. Ethane cloud particles larger than 25 μm can fall to the surface before total evaporation. However, ethane clouds remain only a small sink for tholin particles. At the peak of their cycle, the optical depth of ethane clouds could be comparable to that of tholin in the near-infrared, resulting in a 5% increase in Titan’s albedo for wavelengths between 1 and 2 μm. A number of factors limit our ablility to predict the ethane cloud properties. These factors include the mixing time in the troposphere, the critical saturation ratio for ethane ice, the existence of a surface reservoir of ethane, the magnitude and timing of dynamically driven temperature perturbations, and the abundance and life cycle of methane clouds.  相似文献   

XMM–Newton spectral properties of the narrow-line Seyfert 1 galaxy IRAS 13224 – 3809     

Th. Boller  Y. Tanaka  A. Fabian  W. N. Brandt  L. Gallo  N. Anabuki  Y. Haba  S. Vaughan 《Monthly notices of the Royal Astronomical Society》2003,343(4):L89-L93

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A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143   总被引：1，自引：0，他引：1  

K. A. Pounds  J. N. Reeves  A. R. King  K. L. Page  P. T. O'Brien  M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713

We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s⁻¹. The highest energy lines require a column density of   N _H∼ 5 × 10²³ cm⁻²  , at an ionization parameter of  log ξ∼ 3.4  . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143.  相似文献   

Baryon loading and the Weibel instability in gamma-ray bursts     

M. Fiore  L. O. Silva  C. Ren  M. A. Tzoufras  W. B. Mori 《Monthly notices of the Royal Astronomical Society》2006,372(4):1851-1855

The dynamics of two counter-streaming electron–positron–ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.  相似文献   

Comparison between polar regions of Mars from HEND/Odyssey data     

M.L. Litvak  I.G. Mitrofanov  A.B. Sanin  W.V. Boynton  D. Hamara  R.S. Saunders 《Icarus》2006,180(1):23-37

In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water-ice in subsurface layers and about peculiarities of the CO₂ seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water-ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10-20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO₂ deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3×¹⁰¹⁵ kg, which is larger than the total mass of the seasonal deposition at the north by 40-50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO₂ occurred 10-15 degrees of Ls later than is predicted by the GCM.  相似文献   

B0850+054: a new gravitational lens system from CLASS     

A. D. Biggs  D. Rusin  I. W. A. Browne  A. G. de Bruyn  N. J. Jackson  L. V. E. Koopmans  J. P. McKean  S. T. Myers  R. D. Blandford  K.-H. Chae  C. D. Fassnacht  M. A. Norbury  T. J. Pearson  P. M. Phillips  A. C. S. Readhead  P. N. Wilkinson 《Monthly notices of the Royal Astronomical Society》2003,338(4):1084-1088

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Beam Mode Expansion of Corrugated Conical Horns with Phase Correcting Lens: Application to Radioastronomy Receivers     

E. García  L. De Haro  C. O'Sullivan  G. Cahill  J. A. López Fernández  F. Tercero  B. Galocha  J. L. Besada 《Experimental Astronomy》2003,15(3):173-193

A classical radioastronomy receiver is fed with a corrugated horn and an independent lens, both placed in a cryostat to lower the noise temperature. The beam is focused and directed using a combination of elliptical and plane mirrors. This paper proposes modifying the initial feeding system by placing the lens onto the horn aperture, thereby allowing a size reduction of the horn and lens, and a simplification of their mechanical design. The profiled lens is shaped to correct the phase error on the horn aperture. A quasi-optical model of the horn-plus-lens system has been developed using a Beam Mode Expansion (BME). Results using both a hyperbolic-planar lens and a spherical-elliptical lens, as well as results obtained by using Geometrical Optics (GO) with a Kirchoff–Huygens integration to get the far-field pattern, have been compared with measurements. As a direct application, a full focusing system for the new 40-m radiotelescope at the “Centro Astronómico de Yebes” is presented for the 22, 30 and 45 GHz bands. This paper has developed a QO model for a corrugated conical horn with a phase-correcting lens. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献