Angular diameters, fluxes and extinction of compact planetary nebulae: further evidence for steeper extinction towards the bulge     

P. M. E. Ruffle  A. A. Zijlstra  J. R. Walsh  M. D. Gray  K. Gesicki  D. Minniti  F. Comeron 《Monthly notices of the Royal Astronomical Society》2004,353(3):796-812

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A three-dimensional model of moist convection for the giant planets II: Saturn's water and ammonia moist convective storms     

Ricardo Hueso  Agustín Sánchez-Lavega 《Icarus》2004,172(1):255-271

Moist convective storms constitute a key aspect in the global energy budget of the atmospheres of the giant planets. Among them, Saturn is known to develop the largest scale convective storms in the Solar System, the Great White Spots (GWS) which occur rarely and have been detected once every 30 years approximately. On the average, Saturn seems to show much less convective storms than Jupiter with smaller size and reduced frequency and intensity. Here we present detailed simulations of the onset and development of storms at the Equator and mid-latitudes of Saturn. These are the regions where most of the recent convective activity of the planet has been observed. We use a 3D anelastic model with parameterized microphysics (Hueso and Sánchez-Lavega, 2001, Icarus 151, 257) studying the onset and evolution of water and ammonia moist convective storms up to sizes of a few hundred km. Water storms, while more difficult to initiate than in Jupiter, can be very energetic, arriving to the 150 mbar level and developing vertical velocities on the order of 150 m s⁻¹. Ammonia storms develop easier but with a much smaller intensity unless very large abundances of ammonia (10 times solar) are present in Saturn's atmosphere. The Coriolis forces play a major role in the morphology and properties of water based storms.  相似文献   

Outbursts of young stellar objects     

O. M. Matthews  R. Speith  G. A. Wynn 《Monthly notices of the Royal Astronomical Society》2004,347(3):873-884

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Direct feeding of the black hole at the Galactic Centre with radial gas streams from close-in stellar winds     

Abraham Loeb 《Monthly notices of the Royal Astronomical Society》2004,350(2):725-728

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Gigamasers: the key to the dust-obscured star formation history of the Universe?     

R.H.D. Townsend  R.J. Ivison  Ian Smail  A.W. Blain  D.T. Frayer 《Monthly notices of the Royal Astronomical Society》2001,328(2):L19-L22

A numerical model of the Leonid stream is developed, based on an earlier model which has been applied to the Perseid stream. The results for this model are applied to the 2001 Leonid return. By examining the full three-dimensional dispersion of individual 'streamlets' released from the Leonid parent comet, 55P/Tempel–Tuttle, we have derived an estimate for the temporal change in spatial density of each trail. Using this result along with an estimate for the location of the centres for individual streamlets and fits to previous Leonid storm profiles, we estimate that the activity from the shower will be broad and relatively strong (zenithal hourly rates perhaps in excess of 1000). In particular, streamlets from the 1766 and 1799 ejections contribute to activity peaking near 10 and 12 ut on 2001 November 18, respectively. Additional older material from 1633, 1666 and 1699, as well as more recent ejections from 1866 and 1833, contributes to a much broader secondary maximum near 17.5 ut on November 18. Comparison with other published models of predicted Leonid activity in 2001 shows general agreement in terms of timing, but the models differ significantly in terms of the relative magnitude of the activity (which other models suggest will be larger). Significant anisotropy in the impact hazard exists for satellites in the geostationary belt, with those over western longitudes most likely to be affected. Integrated fluences for the 2001 Leonid return suggest a hazard of order one magnitude greater than occurred for the 1999 Leonid storm.  相似文献   

Radiative acceleration in Schwarzschild space–times     

A. J. Keane  †  R. K. Barrett  J. F. L. Simmons ‡ 《Monthly notices of the Royal Astronomical Society》2001,321(4):661-677

We examine the radial motion of a material particle in the intense radiation field of a static spherically symmetric compact object with spherical emitting surface outside the Schwarzschild radius. This paper generalizes previous work which dealt with radial motion in the Thomson limit, where the radiation force is simply proportional to the radiative flux. In the general case the average time component of the 4-momentum transferred to the particle is not negligible compared with its rest mass. Consequently, we find that the frequency dependence of the radiation force owing to Compton scattering for highly energetic photons gives rise to an increase in the effective mass of the test particle. In this work we outline the effects of this frequency dependence and compare these with the results in the Thomson limit. We present the frequency dependent saturation velocity curves for a range of stellar luminosities and radiation frequencies and present the resulting phase-space diagrams corresponding to the radial test particle trajectories. In particular, the stable equilibrium points which exist in the Thomson limit are found to be absent in the general case.  相似文献   

Streaming motions of galaxy clusters within 12 000 km s⁻¹– III. A standardized catalogue of Fundamental Plane data     

Michael J. Hudson  John R. Lucey  Russell J. Smith  David J. Schlegel  Roger L. Davies 《Monthly notices of the Royal Astronomical Society》2001,327(1):265-295

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On the X-ray time-lags in the black hole candidates     

O. Kotov  E. Churazov  M. Gilfanov 《Monthly notices of the Royal Astronomical Society》2001,327(3):799-807

It is shown that the energy dependence of the time-lags in Cygnus X-1 excludes any significant contribution of the standard reflected component to the observed lags. The conclusion is valid in the     frequency range where time-lags have been detected with sufficient significance. In fact, the data hint that the reflected component is working in the opposite direction, reducing the lags at energies where the contribution of the reflected component is significant.
We argue that the observed logarithmic dependence of time-lags on energy could be due to the small variations of the spectral index in the frame of a very simple phenomenological model. We assume that an optically thin flow/corona, emitting a power law like spectrum, is present at a range of distances from the compact object. The slope of the locally emitted spectrum is a function of distance, with the hardest spectrum emitted in the innermost region. If perturbations with different time-scales are introduced to the accretion flow at different radii, then X-ray lags naturally appear, caused by the inward propagation of perturbations on the diffusion time-scales.  相似文献   

Erratum: Sunyaev–Zel'dovich distortion from early galactic winds     

Subhabrata Majumdar  Biman B. Nath  Masashi Chibaby 《Monthly notices of the Royal Astronomical Society》2001,326(3):1216-1216

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A two-zone model for the broad iron line emission in MCG–6-30-15     

R. Misra 《Monthly notices of the Royal Astronomical Society》2001,320(4):445-450

We reanalyse the ASCA and BeppoSAX data of MCG–6-30-15, using a double-zone model for the iron line profile. In this model, the X-ray source is located around ≈10 Schwarzschild radii and the regions interior and exterior to the X-ray source produce the line emission. We find that this model fits the data with a similar reduced χ ² to the standard single-zone model. Thus we show that the presence of a broad iron line feature does not necessarily require that the X-ray source be located close to the last stable orbit or in the disc rotation axis.
Within the framework of this model, the best-fitting inclination angle of the source     for the intermediate-intensity ASCA data set is compatible with that determined by earlier modelling of optical lines. The observed variability of the line profile with intensity can be explained as variations of the X-ray source size. That several active galactic nuclei with broad lines have the peak centroid near 6.4 keV can be explained under certain conditions.
We also show that the simultaneous broad-band observations of this source by BeppoSAX rule out the Comptonization model which was an alternative to the standard inner-disc one. We thereby strengthen the case that line broadening occurs as a result of the strong gravitational influence of a black hole.  相似文献