Classification of planetary nebulae by their departure from axisymmetry     

NoamSoker  RonHadar† 《Monthly notices of the Royal Astronomical Society》2002,331(3):731-735

We propose a scheme to classify planetary nebulae (PNe) according to their departure from axisymmetric structure. We consider only departure along and near the equatorial plane, i.e. between the two sides perpendicular to the symmetry axis of the nebula. We consider six types of departure from axisymmetry: (1) PNe where the central star is not at the centre of the nebula; (2) PNe having one side brighter than the other; (3) PNe having unequal size or shape of the two sides; (4) PNe where the symmetry axis is bent, e.g. the two lobes in a bipolar PN are bent toward the same side; (5) PNe where the main departure from axisymmetry is in the outer regions, e.g. an outer arc; and (6) PNe that show no departure from axisymmetry, i.e. any departure, if it exists, is on scales smaller than the scale of blobs, filaments and other irregularities in the nebula. PNe that possess more than one type of departure are classified by the most prominent type. We discuss the connection between departure types and the physical mechanisms that may cause them, mainly resulting from the influence of a stellar binary companion. We find that ∼50 per cent of all PNe in the analysed sample possess large-scale departure from axisymmetry. This number is larger than that expected from the influence of binary companions, namely ∼25–30 per cent. We argue that this discrepancy comes from many PNe where the departure from axisymmetry, mainly unequal size, shape or intensity, results from the presence of long-lived and large (hot or cool) spots on the surface of their asymptotic giant branch progenitors. Such spots locally enhance the mass-loss rate, leading to a departure from axisymmetry, mainly near the equator, in the descendent PN.  相似文献   

Partial derivatives of surface wave phase velocities for flat anisotropic models     

V. Maupin 《Geophysical Journal International》1985,83(2):379-398

Summary. Surface wave behaviour in flat anisotropic structures is first illustrated by performing an exact computation on a simple two-layer model. The variational procedure of Smith & Dahlen is then used to compute the partial derivatives of surface wave phase velocities with respect to the elastic parameters in more realistic earth models. Linear relationships between the partial derivatives for a general anisotropic structure and those for a transversely isotropic structure are derived. When considering waves propagating in a fixed direction, there are only four independent derivatives for Rayleigh waves, and two for Love waves. To avoid the lack of resolution in an inverse method, we propose to use physically constrained models. These results are illustrated by using a model with hexagonal symmetry and a symmetry axis oriented either vertically or horizontally. Quasi-Love- and quasi-Rayleigh-wave partial derivatives are computed for both axis orientations. Modes up to the second overtone and periods ranging between 45 and 130 s have been considered. Finally, anomalies of phase velocity are computed in an oceanic model made of 1/6 oriented olivine crystals with horizontal or vertical preferred orientations of the a -axis.  相似文献   

The response of two half spaces to point dislocations at the material interface   总被引：1，自引：0，他引：1  

Yehuda Ben-Zion 《Geophysical Journal International》1990,101(3):507-528

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Microwave observations of Saturn's rings: anisotropy in directly transmitted and scattered saturnian thermal emission     

David E. Dunn  Lawrence A. Molnar  Imke de Pater 《Icarus》2004,171(1):183-198

We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae.  相似文献   

Shock Propagation Through Multiphase Media*     

R. J. R. Williams  D. L. Youngs 《Astrophysics and Space Science》2005,298(1-2):191-196

This paper presents two and three dimensional simulations of the interaction of shocks with media with large numbers of dense inclusions. An approximate model of the interaction of a starburst wind with the surrounding galactic ISM illustrates issues which must be addressed in global models of ISM dynamics. As a step towards developing the sub-grid model of multiphase turbulence, we define and study a form of ‘multiphase Riemann problem’. This allows us to develop macroscopic characteristics of the flows which may be compared to such subgrid models.  相似文献   

X-shaped radio galaxies as observational evidence for the interaction of supermassive binary black holes and accretion disc at parsec scale     

F. K. Liu 《Monthly notices of the Royal Astronomical Society》2004,347(4):1357-1369

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Apsidal corotation in mean motion resonance: the 55 Cancri system as an example   总被引：2，自引：0，他引：2  

Li-Yong Zhou  Harry J. Lehto  Yi-Sui Sun  Jia-Qing Zheng 《Monthly notices of the Royal Astronomical Society》2004,350(4):1495-1502

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The gravitational wave 'probability event horizon' for double neutron star mergers     

D. M. Coward  M. Lilley  E. J. Howell  R. R. Burman  D. G. Blair 《Monthly notices of the Royal Astronomical Society》2005,364(3):807-812

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Disc–Planet Interactions: Migration and Resonances in Extrasolar Planetary Systems     

J. C. B. Papaloizou 《Celestial Mechanics and Dynamical Astronomy》2003,87(1-2):53-83

We consider orbital resonances in multiplanet systems. These are expected to arise during or just after formation in a gaseous disc. Disc–planet interaction naturally produces orbital migration and circularization through the action of tidal torques which in turn may lead to an orbital resonance. The mass and angular momentum content of the disc is likely to be comparable to that in the planets so that it is essential to fully incorporate the disc in the analysis.We study the orbital evolution of two planets locked in 2:1 commensurability through migration tidally induced by the disc using both analytic methods and numerical hydrodynamic simulations. The planets are assumed to orbit in an inner cavity containing at most only a small amount of disc material. Results are found to be sensitive to initial surface density profile, planet masses and disc parameters. The evolution may range between attaining and subsequently maintaining a resonance lock with two angles librating to divergent migration with no commensurability formed. In the former case eccentricities increase monotonically with time while the system undergoes inward migration. If the migration is halted by loss of the disc leaving the planets in a final configuration, there is likely to be a low probability of seeing resonant planets at small radii as well as a sensitive dependence on past history.We have also considered a multiplanet system in secular apsidal resonance. We consider the system as being in just one secular normal mode and include the effects of a gaseous disc. It is suggested that a normal mode may be selected by adding in some weak dissipative process in the disc and that it may remain, involving only the planets, when the disc is slowly removed.  相似文献   

A unique Galactic planetary nebula with a [WN] central star     

D. H. Morgan  Q. A. Parker  Martin Cohen 《Monthly notices of the Royal Astronomical Society》2003,346(3):719-730

We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Hα Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s⁻¹, suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf–Rayet star with a ring nebula.  相似文献