Solar system constraints on multifield theories of modified dynamics     

R. H. Sanders 《Monthly notices of the Royal Astronomical Society》2006,370(3):1519-1528

Any viable theory of modified Newtonian dynamics (MOND) as modified gravity is likely to require fields in addition to the usual tensor field of General Relativity. For these theories, the MOND phenomenology emerges as an effective fifth force probably associated with a scalar field. Here, I consider the constraints imposed on such theories by Solar system phenomenology, primarily by the absence of significant deviations from inverse-square attraction in the inner Solar system as well as detectable local preferred frame effects. The current examples of multifield theories can be constructed to satisfy these constraints and such theories lead inevitably to an anomalous non-inverse-square force in the outer Solar system.  相似文献   

Quasi-spherical collapse with cosmological constant     

Ujjal Debnath  Soma Nath  Subenoy Chakraborty 《Monthly notices of the Royal Astronomical Society》2006,369(4):1961-1964

The junction conditions between static and non-static space–times are studied for analysing gravitational collapse in the presence of a cosmological constant. We have discussed about the apparent horizon and their physical significance. We also show the effect of cosmological constant in the collapse and it has been shown that cosmological constant slows down the collapse of matter.  相似文献   

Toy stars in one dimension     

J. J. Monaghan  D. J. Price 《Monthly notices of the Royal Astronomical Society》2004,350(4):1449-1456

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EGUP与黑洞熵的修正值     

白吉龙  温廷敦 《天文学报》2015,56(1):1-6

目前,人们对黑洞Bekenstein-Hawking熵的量子修正值产生了极大的兴趣,尤其是黑洞熵对数修正项的系数.在广义不确定关系(GUP)的基础上,通过引入了推广的广义不确定关系(EGUP),运用面积定理计算了3类时空的黑洞熵的修正值,得到的黑洞熵的修正项的系数是正的.这种计算方法不仅对单视界时空适用,而且对有内视界的黑洞时空依然成立,并且在EGUP基础上计算出黑洞熵的修正值.相比GUP基础上得到的黑洞熵,EGUP可以应用于大尺度时空下,所以应用范围更广.此计算方法简洁明了,物理意义明确,可为黑洞熵对数修正值系数的确定提供参考.  相似文献   

The fragmentation of condensations of the primordial gas     

D. R. Flower 《Monthly notices of the Royal Astronomical Society》2002,333(4):763-767

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Radiation pressure supported stars in Einstein gravity: eternally collapsing objects     

A. Mitra † 《Monthly notices of the Royal Astronomical Society》2006,369(1):492-496

Even when we consider Newtonian stars, that is, stars with surface gravitational redshift   z ≪ 1  , it is well known that, theoretically, it is possible to have stars supported against self-gravity almost entirely by radiation pressure. However, such Newtonian stars must necessarily be supermassive. We point out that this requirement for excessively large M in the Newtonian case is a consequence of the occurrence of low   z ≪ 1  . However, if we remove such restrictions, and allow for the possible occurrence of a highly general relativistic regime,   z ≫ 1  , we show that it is possible to have radiation pressure supported stars (RPSSs) at an arbitrary value of M . Since RPSSs necessarily radiate at the Eddington limit, in Einstein gravity, they are never in strict hydrodynamical equilibrium. Further, it is believed that sufficiently massive or dense objects undergo continued gravitational collapse to the black hole (BH) stage characterized by   z =∞  . Thus, late stages of BH formation, by definition, would have   z ≫ 1  , and hence would be examples of quasi-stable general relativistic RPSSs. It is shown that the observed duration of such Eddington limited radiation pressure dominated states is   t ≈ 5 × 10⁸ (1 + z ) yr  . Thus,   t →∞  as BH formation  ( z →∞)  takes place. Consequently, such radiation pressure dominated extreme general relativistic stars become eternally collapsing objects (ECOs) and the BH state is preceded by such an ECO phase. This result is also supported by our previous finding that trapped surfaces are not formed in gravitational collapse and the value of the integration constant in the vacuum Schwarzschild solution is zero. Hence the supposed observed BHs are actually ECOs.  相似文献   

Poincarè Chaos and the Dynamical Evolution of Systems of Gravitating Bodies     

A. D. Chernin  M. Valtonen  L. P. Ossipkov  Q.-J. Zheng  S. Wiren 《Astrophysics》2002,45(2):240-247

Some general laws of evolution of a system of a large number of gravitating bodies are discussed. If in the initial stage the dynamics of the system is determined by large-scale perturbations of the gravitational potential associated with excitations of a few collective degrees of freedom, then one can assume, by analogy with chaos in the several-body problem (Poincarè chaos), that randomization will occur in the system over several average crossing times. In the next stage of evolution, the energy of collective modes should be transferred by the cascade mechanism to ever smaller scales, down to invididual particles. Numerical experiments and gross-dynamical considerations that could verify this picture and bring out details are discussed.  相似文献   

Mass transfer in eccentric binary stars     

Enikö Regös  Vernon C. Bailey  Rosemary Mardling 《Monthly notices of the Royal Astronomical Society》2005,358(2):544-550

The concept of Roche lobe overflow is fundamental to the theory of interacting binaries. Based on potential theory, it is dependent on all the relevant material corotating in a single frame of reference. Therefore if the mass losing star is asynchronous with the orbital motion or the orbit is eccentric, the simple theory no longer applies and no exact analytical treatment has been found. We use an analytic approximation whose predictions are largely justified by smoothed particle hydrodynamic simulations (SPH). We present SPH simulations of binary systems with the same semi-major axis   a = 5.55 R_⊙  , masses   M ₁= 1 M_⊙, M ₂= 2 M_⊙  and radius   R ₁= 0.89 R_⊙  for the primary star but with different eccentricities   e = 0.4, 0.5, 0.6  and 0.7. In each case the secondary star is treated as a point mass. When   e = 0.4  no mass is lost from the primary while at   e = 0.7  catastrophic mass transfer, partly through the L₂ point, takes place near periastron. This would probably lead to common-envelope evolution if star 1 were a giant or to coalescence for a main-sequence star. In between, at   e ≥ 0.5  , some mass is lost through the L₁ point from the primary close to periastron. However, rather than being all accreted by the secondary, some of the stream appears to leave the system. Our results indicate that the radius of the Roche lobe is similar to circular binaries when calculated for the separation and angular velocity at periastron. Part of the mass loss occurs through the L₂ point.  相似文献   

2D Fokker–Planck models of rotating clusters     

J. Fiestas  R. Spurzem  E. Kim 《Monthly notices of the Royal Astronomical Society》2006,373(2):677-686

Globular clusters rotate significantly, and with the increasing amount of detailed morphological and kinematical data obtained in recent years on galactic globular clusters many interesting features show up. We show how our theoretical evolutionary models of rotating clusters can be used to obtain fits, which at least properly model the overall rotation and its implied kinematics in full 2D detail (dispersions, rotation velocities). Our simplified equal mass axisymmetric rotating model provides detailed two-dimensional kinematical and morphological data for star clusters. The degree of rotation is not dominant in energy, but also non-negligible for the phase-space distribution function, shape and kinematics of clusters. Therefore, the models are well applicable for galactic globular clusters. Since previously published papers on that matter by us made it difficult to do detailed comparisons with observations, we provide a much more comprehensive and easy-to-use set of data here, which uses as entries dynamical age and flattening of observed cluster and then offers a limited range of applicable models in full detail. The method, data structure and some exemplary comparison with observations are presented. Future work will improve modelling and data base to take a central black hole, a mass spectrum and stellar evolution into account.  相似文献   

Joint modelling of the probability distribution function and power spectrum of the Lyα forest: comparison with observations at z= 3     

Vincent Desjacques  Adi Nusser 《Monthly notices of the Royal Astronomical Society》2005,361(4):1257-1272

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