Model of ejection of matter from non-stationary dense stellar clusters and chaotic motion of gravitating shells

It is shown that during the motion of two initially gravitationally bound spherical shells, consisting of point particles moving along ballistic trajectories, one of the shells may be expelled to infinity at subrelativistic speed   v _exp≤0.25 c   . The problem is solved in Newtonian gravity. Motion of two intersecting shells in the case when they do not runaway shows a chaotic behaviour. We hope that this toy and oversimplified model can nevertheless give a qualitative idea on the nature of the mechanism of matter outbursts from dense stellar clusters .     

New self-similar solutions of polytropic gas dynamics

We explore semicomplete self-similar solutions for the polytropic gas dynamics involving self-gravity under spherical symmetry, examine behaviours of the sonic critical curve and present new asymptotic collapse solutions that describe 'quasi-static' asymptotic behaviours at small radii and large times. These new 'quasi-static' solutions with divergent mass density approaching the core can have self-similar oscillations. Earlier known solutions are summarized. Various semicomplete self-similar solutions involving such novel asymptotic solutions are constructed, either with or without a shock. In contexts of stellar core collapse and supernova explosion, a hydrodynamic model of a rebound shock initiated around the stellar degenerate core of a massive progenitor star is presented. With this dynamic model framework, we attempt to relate progenitor stars and the corresponding remnant compact stars: neutron stars, black holes and white dwarfs.     

The evolution of misaligned accretion discs and spinning black holes

In this paper, we consider the process of alignment of a spinning black hole and a surrounding misaligned accretion disc. We use a simplified set of equations, that describe the evolution of the system in the case where the propagation of warping disturbances in the accretion disc occurs diffusively, a situation likely to be common in the thin discs in active galactic nuclei (AGN). We also allow the direction of the hole spin to move under the action of the disc torques. In such a way, the evolution of the hole–disc system is computed self-consistently. We consider a number of different situations and we explore the relevant parameter range, by varying the location of the warp radius R _w and the propagation speed of the warp. We find that the dissipation associated with the twisting of the disc results in a large increase in the accretion rate through the disc, so that AGN accreting from a misaligned disc are likely to be significantly more luminous than those accreting from a flat disc. We compute explicitly the time-scales for the warping of the disc and for the alignment process and compare our results with earlier estimates based on simplified steady-state solutions. We also confirm earlier predictions that, under appropriate circumstances, accretion can proceed in a counter-aligned fashion, so that the accreted material will spin-down the hole, rather than spinning it up. Our results have implication in a number of different observational features of AGN such as the orientation and shape of jets, the shape of X-ray iron lines and the possibility of obscuration and absorption of X-ray by the outer disc as well as the general issue of the spin history of black holes during their growth.     

Detection of sporadic impact flashes on the Moon: Implications for the luminous efficiency of hypervelocity impacts and derived terrestrial impact rates

We present the first redundant detection of sporadic impact flashes on the Moon from a systematic survey performed between 2001 and 2004. Our wide-field lunar monitoring allows us to estimate the impact rate of large meteoroids on the Moon as a function of the luminous energy received on Earth. It also shows that some historical well-documented mysterious lunar events fit in a clear impact context. Using these data and traditional values of the luminous efficiency for this kind of event we obtain that the impact rate on Earth of large meteoroids (0.1-10 m) would be at least one order of magnitude larger than currently thought. This discrepancy indicates that the luminous efficiency of the hypervelocity impacts is higher than 10⁻², much larger than the common belief, or the latest impact fluxes are somewhat too low, or, most likely, a combination of both. Our nominal analysis implies that on Earth, collisions of bodies with masses larger than 1 kg can be as frequent as 80,000 per year and blasts larger than 15-kton could be as frequent as one per year, but this is highly dependent on the exact choice of the luminous efficiency value. As a direct application of our results, we expect that the impact flash of the SMART-1 spacecraft should be detectable from Earth with medium-sized telescopes.     

Neutron-loaded outflows in gamma-ray bursts

The force-free limit of magnetohydrodynamics (MHD) is often a reasonable approximation to model black hole and neutron star magnetospheres. We describe a general relativistic force-free (GRFFE) formulation that allows general relativistic magnetohydrodynamic (GRMHD) codes to directly evolve the GRFFE equations of motion. Established, accurate and well-tested conservative GRMHD codes can simply add a new inversion piece of code to their existing code, while continuing to use all the already-developed facilities present in their GRMHD code. We show how to enforce the   E · B = 0  constraint and energy conservation, and we introduce a simplified general model of the dissipation of the electric field to enforce the   B ²− E ² > 0  constraint. We also introduce a simplified yet general method to resolve current sheets, without much reconnection, over many dynamical times. This formulation is incorporated into an existing GRMHD code ( harm ), which is demonstrated to give accurate and robust GRFFE results for Minkowski and black hole space–times.     

The growth history of supermassive black holes and the origin of the radio-loud–radio-quiet dichotomy

Monte Carlo simulations of the growth of supermassive black holes in semi-analytic models of galaxy formation are used to reconstruct characteristic merging and accretion histories. This paper shows that the growth pattern depends on the environment. In field galaxies black holes acquire most of their mass in a single accretion event. Refuellings and mergers with other black holes become important in groups and clusters. I also investigate whether the assumption that radio jets are powered by rotating black holes can explain the observed radio-loud–radio-quiet dichotomy. Wilson & Colbert speculated that rapidly rotating black holes are the natural product of major mergers, while normal accretion through a disc may not give rapidly spinning black holes if the conversion of rotational energy into electromagnetic energy is very efficient. Here I derive predictions for this model including the fraction of radio-loud quasars as a function of redshift and luminosity and compare these predictions with those of alternative proposals.