Accretion process

New computer simulations of the accretion process of the protoearth indicate that only a few large bodies with a high water concentration collided with the Earth during the later bombardment. They apparently came from the same region of the asteroid belt as the carbonaceous chondrites. 

The discovery of the average metal-rich nature of planet-harbouring stars with regard to disc stars (i.e. [1],[2], [3]) has revealed the key role that metallicity plays in the formation and evolution of planetary systems. If the accretion processes were the main responsible for the iron excess found in planet host stars, volatile abundances should show clear differences in stars with and without planets, since volatiles (with low Tc) are expected to be deficient in accreted materials [4]. Previous studies of the abundance trends of the volatiles N, C, S and Zn [5, 6] have obtained no anomalies for a large sample of planet host stars. 

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

The first simulations of the collapsar scenario have been performed using 2D Newtonian, hydrodynamics (MacFadyen Woosley 1999) exploring the collapse of helium cores of more than 10 M . In their 2D simulation MacFadyen Woosley found the jet to be collimated by the stellar material into opening angles of a few degrees and to transverse the star within 10 s. The accretion process was estimated to occur for a few tens of seconds. In such a model variability in the lightcurve could result for example from (magneto-) hydrodynamic instabilities in the accretion disk that would translate into a modulation of the neutrino emission/annihilation processes or via Kelvin-Helmholtz instabilities at the interface between the jet and the stellar mantle. 

Osteoid Etidronate suppresses bone turnover and may retard mineralization of osteoid laid down during the bone accretion process. In patients with fractures, especially of long bones, it may be advisable to delay or interrupt treatment until callus is evident. 

Here, a more directed and expeditious approach is recommended. Managers who need to develop cannot rely on sudden Insight to occur, though it may. Nor can they wait for the slow accretive process to occur, though they should not impede it. 

This 4.53 Gyr represents the time that has elapsed since the last high-temperature event, which determined the time t = 0. For the oldest meteorites, the formation interval can also be estimated. Short half-life (1-100 Myr) radionuclides were present in the young parent body the daughters can be detected. This is the case of 129Xe (daughter of 129I with a half-life of 16 Myr). Since 129I was frozen in the parent body matrix, this means that the accretion process took a short time (a few million years). 

The Synthesis of Organic Molecules in the Protosolar Nebula and During the Accretion Process... 

Moreover, the isomeric distributions of aminoacids and hydroxyacids seem to suggest that radical reactions, must also be taken into account (see paragraph 5.4). Presently, it remains impossible to determine if this kind of processes played a role during the accretion process or later. 

Churazov et al. (2002) developed a model to explain the self-regulation of the accretion process around supermassive black holes. In this simple model, the supermassive black hole lies at the bottom of the gravitational potential surrounded by the lowest entropy gas. When cooling dominates, the gas entropy decreases and accretion increases. The higher accretion rate increases the energy ouput of the central black hole and the energy input to the gas increases, its entropy decreases, and the accretion declines. 

Safronov (1979) suggested that Mars- to Earth-mass planetary embryos left over from the accretion process could have caused the excitation and depletion of the Asteroid Belt. In this model, these embryos were scattered inwards by Jupiter onto orbits that traveled through the Asteroid Belt. Petit et al. (1999) modeled this scenario in detail and showed that it cannot fully explain the observed mass depletion and dynamical excitation of the Asteroid Belt, especially the orbital inclinations. 

The exception is chemical reactions that form mineral rocks. These reactions have been explored by geochemists, and some interesting processes have been proposed to exploit them. A prominent example is the mineral accretion process, developed by W. H. Hilbertz, an architect, to build homes in the ocean [17]. Hilbertz proposes to use this process to... 

Briefcase, a sculpture made with mineral accretion process (Courtesy Chris Scala, the artist). 

The above explanation does not account for the elevated ° Pb/ °" Pb ratios of continental rocks and their sedimentary derivatives relative to mantle-derived basalts (Figures 5(a) and 21). This special feature can be explained by a more complex evolution of continents subsequent to their formation. New continental crust formed during Archean time by subduction and accretion processes must have initially possessed a U/Pb ratio slightly higher than that of the mantle. At that time, the terrestrial ratio was signifi-... 

I have chosen to discuss three separate categories of black holes those that arise from stellar collapse, those that may come about from merging and accretion processes and are termed supermassive black holes (SMBH), and those which may be relics of the dense highly inhomogeneous medium in the early universe (primordial black holes—PBH). 

---