Layered accretion

Ionization structure and layered accretion. MRI is a powerful phenomenon, but is limited to affecting... 

When the mass of the helium-layer accreted into the low-entropy core is sufficiently high, helium ignition will occur at the boundary between the non-degenerate and degenerate material. The star will expand on the short thermal timescale of the envelope ( 103 y) to become a yellow supergiant -a stage which will be described in more detail in Sect. 15.7. 

A scenario referred to as a sub-Chandrasekhar-mass supernova envisions a C-O WD capped with a helium layer accreted by a companion, and which explodes as the result of a hydrodynamical burning before having reached the Chandrasekhar limit. This type of explosions may exhibit properties which do not match easily the observed properties of typical SNIa events. It cannot be excluded, however, that they are responsible for some special types of events, depending in particular on the He accretion rate and on the CO-sub-Chandrasekhar WD (SCWD) initial mass (e.g. [85]). Unidimensional simulations of He cataclysmics characterized by suitably selected values of these quantities reach the conclusion that the accreted He-rich layer can detonate. Most commonly, this explosion is predicted to be accompanied with the C-detonation of the CO-SC WD. In some specific cases, however, this explosive burning might not develop, so that a remnant would be left following the He detonation. Multidimensional calculations cast doubt on the nature, and even occurrence, of the C-detonation in CO-SC WD (e.g. [86]). 

Fig. 36. Snapshots in the nuclidic chart of flow patterns in a ID model of a detonating He layer accreted onto a 0.8M WD. The selected times and corresponding temperatures or densities are given in different panels. The stable nuclides are indicated with open squares. The magic neutron and proton numbers are identified by vertical and horizontal double lines. The drip lines predicted by a microscopic mass model are also shown. The abundances are coded following the grey scales shown in each panel. At early times (bottom left panel), an r-process type of flow appears on the neutron-rich side of the valley of nuclear stability. At somewhat later times (top left panel), the material is pushed back to the neutron-deficient side rather close to the valley of /3-stability. As time passes (two right panels), a pn-process [87] develops...

There is no definite answer to this key question. This enrichment of the accreting WD might result from its past AGB history if indeed some of its outer s-process enriched layers could be mixed convectively with part at least of the accreted He-rich layers before the detonation. Alternatively, the He-rich layer accreted by the WD could be (or become) enriched in s-process elements. Such speculations need to be confirmed by detailed simulations. 

The precipitation of calcium carbonate in the boiler involves a number of discrete processes, including nucleation, followed by the formation of microcrystals which compete to grow into larger crystals (accretion) that eventually form layers of dense, crystalline scale on the heat exchange surface. 

The corrosion of metals and alloys generally starts at the surface with the formation of an outer layer, which may develop into a crust of corrosion products. If a crust is formed, it generally has a layered structure comprising two or more compounds (1) an outer, rather stable, mineralized layer that often covers entirely the surface of the objects, and underneath, (2) a less mineralized, unstable, and chemically active layer. Some corrosion layers may also bind ugly and disfiguring earthy accretions. 

Raisbeck et al. have reported on the application of the Grenoble cyclotron for the measurement of 10Be in artifically enriched samples [9]. Later experiments have measured 10Be in melted arctic glacier ice cores [10], marine sediments [32] and ocean surface layers [33]. The Yale group, Turekian et al., [11] have measured the 10Be content in magnesium nodules and demonstrated that these nodules accrete at the rate of approximately 4.5 mm/106 years. 

The observation that nodules grow at widely varying rates provides further support for the existence of multiple formation mechanisms. The nodules that accrete most slowly (1 mm per million years) appear to have formed primarily by the process of hydrogenous precipitation. This accretion rate is equivalent to the annual deposition of a layer that is only one atom deep. These slow rates cause a significant amount of metal-rich seawater to become occluded between the Fe-Mn oxide layers. 

Solute or particulate accumulation onto an aggregated phase (or solid state) that grows together by the addition of material at the periphery. Both cohesive and adhesive forces are thought to be driving forces in accretion. Sea shells and kidney stones are also known to form as layers of crystallites and amorphous components by accretion of external substances. 

The accretion flow with this instability will produce strong accretion shocks, turbulence, and heating of the outer layer of the star. Matter will also be ejected, and this could account for much of the mass and energy from young stars. The luminous FU Ori flare-ups may be important energy source for driving molecular out flows around low mass young stars. 

Finally, we tried a numerical experiment where 56Ni exists only in the outermost 0.2 M layer beneath the surface. The escape of X-rays and 7-rays are significant and the resulting optical light curve (the dash-dotted curve in Fig. 11) is as narrow as SN 1964L. Such a composition inversion is not realistic for helium stars but a natural outcome from the off-center detonation in accreting white dwarfs. 

When the pH of a suspension of microspheres of acidic proteinoid is raised by 1-2 units, diffusion of material from the interior to the exterior, fission into two particles, and the appearance of a double layer in the boundary are observed 2 Proteinoid microspheres shrink or swell on transfer to hypertonic or to hypotonic solutions respectively. Some experiments show that polysaccharides are retained under conditions in which monosaccharides diffuse out2. Some proteinoid microspheres possess the intrinsic capacity to grow by accretion, to proliferate through budding, and to form junctions 2). The morphology and other characteristics of proteinoid microspheres are altered by the inclusion of other materials such as polynucleotides, lipids or salts. 

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