Accretion discs

Accretion disc The collection of matter around a star that will form into a solar system. 

The heterogeneities of the protosolar nebula are considered as very significant by some authors I7), and as quantitatively not important by others 9). Nevertheless, it remains a fact that the model of a homogeneous accretion disc where equilibrium reaction takes place has now been replaced by a much more subtle description where a lack of homogeneity and equilibrium are facts that must be taken into account. This problem will be discussed below. 

Class II. Sources with spectral index —1.5 < cyir < 0. These are pre-main-sequence stars with observable accretion discs (classical T Tauri stars). 

Theory of Black Hole Accretion Discs, edited by M. A. Abramowicz, G. Bjomsson and J. E. Pringle (ISBN 0 521 62362 6)... 

Pulsars are believed to be rapidly rotating magnetized neutron stars and many X-ray sources are thought to be neutron stars in binary systems with another star, from which material is drawn into an accretion disc. This material, heated to a very high temperature, emits radiation in the X-ray region. 

The paramount importance of carbon in the cosmos is shown by the fact that more than 75% of the approximately 120 interstellar and circumstellar molecules so far identified are carbon containing (Henning and Salama, 1998). Molecules apparently travel from the ISM via protoplanetary discs to the planetesimals and from there, via accretion, to the heavenly bodies formed. The molecules so far identified in ISM come from quite different types of compounds ... 

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. 

Figure 6.2 Formation of the solar system (a) unstable molecular cloud possessing some angular momentum (b) angular moment conservation produces the disc shape under collapse (c) matter accretion forms the planets (d) the mature system of planets seen today evolves after 4 Myr...

In our scenario, we consider a purely hadronic star whose central pressure is increasing due to spin-down or due to mass accretion, e.g., from the material left by the supernova explosion (fallback disc), from a companion star or from the interstellar medium. As the central pressure exceeds the threshold value Pq at static transition point, a virtual drop of quark matter in the Q -phase can be formed in the central region of the star. As soon as a real drop of Q -matter is formed, it will grow very rapidly and the original Hadronic Star will be converted to and Hybrid Star or to a Strange Star, depending on the detail of... 

Under Ya.B. s guidance the theory of disc accretion was developed and received recognition and experimental verification. We note that all this work was basically performed before the experimental discoveries. Still awaiting experimental confirmation is the burst of neutrino radiation accompanying the collapse of a star, which Ya.B. examined together with O. Kh. Guseinov... 

Gail H.-P. (2001) Radial mixing in protoplanetary accretion disks I. Stationary disc models with annealing and carbon combustion. Astron. Astrophys. 378, 192-213. 

The planets of our solar system probably formed from a disc-shaped cloud of hot gases, the remnants of a stellar supernova. Condensing vapours formed solids that coalesced into small bodies (planetesimals), and accretion of these built the dense inner planets (Mercury to Mars). The larger outer planets, being more distant from the sun, are composed of lower-density gases, which condensed at much cooler temperatures. 

FIGURE 2.6 Midplane solar nebular temperatures (K) calculated for 0.04 and 0.02 solar masses in the accreting disk, and estimated temperatures from meteorites and comets, plotted against distance from the sun, expressed both as astronomical units (1 AU = Earth-Sun distance) (bottom) and planetary distance (top). The temperatures presented here are indicative only, as they are dependent upon the size of the disc and the thermal model used (after Boss, 1998). 

---