Accretion Processes

Accretion events play a major role in many astrophysical systems starting from protostar formation, close binary systems with mass transfer to the centres of galaxies. In particular I studied the following topics:

  • Internal structure of Accretion Discs:

      Accretion discs are driven by angular momentum transport by a turbulent viscosity. This causes mass to flow inwards towards the central object and angular momentum to be transported outwards. The action of turbulence is usually modeled using a Reynolds stress approximation and for the viscosity coefficient an alpha-ansatz is being used. The internal dynamics of the disc flow may be rather complicated and I have performed two-dimensional, numerical studies of the r-z structure of discs.

    1. Meriodional Circulation

      Firstly, the case of convectively stable discs was considered. It was found that the generic internal flow field of an accretion disc consists of radial outflow in near the midplane of the disc and inflow at the surfaces. Just as in the case of rotating stars where the von Zeipel paradoxon prevents the star from being strictly in hydrostatic equilibriuum, in the disc case a meridional circulation is required as well. However, the velocities are adjusted in such a way as to maintain a constant mass accretion rate at each radius. Details of the computations can be found in ApJ 397, 600 (1992).

    2. Pulsational Overstability

      As was shown initially by Kato (1978), accretion discs are subject to a radial pulsational instability. This occurs if the viscosity coefficient increases sufficiently strongly upon compression. For standard accretion disc models this means that the viscosity coefficient alpha has to large enough, for typical parameter of Cataclysmic Variable systems we find that alpha must be larger than about 0.06. The results of the models are published in ApJ 409, 739 (1993).

    3. Convection

      For study convectively unstable discs we analyzed the disc structure for parameter typical in a protostellar environment and modeled a section of a disc in the region near Jupiter. The flow field shows convection cells penetrating the discs midplane, and the typical temperature behaviour in convective motion: Hot elements rise while cool ones flow to the midplane. Note that in discs there is a gravitational force directed towards the midplane of the disc. More details on convection in protostellar discs are found on my Star Formation page.

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