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Astron. Astrophys. 317, 793-814 (1997)

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9. Conclusions

For the first time a comprehensive numerical three -dimensional study is presented of wind-accretion with a velocity gradient using a high resolution hydrodynamic code. I vary the following parameters: Mach number of the relative flow (Mach 3 and 10), strength of the velocity gradient perpendicular to this flow (3% and 20% over one accretion radius), radius of the accretor (0.02, 0.1 and 1 accretion radius), and adiabatic index (5/3 and 4/3). The results are compared among the models with differing parameters, to some previously published simulations, and also to the analytic estimates of the specific angular momentum of the matter that is accreted (Eq. (7), which assumes that all angular momentum in the accretion cylinder is actually accreted).

  1. All models with a small enough accretor (with a size less or equal than 0.1 accretion radii) exhibit active unstable phases, very similar to the models without gradients. The accretion rates of mass, linear and angular momentum fluctuate with time, although not as strongly as published previously for 2D models (e.g. Fryxell & Taam 1988). Similarly to the 2D simulations, transient disks form around the accretor that alternate their direction of rotation with time.
  2. Depending on the model parameters, the average specific angular momentum accreted is roughly between 7% and 70% of the analytical estimate. For the models with small velocity gradients (3%) the accreted specific angular momentum is roughly a factor of 10 smaller than the value of a vortex with Kepler velocity around the surface of the accretor. This factor is roughly 3 for models with a large gradient of 20%.
  3. The mass accretion rates of all models with velocity gradients are equal, to within the fluctuation amplitudes, to the rates of the models without gradients (published previously).
  4. The fluctuations of the mass accretion rate in the models with small gradients (3%) are also similar to the values of the models without gradients, while the models with large gradients (20%) exhibit larger fluctuations. So large gradients either amplify existing instability mechanisms or generate new ones.
  5. Marginal correlations are found, connecting the mass accretion rate, the specific angular momentum, and the specific entropy during the temporal evolution. The mass accretion rate is maximal when the specific angular momentum is zero, while the specific entropy tends to be smaller when the disks are prograde (i.e. when the specific angular momentum is negative, in our units).

Movies in mpeg format of the dynamical evolution of some models are available in the WWW at http://www.mpa-garching.mpg.de/~mor/bhla.html

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998