Astron. Astrophys. 317, 793-814 (1997)
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).
- 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
- 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%.
- 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).
- 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.
- 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
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998