Astron. Astrophys. 346, 861-877 (1999)
For the first time a comprehensive numerical three
-dimensional study is presented of wind-accretion with a
density 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
density gradient perpendicular to this flow
(=3%, 20% and 100% over one accretion
radius), and adiabatic index (=5/3,
4/3, and 1.01). The results are compared (a) among the models with
different parameters, (b) to some previously published simulations of
models with density and velocity gradients, and (c) also to the
analytic estimates of the specific angular momentum.
All models exhibit active unstable phases, which are very similar
to the models without gradients. Only the mildly supersonic case
(=1.4) displayed a relatively steady
flow (as compared to the faster flow models). The accretion rates of
mass, linear and angular momentum fluctuate with time, although not as
strongly as published previously for 2D models.
Depending on the model parameters, the average specific angular
momentum accreted is roughly between zero and 70% of the analytical
estimate, which assumes that all angular momentum within the accretion
cylinder is actually accreted.
The mass accretion rates of all models with density gradients are
equal, to within the fluctuation amplitudes, to the rates of the
models without gradients (published previously), although the
accretion rates might seem to decrease slightly when increasing the
density gradient. The fluctuations of the mass accretion rate in all
models hardly vary with gradient strength.
The overall qualitative flow dynamics as well as the mass accretion
rates are very similar to what has been published on models with
velocity gradients. Of course the accretion rate of angular
momentum and its specific values (i.e per mass unit) are reduced
if one compares equal gradients for both velocity and density, well in
accordance with the analytic estimates. This reduction means that in
the density gradient models the fluctuations due to the unstable
accretion flow have a greater influence on the angular momentum
accretion than for the velocity gradient models.
The models with small gradients
(=0.03) display an initially quiet
stable phase, in which the specific angular momentum of the matter
accreted is within 10% of the analytic estimate. Thus for the
quiescent phases the analytic values are appropriate. The average
drops when the flow becomes unstable.
The model with very large density gradient
( over one accretion radius) was the
only one for which the accreted angular momentum was always prograde
with respect to the angular momentum available in the incoming flow.
Here the amplitude of the perturbation due to the unstable flow is
much smaller than the average angular momentum accreted. However the
specific angular momentum of the incoming flow in this case is larger
than the maximum given by the Kepler velocity times the radius of the
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999