*Astron. Astrophys. 346, 861-877 (1999)*
## Non-axisymmetric wind-accretion simulations
### II. Density gradients
**
M. Ruffert **^{*}
Department of Mathematics & Statistics, University of Edinburgh, Scotland EH9 3JZ, UK
*Received 29 May 1998 / Accepted 19 March 1999*
**Abstract**
The hydrodynamics of a variant of classical Bondi-Hoyle-Lyttleton
accretion is investigated: a totally absorbing sphere moves at various
Mach numbers (3 and 10) relative to a medium, which is taken to be an
ideal gas having a *density* gradient (of 3%, 20% or 100% over
one accretion radius) perpendicular to the relative motion. I examine
the influence of the Mach number, the adiabatic index, and the
strength of the gradient upon the physical behaviour of the flow and
the accretion rates of the angular momentum in particular. The
hydrodynamics is modeled by the "Piecewise Parabolic Method" (PPM).
The resolution in the vicinity of the accretor is increased by
multiply nesting several grids around the sphere.
Similarly to the 3D models published previously, both with
*velocity* gradients and without, the models with a
*density* gradient presented here exhibit non-stationary flow
patterns, although the Mach cone remains fairly stable. The accretion
rates of mass, linear and angular momenta do not fluctuate as strongly
as published previously for 2D models. No obvious trend of the
dependency of mass accretion rate fluctuations on the density gradient
can be discerned. The average specific angular momentum accreted is
roughly between zero and 70% of the total angular momentum available
in the accretion cylinder in the cases where the average is prograde.
Due to the large fluctuations during accretion, the average angular
momentum of some models is retrograde by up to 25%. The magnitude is
always smaller than the value of a vortex with Kepler velocity around
the surface of the accretor.
The models with small density gradients *initially* display a
transient quasi-stable accretion phase in which the specific angular
momentum accreted is within 10% of the total angular momentum
available in the accretion cylinder. Later, when the flow becomes
unstable, the average decreases. I conclude that for accretion from a
medium with both density and/or velocity gradients, most of the
angular momentum that is available in the accretion cylinder is
accreted together with mass. Small gradients hardly influence the
average accretion rates as compared to accretion from a homogeneous
medium, while very large ones succeed to dominate and form an
accretion flow in which the sense of rotation is not inverted.
**Key words:** accretion, accretion
disks
hydrodynamics
instabilities
shock waves
methods:
numerical
stars: binaries: close
* e-mail: m.ruffert@ed.ac.uk
This article contains no SIMBAD objects.
### Contents
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999
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