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Astron. Astrophys. 346, 861-877 (1999)

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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


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

© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999