## 1. IntroductionThe simplicity of the classic Bondi-Hoyle-Lyttleton (BHL) accretion
model makes its use attractive in order to roughly estimate accretion
rates and drag forces in many different astrophysical contexts,
ranging from wind-fed X-ray binaries (e.g. Anzer &
Börner 1995), over supernovae
(e.g. Chevalier 1996), and galaxies moving through
intracluster gas in a cluster of galaxies (Balsara et al. (1994),
to the black hole believed to be at the center of our Galaxy (Ruffert
& Melia 1994; Mirabel et al. 1991). In the BHL scenario
a totally absorbing sphere of mass The BHL recipe for accretion in the axisymmetric case for
pressureless matter is the following. A ring of material with radius
where I will refer to the volume upstream of the accretor from which matter is accreted as accretion cylinder. However, if the assumption of homogeneity of the surrounding medium is dropped, e.g. by assuming some constant gradient in the density or the velocity distribution, the consequences on the accretion flow remain very unclear. Using the same conceptual procedures, one can calculate (Dodd & McCrea 1952; Illarionov & Sunyaev 1975; Shapiro & Lightman 1976; Wang 1981) how much angular momentum is present in the accretion cylinder for a non-axisymmetric flow which has a gradient in its density or velocity perpendicular to the mean velocity direction. Then, assuming that the angular momentum will be accreted together with the mass, it is only a small step to conclude that the amount of angular momentum accreted is equal to (or at least is a large fraction of) the angular momentum present in the accretion cylinder. Note, that if the velocity is a function of position, then by virtue of Eq. (1) also the accretion radius varies in space. Thus the cross section of the accretion cylinder (perpendicular to the axis) is not circular. However, the reasoning of BHL calls for a cancelling of linear momentum perpendicular to the radial accretion line before matter is accreted. Together with this linear momentum also angular momentum is cancelled and so the matter accreted has zero angular momentum by construction! This point was first discussed by Davies & Pringle (1980), who were able to construct two-dimensional flows with small non-vanishing gradients for which the accreted angular momentum was exactly zero, by placing the accretion line appropriately. Thus, following these analytic investigations two opposing views are voiced about how much angular momentum can be accreted: either a large or a very small fraction of what is present in the accretion cylinder. Numerical simulations thus are called for to help solve the problem. In this paper I would like to compare the accretion rates of
several quantities (especially angular momentum) of numerically
modeled accretion flows with gradients to the previous results of
accretion without gradients (e.g. Ruffert 1994). One has to
change some of the parameters of the flow (Mach number, size of the
accretor) in order to get a good overview of which features are
generic and which specific to that combination of parameters. Although
several investigations of In Sect. 2 I give only a short summary of the numerical procedure used. Sects. 4 to 6 present the results, which I analyze and interpret in Sect. 8. Sect. 9 summarizes the implications of this work. © European Southern Observatory (ESO) 1997 Online publication: July 8, 1998 |