## 1. IntroductionThe simplicity of the classic Bondi-Hoyle-Lyttleton (BHL) accretion
model makes its use attractive in order to estimate roughly accretion
rates and drag forces applicable in many different astrophysical
contexts. Various aspects of the BHL flow have repeatedly been
investigated in the past by many authors. In the BHL scenario a
totally absorbing sphere of mass We recall that the largest radius from which matter is still accreted by the BHL-procedure turns out to be the so-called Hoyle-Lyttleton accretion radius where I will refer to the volume upstream of the accretor from which matter is accreted as accretion cylinder. One can additionally 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 distribution perpendicular to the mean velocity direction. Then, assuming no redistribution of angular momentum, the amount accreted is equal to (or at least is a large fraction of) the angular momentum present in the accretion cylinder. The uncertainty about how much angular momentum can actually be accreted in a BHL flow stems from these two opposing views involving either a large or a very small fraction of what is present in the accretion cylinder. In the first paper R1, I showed that the answer is not clear cut, but depends on the initial and boundary conditions. Roughly 7% to 70% of the total angular momentum available in the accretion cylinder is accreted. In this second paper I would like to compare the accretion rates of
the angular momentum of numerically modeled accretion flows with
In order to be able to compare the new results to previous models of R1, I will use the same values for the gradients as in R1. However, as will be mentioned in connection with Eq. 6 (which had already been presented in R1), the expected angular momentum accretion is reduced by a factor of six, when changing from velocity to density gradients. Thus the clear separation between angular momentum from the bulk flow one from the unstable, fluctuating flow blurs. On the other hand arbitrarily large specific angular momenta cannot be accreted because of the ang. mom. barrier felt by matter spiralling into the accretor. A simulation with a very large density gradient was done, too, mainly to be able to compare to a previous 2D model (Fryxell & Taam, 1988). In Sect. 2 I give an only short summary of the numerical procedure used. Sects. 3 and 4 present the results, which I summarise in Sect. 5. © European Southern Observatory (ESO) 1999 Online publication: June 17, 1999 |