THE VISCOUS EVOLUTION OF ELLIPTIC ACCRETION DISKS

We examine critically the widely accepted assumption that an accretion flow should circularize on a time-scale much shorter than the viscous time-scale. We provide an analytic calculation, supplemented by a generalization of the circular discs of Shakura & Sunyaev, and some 'sticky-particle' numerical simulations, both of which suggest that the converse might be the case (i.e. that initially elliptical discs could retain their shape over a viscous time-scale, or even become more elliptical). We argue that there are astrophysical implications of such a result for initially eccentric streams of gas, either in a Kepler potential, or where the accretion rate is high compared to the rate of differential precession of streamlines.