INSTABILITY OF SPHERICAL ACCRETION - I - SHOCK-FREE BONDI ACCRETION

We examine the spatial stability of spherical adiabatic Bondi accretio n on to a point gravitating mass against external perturbations. Both transonic critical and subsonic subcritical accretion are shown to be stable against purely radial acoustic, vortex or entropy perturbations . In the case of non-radial perturbations the amplitude of the perturb ations grows without limit with smaller radii. Instability manifests i tself only if the size of the accreting body is much less than the Bon di radius so that the inflow is highly supersonic or highly subsonic a t the surface of the accretor in the case of critical or subcritical a ccretion respectively. These asymptotics hold and consequently the ins tability may develop for adiabatic index of accreting gas gamma < 5/3. We suggest that this instability may lead to an essential thermalizat ion of accreting flow thus, particularly, solving the problem of other wise inefficient energy release in spherical accretion on to a black h ole.