CORED APPLE BIPOLARITY - A GLOBAL INSTABILITY TO CONVECTION IN RADIAL ACCRETION

We propose that the prevalence of bipolarity in young stellar objects is due to the fine tuning that is required for spherical accretion of an ambient medium on to a central node. It is shown that there are two steady modes that are more likely than radial accretion, each of which is associated with a hyperbolic central point in the meridional stream lines, and consequently with either an equatorial inflow and an axial ejection or an axial inflow and an equatorial ejection. In each case the stream lines pass through a thick accretion torus, which is better thought of as a standing pressure wave rather than as a relatively inert Keplerian structure. We base our arguments on a simple analytic example, which is topologically generic, wherein each bipolar mode is created by the `rebound' of accreting matter under the action of the thermal, magnetic, turbulent and centrifugal pressures created in the flow. In both bipolar modes the presence of non-zero angular momentum implies axial regions wherein the pressure is first reduced below the value at infinity and then becomes negative, where the solution fails because rotating material cannot enter this region without `suction'. The models thus have empty `stems' where the activity of the central source must dominate. The basic engine of the bipolar flow discussed here is therefore simply the rebound of freely falling material from a thick pressure disc into an axial low-pressure region. The low-mass, high-velocity outflow must be produced in this region by an additional mechanism. This is reminiscent of the `cored apple' structure observed recently in the very young bipolar source VLA 1623.