CORED APPLE BIPOLARITY - A GLOBAL INSTABILITY TO CONVECTION IN RADIALACCRETION

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 whic h is associated with a hyperbolic central point in the meridional stre am lines, and consequently with either an equatorial inflow and an axi al ejection or an axial inflow and an equatorial ejection. In each cas e the stream lines pass through a thick accretion torus, which is bett er 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 t hermal, magnetic, turbulent and centrifugal pressures created in the f low. In both bipolar modes the presence of non-zero angular momentum i mplies axial regions wherein the pressure is first reduced below the v alue at infinity and then becomes negative, where the solution fails b ecause rotating material cannot enter this region without `suction'. T he models thus have empty `stems' where the activity of the central so urce must dominate. The basic engine of the bipolar flow discussed her e is therefore simply the rebound of freely falling material from a th ick pressure disc into an axial low-pressure region. The low-mass, hig h-velocity outflow must be produced in this region by an additional me chanism. This is reminiscent of the `cored apple' structure observed r ecently in the very young bipolar source VLA 1623.