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We propose that the prevalence of bipolarity in Young Stellar Objects is dueto the fine tuning that is required for spherical accretion of an ambientmedium onto a central node.It is shown that there are two steady modes that aremore likely than radial accretion, each of which is associated with ahyperbolic central point in the meridional stream lines, and consequently witheither an equatorial inflow and an axial ejection or vice versa. In each casethe stream lines pass through a thick accretion torus, which is better thoughtof as a standing pressure wave rather than as a relatively inert Keplerianstructure.We base our arguments on a simple analytic example,which is topologically generic,wherein each bipolarmode is created by the rebound of accretingmatter under the action of the thermal,magnetic,turbulent and centrifugalpressures created in the flow. In both bipolar modes the presence of non-zeroangular momentum implies axial regions wherein the pressure is first reducedbelow the value at infinity and then becomes negative, where the solution failsbecause rotating material can not enter this region without suction.The modelthus has empty stems where the activity of the central source must dominate.Sothe basic engine of the bipolar flow discussed here is simply the rebound offreely falling material from a thick pressure disc into an axial low pressureregion.The low mass,high velocity outflow must be produced in this region by anadditional mechanism. This is reminiscent of the cored apple structure observedrecently in the very young bipolar source VLA 1623.Comment: PostScript, 10 page

‘Cored apple’ bipolarity: a global instability to convection in radial accretion?