A disc-wind model with correct crossing of all magnetohydrodynamic critical surfaces

The classical Blandford & Payne model for the magneto-centrifugal accelerat ion and collimation of a disc-wind is revisited and refined. In the origina l model, the gas is cold and the solution is everywhere subfast magnetosoni c. In the present model the plasma has a finite temperature and the self-co nsistent solution of the MHD equations starts with a subslow magnetosonic s peed which subsequently crosses all critical points, at the slow magnetoson ic, Alfven and fast magnetosonic separatrix surfaces. The superfast magneto sonic solution thus satisfies MHD causality. Downstream of the fast magneto sonic critical point the poloidal streamlines overfocus towards the axis an d the solution is terminated. The validity of the model to disc winds assoc iated with young stellar objects is briefly discussed.