SIMBAD references

We present the kinematics of a sample of bipolar planetary nebulae (PNs) which cover a wide range of observed morphologies and collimation degrees, from bipolar PNs (BPNs) with a marked equatorial ring and wide lobes to highly collimated objects. We use an empirical model in order to derive the expansion velocity, collimation degree, and inclination angle of the PN with respect to the plane of the sky. The equatorial expansion velocities measured in the objects in our sample are always in the low-to-medium range (3-16 km/s), while their polar expansion velocities range from low to very high (18-100 km/s). None of the objects in our sample, even those that show an extreme collimation degree, seem to be (kinematically) younger than ≃10³ yr. We compare our results with the state-of-the-art theoretical models for the formation of BPNs. We find good agreement between the observed expansion velocities and numerical models that use magnetic fields with stellar rotation as a collimation mechanism.