SIMBAD references

We introduce a new method to analyze kinematic proper motion data. The method is called "criss-cross" mapping. It emphasizes regions where proper motion vector extensions cross or converge. From a superposition of lines through the vectors a map is generated which helps to interpret the kinematic data. The new mapping technique is applied to the young planetary nebula BD+30 3639. The data are more than 200 internal proper motion measurements from Li et al. From the criss-cross mapping of BD+30 3639, we conclude that the kinematic center is approximately 0.5 arcsec off-set to the southeast from the central star. The mapping also shows evidence for a non-homologous expansion of the nebula that is consistent with a disturbance due to the bipolar molecular bullets.