New observations of the halo radial temperature structure in NGC7662
  We report on our studies of the physical structure of the planetary nebula (PN) NGC7662. Using (3D) Integral Field Spectroscopy we have been able to measure the electron temperature more accurately and at a larger number of radial locations than before. Here we briefly present our method by which we find a strong positive temperature gradient with increasing radius. According to hydrodynamic models a hot halo, when compared to the central star, can be the product of the passage of an ionization front (e.g. Marten 1993). Such a gradient is not found in equilibrium models, and this finding - when confirmed for other objects - strongly advocates the use of hydrodynamic models when modeling PN halos. 
C. Sandin, D. Schoenberner, M.M. Roth, M. Steffen, A. Monreal-Ibero,   P. Boehm, U. Tripphahn