The Early Interaction of the Planetary Nebula NGC 40 with the   Interstellar Medium
  We report the detection of an Halpha emission-line structure in the upstream side of the planetary nebula NGC 40, which is predicted by numerical simulations, and which is attributed to Rayleigh-Taylor instability. Such a Rayleigh-Taylor instability is expected to occur at early stages of the interaction process between the interstellar medium (ISM) and a fast moving planetary nebula, as is the case for NGC 40. We resolved the Rayleigh-Taylor instability `tongues', as well as the flatness of the nebula around the `tongues', which results from the deceleration by the ISM. 
Jodie Martin (1), Kiriaki Xilouris (1), Noam Soker (1 and 2) ((1)   University of Virginia, (2) University of Haifa at Orani...