We report the detection of an Ha 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.
- ISM: structure
- Planetary nebulae: general
- Planetary nebulae: individual: NGC 40