van Hoof, P.

published in Herschel Space Observatory Proposal, id.2067, pp. 2067 (2011)

Abstract: We propose to observe a sample of highly evolved planetary nebulae that we believe to have ongoing molecular chemistry inside dense knots that formed only a few thousand years ago inside recombining gas. The proposed Herschel observations will allow us to either prove or disprove this new, still controversial, formation scenario of the knots. If proven correct, these knots will allow us a unique opportunity to test the theory of time-dependent molecular chemistry. Two famous examples of such objects are the Helix nebula (NGC 7293) and the Ring nebula (NGC 6720). They have knots that are currently embedded in the ionized gas as the ionization front has moved outwards since the knots formed. The Helix nebula has very strong H_2 emission coming from the knots. A static photoionization model cannot explain this emission, but a hydrodynamic model can. This model indicates that the knots are quickly eroded by the radiation field of the central star. This poses a problem for rival theories as they assume that the knots formed much earlier, and must have survived through the entire photoionized phase of the nebula. We believe that the knots cannot survive that long in such a harsh environment and formed after the central star entered the cooling track and the nebulae started to recombine. In order to prove this we need more accurate models of the advection flows off the knots that need to be constrained by Herschel observations of the full CO emission line spectrum. To sample various stages of evolution, we have searched for evolved planetary nebulae with knots which were sufficiently bright. After removal of duplications with earlier proposals, we were left with a sample of 5 planetary nebulae, including the Helix nebula. We propose to obtain SPIRE full range spectroscopy and PACS deep line scans on individual CO lines, allowing us to observe the CO spectrum from the 4--3 line up to the 24--23 line.

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