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Published online by Cambridge University Press: 25 May 2016
The nature and origin of the hundreds of ionized knots, with cometary tails, in the Helix planetary nebula are both being unravelled by recent observations. Healey and Huggins (1990, AJ, 100, 511), Meaburn et al (1992, MNRAS, 255, 177) and Walsh and Meaburn (1993, ESO Messenger, 73, 35) all showed, in various ways, that these knots have dense (106 cm–3), dusty, molecular cores with ionized arcs of gas on the surfaces pointing towards the ionizing star. This structure is confirmed in detail by the HST imagery of O'Dell and Handron (1996, ApJ, in press).