High resolution molecular hydrogen imaging of the Ring Nebula

Extract

The relative morphologies and structures of molecular and ionized gas emission from planetary nebulae (PNe) allow a better understanding of the nature and evolution of these objects. The classical paradigm for the structure of PNe is that of an ionized gas bubble bounded by neutral gas and molecules. However, it has been shown that molecular gas exists within ionized regions, leading to a re-evaluation of the classic structure. In the Helix Nebula (NGC 7293) dense condensations known as cometary knots are known to exist in the main ionized nebula. The molecules in these knots are shielded from the ionizing radiation and thus survive within the ionized zone. Another PN in which H₂ emission is seen to originate from within the ionized nebula in NGC 6720 (the Ring Nebula).