NGC 7293, the Helix nebula, represents one of the rare instances in which theoretical predictions of stellar evolution can be accurately tested against observations since the precise parallax distance and the velocity and proper motion of the star are well known. We present numerical simulations of the formation of the Helix PN that are fully constrained by the progenitor stellar mass, stellar evolution history, and star-interstellar medium (ISM) interaction. In the simulations, multiple bow-shock structures are formed by fragmentation of the shock front where the direct interaction of the stellar wind with the ISM takes place.

The stellar life cycle is dominated by phases such as the hydrogen-burning stage and the remnant white dwarf cooling phase. However, between these two stages, stars dramatically transform themselves by losing the bulk of their mass. Planetary nebulae (PNe) provide a powerful clue to the processes involved in this transformation, but they are very complex. Over the past 15 years, a new wave of imaging and spectroscopy programs have uncovered the remnants of PNe, white dwarfs, in a wide range of well-measured environments. With this we can map the masses and temperatures of the stellar remnants to the properties of their progenitors. This work has now led to the first uniform mapping of the initial-final mass relation from 1.5 to 7 M⊙. The resulting relation is a fundamental input to our understanding of stellar evolution for low and intermediate-mass stars that produce PNe and has a wide range of applications.

Water maser emission has been detected only toward three planetary nebulae (PNe). In particular, in K3-35, the first PN where water vapor maser emission was detected, the components are located in a torus-like structure with a radius of 85 AU and also at the surprisingly large distance of 5000 AU from the star, in the tips of the bipolar lobes. The existence of these water molecules in PNe is puzzling, probably related to some unknown mechanism shielding them against the ionizing radiation. We report the detection of HCO+ (J = 1 − 0) emission toward K 3-35, that not only suggests that dense molecular gas (~105 cm−3) is present in this PN, but also that this kind of PN can enrich their surroundings with organic molecules.