Commission H3 Planetary Nebulae

Scientific Objectives

Planetary nebulae trace the end phase of the life of low-mass and intermediate-mass stars. These stars evolve up the giant branches, and eject their envelopes at the tip of the Asymptotic Giant Branch (AGB). The star, now reduced to the core, quickly evolves to higher temperatures, before nuclear burning ceases and the remnant joins the white dwarf cooling track. The expanding ejecta become ionized by the hot star, and this forms the planetary nebula. Planetary nebulae (PNe) are short lived (but much loved), very bright, and can be detected at very large distances where the original star would be undetectable. The emission comes out in strong forbidden lines, which further aids the visibility.

Planetary nebulae provide excellent tools for several important scientific problems, in stellar astrophysics, the interstellar medium and galaxies.

1. The ejection mechanism on the AGB is still very poorly understood. PNe provide a unique record of the mass-loss history of the progenitor star.

2. The shaping of the ejecta is a topic of high scientific interest. PNe show a large variety in shapes, and in their structures can combine tori, disks, jets (sometimes with precession), globules, clumps and tails, halos and asterospheres. PNe are used to study the evolution of the structures, both using observations and hydrodynamic models. The origins of the asymmetries are studied, with a main focus on binary interaction, angular momentum, and magnetic fields.

3. Common-envelope evolution plays an important in some PNe. These systems contain short-period binaries, with periods of order 1 day. The mass ejection during a common envelope is still very uncertain, and PNe are the best tracers of the end phase of their evolution, in terms of the final orbit and the ejecta.

4. Abundances of light elements can be readily measured using both forbidden and recombination lines. These trace the abundances of the progenitor star, and any nuclear processing. PNe provide accurate tracers also in older stellar populations where no HII regions are present, and can be used also in other galaxies.

5. PNe are strong dust emitters, and show many dust components including some not seen elsewhere. The dust is known to form in situ. PNe can be used to study dust formation and evolution, as well as large molecules such as fullerenes.

6. The bright lines allow for accurate velocity determination. PNe are used to measure velocity distributions of stellar populations in distant galaxies, to measure the galactic potential.

Planetary nebulae are tools for many important topics in astrophysics. The Commission supports research in planetary nebulae with particular emphasis on its applications in other fields.

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