Physics and Chemistry of the late stages of stellar evolution

Sun Kwok

The University of Hong Kong, Hong Kong

The asymptotic giant branch (AGB) is characterized by two major events: one is the nucleosynthesis of carbon by triple-a reaction in the core and the other is the initiation of a strong stellar wind at the surface. Through convection, the element carbon is dredged up to the surface. As the surface temperature decreases as the result of envelope expansion, simple molecules begin to form in the photosphere and solid-state materials condense in the extended atmosphere. Radiation pressure on grains drags the gas along, resulting in large-scale mass loss of the stellar envelope. Over 80 different molecules have been observed to form in the outflow and AGB stars are now recognized as prolific molecular factories.

When the hydrogen envelope is completely depleted by mass loss and the core is exposed, the star will become hotter and bluer and this massive stellar wind ceases. A separate less massive but faster, and probably directional, stellar wind occurs, compressing and cutting through the previous stellar wind. This interaction of stellar winds causes morphological changes of the circumstellar materials and creates the bipolar and multipolar structures that we observe.

Although planetary nebulae are easily observable in the visible because of the recombination lines of H and He and recombination lines of metals, most of the mass of planetary nebulae is in fact contained in the neutral component. Molecular-line imaging of planetary nebulae has revealed the relationship between the ionized and neutral components, which can provide useful insights into the shaping mechanism.

Most interestingly, planetary nebulae are now known to be major sources of complex organics in the Galaxy. Organic compounds of mixed aromatic and aliphatic structures have been observed to form in the post-AGB evolution over time scales as short as hundreds of years. There is likely that these stellar organics can journey through the Galaxy and were embedded in early Solar System and may have played a role in the origin of life on Earth.

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