The AGB-star Phenomenon: Setting the Stage

The asymptotic giant branch (AGB) evolution of stars is interesting from many points of view. It is the nal evolutionary phase for the large majority of all stars in the Universe that have left the main sequence. During this stage they contribute to the chemical evolution of galaxies, contribute to the integrated starlight of many galaxies, may provide the only interstellar matter there is in some galaxies, and can, due to their high luminosities and ages, be used as important probes of galac-tic structure and dynamics. It is also the nal evolutionary stage of our own star, the Sun, and hence touches on aspects beyond the immediate astronomical interest. Furthermore, these stars provide us with fascinating systems, where intricate interplays between physical and chemical processes take place, that we simply would like to understand. In this introductory outline our present knowledge of the AGB-star phenomenon is reviewed. 1. AGB-stars Red giant stars have eeectively divided themselves into two parts: a small, very hot (10 8 cm, 10 8 K) core that is strongly gravitationally bound, and a huge, much cooler mantle (10 13 cm, 10 3 K at the surface) where the external parts are only weakly gravitationally bound. We are here only interested in those red giants that have experienced central He-burning, and now populate the asymp-totic giant branch (AGB). These are much rarer than those red giants that have only experienced central H-burning (the RGB-stars). The AGB-stars experience substantial mass loss from the surface, and huge circumstellar envelopes (CSEs) of gas and dust are formed with extents approaching 10 19 cm and temperatures below 10 K in the external parts. This can be regarded as the third part of the star, and hence an AGB-star stretches more than 11 orders of magnitude in size and 7 orders of magnitude in temperature. The evolution of the star on the AGB depends on an intricate interplay between processes taking place in the three parts. In this outline only the pure AGB-stars are considered, but it will set the stage also for the understanding of other fascinating aspects of the AGB-star phenomenon: binarity (Jorissen these proceedings, hereafter th.pr.), post-AGB objects (van Winckel th.pr.), galactic structure (Dejonghe & Van Gaelenberg th.pr.), population studies (Lann con th.pr.), chemical evolution (Gustafsson & Ryde 1998). A limited number of subjectively chosen references are given. More