Hot Bottom Burning in Intermediate Mass Stars

We discuss nucleosynthesis within intermediatemass stars by consideringa 6M model with Z = 0:02 as a typical example. The emphasis is on the AGB phase of evolution, with particular reference to nuclear burning in the bottom of the convective envelope. We nd strong CN cycling, with substantial Al production. Further we connrm previous calculations of lithium production, and show that this model should produce Fl depletions rather than enhancements. Our knowledge of the AGB phase of evolution has increased dramatically over the last decade or so. This has been driven primarily by observational studies, with theory lagging somewhat behind. An area where theoretical studies are desperately needed is in quantifying the nucleosynthesis which occurs in intermediate mass stars during their thermally pulsing evolution. While there are many calculations of the thermal pulses themselves, and associated dredge-up, there are few calculations which include a large nucleosynthesis network, or which include the recently discovered \Hot Bottom Burning". Hot Bottom Burning (hereafter HBB) is the colourful name given to the circumstance where the bottom of the convective envelope of a star reaches temperatures suuciently high for signiicant nucleosynthesis to take place. It is perhaps better to think of this as the bottom of the envelope reaching into the top of the hydrogen shell, although the two are of course equivalent, due to the monotonic variation of temperature within the star (at least in this region). Thus there is a thin layer at high temperatures, and material is mixed through this region and then into the outer envelope where the lower temperatures shutdown any nuclear processing which may occur at the deepest points in the envelope. For nuclear reactions with timescales similar to (or shorter than) the mixing timescale , we cannot make the usual assumption of instantaneous mixing at an average burning rate. The historical motivation for realising the importance of HBB can be brieey sum-marised thus: When the brightest AGB stars in the Magellanic Clouds were observed by Wood, Bessell and Fox (1983) they were surprised to discover that they were not carbon stars. Thus they suggested that perhaps the carbon dredged to their surface after thermal pulses had been burned, by HBB, into 13 C and 14 N. Further observations of these