Dredge third

However, as pointed out in [2], it remains to be seen to what extent the Meynet Maeder [4] yields for N in the intermediate mass star range would increase once the hot bottom burning (HBB) is taken into account. Although Meynet Maeder did not formally include the third dredge-up and HBB, they predict an important N production in low and intermediate mass stars, at low metallicities. In absence of a real quantitative assessment of the importance of the HBB it is interesting to study the importance of this new process, which produce non-parametric yields, independently of HBB. 

Thermal pulses of the He shell drive the convective mixing of third dredge-up... 

There are four processes which can produce chemical peculiarities in AGB stars the first, second and third dredge-up events and envelope (or hot-bottom) burning. 

The third dredge-up occurs as a result of the envelope convection extending downward in mass during a helium shell flash, with the result that the products of both the CNO cycle and the 3-a process are brought to the stellar surface (Iben 1975). As a result of the third dredge-up, the stellar surface is enriched in 12C and s-process elements. 

A detailed review of the above processes is given by Iben and Renzini (1983). Here only effects of the third dredge-up and envelope burning will be considered. 

At luminosities above the MS and S stars in the Magellanic Cloud clusters, we find cool (N type) carbon stars in which the third dredge-up has increased the C/O ratio to >1. There have been many studies of the carbon stars in the Magellanic Clouds a good review and comparison of this work with theory is given by Iben and Renzini (1983), with some more recent work being found in Lattanzio (1986). 

TDU third dredge-up in an AGB star, a powerful convective process that enriches the outer layers of the star with nucleosynthesis products. 

Recently, Abia et al. (2001 - see also Abia Wallerstein, 1998) measured the Rb, Sr, Y, and Zr abundances in a sample of N-type carbon stars. Carbon enrichement is attributed to salting of the atmosphere with 12C from the He-shell of an AGB star. The authors conclude that most of the carbon stars are of low mass, experiencing s-process nucleosynthesis phenomena dominated by the neutron source provided by a-capture s on 13C in radiative intershell layers. It is pleasing that the neutron density inferred from carbon stars is consistent with that from MS and S stars because the sequence of third dredge-ups is predicted to raise the low C/O ratio of MS and S stars to a value C/O in excess of unity which defines a cool carbon star. A similarity in mass is then expected. 

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