Third dredge-up

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. 

Pequignot et al. (2000) discuss two PNe in the Sgr B2 galaxy, He 2-436 and Wray 16-423, whose nuclei are interpreted as belonging to the same evolutionary track. The authors perform a differential analysis of these two PNe, based on tailored photoionization modelling, and argue that while systematic errors may substantially shift the derived abundances, the conclusions based on differences between the two models should not be influenced. The main conclusion is that third dredge up O enrichment is observed in He 2-436, at the 10 % level. 

We now focus on the thermally-pulsing AGB phase of evolution, which alters the surface abundances of the models in two distinct and important ways. The first is through the operation of the third dredge-up, which can occur periodically after each thermal pulse (TP) and is the mechanism for turning... 

The third dredge-up phase, which is when the convective envelope moves inwards into regions previously mixed by the flash-driven convection. Carbon and other He-burning products are mixed to the stellar surface. 

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