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Hot-bottom burning

Theoretical models for nucleosynthesis in asymptotic giant branch stars predict a large contribution to the cosmic nitrogen abundance from intermediate-mass stars [1], In particular, hot-bottom-burning in stars above a certain mass produces [C/N] —1 [2]. However, observations of C and N abundances in C-rich, metal-poor stars, usually using the CH and CN bands, show [C/N] values that vary between —0.5 and 1.5. (Fig. 1). If any of these stars have been polluted by intermediate mass AGB stars, then they should have lower [C/N] ratios. However, most of the CH stars with detailed abundances have [C/Fe] > 1.0, and it is more likely than stars mildly enhanced in C have been polluted by N-rich stars. [Pg.120]

We do not find any stars with low [C/N] ratios and the range of [C/N] we find is much more restricted than predicted. If a standard initial mass function is used, we would expect about one in five AGB stars to undergo hot-bottom-burning, a ratio not seen in Fig. 1. It is possible that some of the C-rich stars in our sample were polluted by nucleosynthesis sources other than AGB stars, and we are currently observing a larger sample of stars. There are also many assumptions which need to re-evaluated, such as mass loss being unaffected by... [Pg.120]

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. [Pg.371]

Planetary nebulae are often even more rich in carbon than cool carbon stars, and those classified by M. Peimbert as Type I are rich in nitrogen, indicating effects of hot-bottom burning in intermediate-mass progenitor stars. The s-process elements are not normally detectable in PN or their central stars, but a remarkable case is that of FG Sagittae, the central star of a fossil planetary nebula, which has cooled in the course of the twentieth century from around 25 000 K to around 5000 K at constant bolometric luminosity. This star suddenly showed an enhancement of s-process elements in its atmosphere between 1965 and 1972 (see Jeffrey Schoenberner 2006, and references therein). [Pg.216]

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. [Pg.31]

Boothroyd A. I., Sackmann I.-J., and Wasserburg G. J. (1995) Hot bottom burning in asymptotic giant branch stars and its etfect on oxygen isotopic abundances. Astrophys. J. 442, L21-L24. [Pg.38]

Lattanzio J. C., Frost C. A., Cannon R. C., and Wood P. R. (1997) Hot bottom burning nucleosynthesis in 6Mq stellar models. Nuclear Phys. A621, 435c-438c. [Pg.40]

Hot bottom burning first alters the envelope abundance of the CNO isotopes by the CN cycle and later, by the ON cycles. There is also a fourth cycle involving the destruction of 19F to produce lsO. The CN cycle burns 12C first into 13C and later into 14N, which reduces the 12C/13C ratio from the pre-AGB value of 20 to close to 4 — 5, see Figure 27. The 14N abundance... [Pg.145]


See other pages where Hot-bottom burning is mentioned: [Pg.353]    [Pg.358]    [Pg.194]    [Pg.196]    [Pg.232]    [Pg.232]    [Pg.135]    [Pg.136]    [Pg.141]    [Pg.32]    [Pg.35]    [Pg.36]    [Pg.123]    [Pg.275]    [Pg.276]    [Pg.26]    [Pg.36]    [Pg.150]    [Pg.150]    [Pg.150]    [Pg.107]    [Pg.124]    [Pg.126]    [Pg.133]    [Pg.143]    [Pg.143]    [Pg.158]   
See also in sourсe #XX -- [ Pg.193 , Pg.196 , Pg.215 , Pg.232 ]




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Bottom burning

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