Nucleosynthesis

First proposal of stellar nucleosynthesis by proton fusion to helium and heavier nuclides... 

Nucleosynthesis is the formation of elements. Hydrogen and helium were produced in the Big Bang all other elements are descended from these two, as a result of nuclear reactions taking place either in stars or in space. Some elements—among them technetium and promethium—are found in only trace amounts on Earth. Although these elements were made in stars, their short lifetimes did not allow them to survive long enough to contribute to the formation of our planet. However, nuclides that are too unstable to be found on Earth can be made by artificial techniques, and scientists have added about 2200 different nuclides to the 300 or so that occur naturally. 

New elements and isotopes of known elements are made by nucleosynthesis the repulsive electrical forces of like-charged particles are overcome when very fast particles collide. 

Fig. 2-3 Schematic showing the path of the s process. The isotopes Xe, Xe, and Ce are beyond the reach of s process nucleosynthesis and are only produced by the r process.

Jorgensen CK, Kauffmann GB (1990) Crookes and Marignac - A Centennial of an Intuitive and Pragmatic Appraisal of Chemical Elements and the Present Astrophysical Status of Nucleosynthesis and Dark Matter . 73 227-254 Jorgensen CK, Reisfeld R (1982) Uranyl Photophysics. 50 121-171... 

Fig. 3.9 Greatly simplified representation of the path taken by the material under study, beginning with nucleosynthesis and ending with laboratory analysis. Circumstellar dust (a component of the primeval presolar nebula) which was contained in asteroids or comets came to Earth in meteorites and was then available for exact study (Lugmair, 1999)...

Abstract. AGB stars, in particular those of carbon types, are excellent laboratories to constraint the theory of stellar structure, evolution and nucleosynthesis. Despite the uncertainties still existing in the chemical analysis of these stars, the determination of the abundances of several key species in their atmospheres (lithium, s-elements, carbon and magnesium isotopic ratios etc.) is an useful tool to test these theories and the mixing processes during the AGB phase. This contribution briefly review some recent advances on this subject. 

Planetary nebulae (PNe) offer the opportunities 1) to study stellar nucleosynthesis in the advanced phases of stellar evolution of stars in the wide mass range - -O. S to Mq and 2) to probe radial and as well horizontal/vertical chemical gradients in spiral galaxies by the time of formation of their progenitors. 

Three sources have been proposed to produce fluorine in the Galaxy. The first was suggested by Forestini et al. (1992) and refers to production in low-mass stars during the AGB phase while two others are related to massive stars production in Wolf-Rayet stars (Meynet Arnould 2000) and in type II Supernovae, via the neutrino-induced nucleosynthesis (Woosley et al. 1990). 

Abstract. We present the results from our non-LTE investigation for neutral carbon, which was carried out to remove potential systematic errors in stellar abundance analyses. The calculations were performed for late-type stars and give substantial negative non-LTE abundance corrections. When applied to observations of extremely metal-poor stars, which within the LTE framework seem to suggest a possible [C/O] uprise at low metallicities (Akerman et al. 2004), these improvements will have important implications, enabling us to understand if the standard chemical evolution model is adequate, with no need to invoke signatures by Pop. Ill stars for the carbon nucleosynthesis. 

The discussion on abundances will focus on metallicities, a- and r-process elements, as probes of the nucleosynthesis history in the bulge, and timescale of bulge formation. 

Most of the work reported here has been conducted within the ESO Large Programme 165.N-0276 Galaxy Formation, Early Nucleosynthesis, and the First Stars , which has covered 4 periods 65-68, from April 2000 to November 2001, with a total of 38 nights in visitor mode. The team had R. Cayrel as PI, and 13 Cols ... 

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. 

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... 

We have used the infra-red triplet (921.286 nm, 922.809 nm and 923.754 nm) to determine the sulphur abundance in the 32 giants from ESO s Large Program Galaxy Formation, Early Nucleosynthesis, and First Stars . 

From C-Enhanced, Metal Poor Stars to AGB Nucleosynthesis... 

Another possibility might be that the nucleosynthesis of the donor star is affected by the presence of a close companion. In fact, Herwig et al. (2004) recently noted that the most extreme abundance patterns among CEMP-s are found among short period binaries. Hence, the comparison of the CEMP-s with models of single AGB star nucleosynthesis might not appropriate. 

M. Asplund, N. Grevesse, A. Jacques Sauval The solar chemical composition . In Cosmic abundances as records of stellar evolution and nucleosynthesis, ed. by F.N. Bash, T.G. Barnes (ASP San Francisco 2005), in press... 

Big efforts have been devoted in the last years to the study of light elements abundances. Definitively their importance is strongly related to cosmology as well as to stellar structure and evolution. In fact hints on the primordial nucleosynthesis can be achieved from Li, Be and B primordial abundances. Moreover these studies can be a precious tool for testing and understanding the inner stellar structure, especially for what regards the mixing processes in stellar envelopes [11-... 

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