Nucleosynthesis, stellar

In Fig. 1 we display the log(N/0) vs. time and log(N/0) vs. log(0/H)+12 behaviours predicted by two successful models for NGC 1569 and NGC 1705. These models well reproduce several observational constraints - the present-day gaseous and total masses as well as the overall metallicity and oxygen content of the gas - by adopting the same prescriptions on the stellar nucleosynthesis, stellar IMF and galactic outflow onset conditions and efficiency. However, as can be seen from Fig. 1, the present-day N/O ratio is reproduced only for NGC 1569, whilst for NGC 1705 the theoretical N/O ratio during the last 4 Gyr of galaxy s evolution is 0.3-0.4 dex higher than observed at the present time. 

The potential of presolar grains to provide information about nucleosynthesis, stellar evolution, galactic chemical evolution, interstellar processes, and nebular processes is only beginning to be tapped. But as new instrumentation is developed, more and more of the information that they carry will be extracted. 

Cosmochemistry, however, remains a fertile area of research, as there remain many outstanding problems. A comprehensive approach to cosmochemistry requires a combination of a number of topics like primordial nucleosynthesis, stellar nucleosynthesis, explosive nucleosynthesis and solar abundance. The Kodai school on Synthesis of elements in stars was organized to provide a glimpse of this exciting area of research to astrophysicists of tomorrow, motivated young students from India and abroad. The lectures are thus aimed at researchers who would like to venture deeper into this exciting arena. 

Abstract This chapter provides the necessary background from astrophysics, nuclear, and particle physics to understand the cosmic origin of the chemical elements. It reflects the year 2008 state of the art in this extremely quickly developing interdisciplinary research direction. The discussion summarizes the nucleosynthetic processes in the course of the evolution of the Universe and the galaxies contained within, including primordial nucleosynthesis, stellar evolution, and explosive nucleosynthesis in single and binary systems. 

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

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. 

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. 

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-... 

McWilliam, Smecker-Hane, Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis ASP Conference Series, Ed. Bash and Barnes, (2005)... 

Detailed elemental abundances are now available for several individual stars in the Galaxy s dwarf satellites (Shetrone et a1. 2001, 2003 Geisler et al. 2005 also see the reviews in this proceedings). A comparison of these abundance ratios to those of stars in the Galaxy can be used to address several questions related to galaxy formation and evolution, as well as stellar nucleosynthesis. 

Abstract. We have performed the chemical analysis of extragalactic carbon stars from VLT/UVES spectra. The derived individual abundances of metals and s-elements as well as the well known distance of the selected stars in the Small Magellanic Cloud and the Sagittarius dwarf galaxies permit us to test current models of stellar evolution and nucleosynthesis during the Asymptotic Giant Branch phase in low metallicity environments. 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

D.D. Clayton Principles of stellar evolution and nucleosynthesis, 2nd edn. (Univ. of Chicago Press, Chicago 1983)... 

Abstract. We recall the emergence of the 3He problem , its currently accepted solution, and we summarize the presently available constraints on models of stellar nucleosynthesis and studies of Galactic chemical evolution from observations of the He isotopic ratio in the Galaxy. 

Fig. 2. Abundance of 3He vs. main-sequence masses (determined by Galli et al. 1997) for the six PN of the sample of Balser et al. (1997) and Balser, Rood, Bania (1999). The curves labeled Pop I and Pop II show the standard abundance of 3He computed by Weiss, Wagenhuber, Denissenkov (1996) for two metallicities. The curves labeled with extra-mixing show the results of stellar nucleosynthesis calculations with deep mixing by Boothroyd Sackmann (1999) (upper curve) and the equilibrium value 3He/H = 10-B for M < 2.5 M (lower curve).

We adopt the following nucleosynthesis prescriptions for stars of all masses and take into account the stellar lifetimes. 

Where Ay (IS) is the correction for the ionization structure [6] by model calculations, depending on a star effective temperature (Teft) and dust y+= N(He+)/N(H+), y = N(He)/N(H). Correction for a stellar nucleosynthesis He production was either using Y Z linear dependence with the slope value of [3] or for distant source Ay = -(0.5 0.5)% being accepted as half of [2] calculation. 

D. D. Clayton, Principles of Stellar Evolution and Nucleosynthesis, McGraw-Hill 1968, University of Chicago Press 1984. This classic text is particularly well written and gives more accurate and rigorous arguments for results derived here often in a somewhat heuristic manner. A more up-to-date (though in some ways less complete) description of nuclear astrophysics is given in... 

Progress in understanding stellar evolution and nucleosynthesis, and the discovery by Merrill (1952) of the unstable element technetium in the S star R Andromedae, demonstrating the occurrence of stellar nucleosynthesis within a few half-lives of Tc (i.e. < about 1 Myr see Fig. 1.8), has led to acceptance of the idea that abundance variations among stars are perfectly natural as a consequence of three main effects (see Fig. 3.37) ... 

Helium is the second most abundant element in the visible Universe and accordingly there is a mass of data from optical and radio emission lines in nebulae, optical emission lines from the solar chromosphere and prominences and absorption lines in spectra of hot stars. Further estimates are derived more indirectly by applying theories of stellar structure, evolution and pulsation. However, because of the relative insensitivity of Tp to cosmological parameters, combined with the need to allow for additional helium from stellar nucleosynthesis in most objects, the requirements for accuracy are very severe better than 5 per cent to place cosmological limits on Nv and better still to place interesting constraints on t] or One can, however, assert with confidence that there is a universal floor to the helium abundance in observed objects corresponding to 0.23 < Fp < 0.25. 

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