Giant stars nucleosynthesis

Carbon rich giant stars are highly evolved stars that have a greater concentration of carbon than oxygen in their atmospheres. This superficial enrichment is the consequence of nuclear reactions that take place in their interiors and of the efficient convection that transports the nucleosynthesis products to the most external layers. Carbon stars have effective temperatures in the range 2,000-3,000 K and are some 10,000 times brighter than the Sun. They often present important mass loss... 

Intermediate-mass red giant stars are understood to be the primary source both of and of the heavy s-process (slow neutron capture) elements, as well as a significant source of and other less abundant CNO isotopes. Their contributions to galactic nucleosynthesis are... 

Many CAIs, together with some chondrules and samples of differentiated asteroids, contained short-lived radioactive isotopes at the time they formed. This is deduced from the abundances of the daughter isotopes seen in modern meteorites. The short-lived isotopes include " Ca, A1, °Be, e, Mn, and ° Pd, with half-lives (in units of Myr) 0.13, 0.7, 1.5, 1.5, 3.7, and 6.5, respectively. Many of these isotopes could have been produced from stable ones by absorption of neutrons in a supernova or the outer layers of a giant star. In particular, °Fe can only be produced efficiently by stellar nucleosynthesis and so must have come from an external source (Shukolyukov and Lugmair, 1993). Conversely, some isotopes such as Be almost certainly formed in the protoplanetary nebula when material was bombarded by solar cosmic rays (McKeegan et al., 2000). Multiple sources are possible for some short-lived isotopes. The abundances of the decay products of... 

This chapter briefly introduces the chemistry in circumstellar envelopes (CSE) around old, mass-losing stars. The focus is on stars with initial masses of one to eight solar masses that evolve into red giant stars with a few hundred times the solar radius, and which develop circumstellar shells several hundred times their stellar radii. The chemistry in the innermost circumstellar shell adjacent to the photosphere is dominated by thermochemistry, whereas photochemistry driven by interstellar UV radiation dominates in the outer shell. The conditions in the CSE allow mineral condensation within a few stellar radii, and these grains are important sources of interstellar dust. Micron-sized dust grains that formed in the CSE of red giant stars have been isolated from certain meteorites and their elemental and isotopic chemistry provides detailed insights into nucleosynthesis processes and dust formation conditions of their parent stars, which died before the solar system was bom 4.56 Ga ago. 

Stars with initial masses of -1-8 M evolve into red giant stars and lose their outer atmospheres through stellar winds. The lost material creates huge circumstellar shells. The overall composition of a CSE is determined by the ongoing nucleosynthesis in the star. Most importantly, production and dredge-up of C in AGB stars changes the surface composition from oxygen rich (C/0<1 in M stars) to carbon-rich (C/0>1 in C stars). The C/0 ratio determines the gas chemistry in the CSE and which condensates (e.g., silicates or carbides) appear. 

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 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 . 

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. 

Almost all of the elements heavier than He are synthesized in the interiors of stars. The work of Burbidge et al. (1957) gives the theoretical framework for the synthesis of the elements. The experimental evidence of active nucleosynthesis came from the discovery of the unstable nuclei of technetium in the spectra of red giants (Merrill 1952). The solar elemental and isotopic abundances which are taken from the primitive carbonaceous chondrites constitute the guidelines for testing such models (Anders and Grevesse 1989). A minimum of eight basic processes are required to reproduce the observed compositions. Nucleosynthetic... 

Busfield A, Gilmour JD, Whitby JA, Turner G (2004) Iodine-xenon analysis of ordinary chondrite halide implications for early solar system water. Geochim Cosmochim Acta 68 195-202 Busso M, Gallino R, Wasserburg GJ (1999) Nucleosynthesis in asymptotic giant branch stars relevance for galactic enrichment and solar system formation. Annu Rev Astronom Astrophys 37 239-309 Cameron AGW (1969) Physical conditions in the primitive solar nebula. In Meteorite Research. Millman PM (ed) Reidel, Dordrecht, p 7-12... 

The compositions of several varieties of peculiar warm giants are also listed in Table 1. These stars differ from normal G and K giants in a variety of ways lithium is unusually high (or low) carbon is enriched through triple-a nucleosynthesis or depleted through CN-cycle processing and/or s-process elements... 

Observations of isotopic abundances provides information on the nucleosynthesis operating in the compact core of stars and supernova explosions and on the chemical evolution of the Galaxy. The CNO nuclides in late-type stars are affected by freshly synthesized core material brought up by dredge-up events. On the other hand, the Si isotopes are involved in later phases of nuclear burning, a narrow span of the red giant lifetime before planetary nebulae or supernovae. Therefore relative abundances of Si isotopes we observe remain unchanged from those of interstellar matter from which a star was formed. 

---