Asymptotic giant branch stars evolution

Herwig, F. (2005) Evolution of asymptotic giant branch stars. Annual Reviews of Astronomy and Astrophysics, 43, 435-479. An excellent discussion of the evolution of AGB stars. 

Asymptotic Giant Branch Stars (AGB) A period of stellar evolution. They are named for their place in a region on the Hertzsprung-Russell Diagram dominated by evolving low to medium-mass stars (0.6-10 solar masses). 

The most metal-rich stars in dwarf spheroidals (dSph) have been shown to have significantly lower even-Z abundance ratios than stars of similar metallicity in the Milky Way (MW). In addition, the most metal-rich dSph stars are dominated by an s-process abundance pattern in comparison to stars of similar metallicity in the MW. This has been interpreted as excessive contamination by Type la super-novae (SN) and asymptotic giant branch (AGB) stars ( Bonifacio et al. 2000, Shetrone et al. 2001, Smecker-Hane McWilliam 2002). By comparing these results to MW chemical evolution, Lanfranchi Matteucci (2003) conclude that the dSph galaxies have had a slower star formation rate than the MW (Lanfranchi Matteucci 2003). This slow star formation, when combined with an efficient galactic wind, allows the contribution of Type la SN and AGB stars to be incorporated into the ISM before the Type II SN can bring the metallicity up to MW thick disk metallicities. 

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. 

Becker S. A. and Iben L, Jr. (1979) The asymptotic giant branch evolution of intermediate-mass stars as a function of mass and composition I. Through the second dredge-up phase. Astrophys. J. 232, 831-853. 

Fig. 9. A simplified view of the internal chemical structure of a star during the major phases of stellar evolution. The panels represent the main-sequence, through the giant-branch, helium-burning, asymptotic-giant branch and white dwarf phases. Filled circles and thick lines represent nuclear-burning regions. Not to scale...

The remaining classes of hydrogen-deficient star are associated with very late stages of evolution, either low-mass stars approaching the horizontal-branch, or more massive stars which have already past the asymptotic-giant branch and are evolving to become white dwarfs, or yet older stars which have been white dwarfs and somehow found a second life. The first to consider are the low-mass supergiants. 

Such a diverse collection of exotic stars places severe demands on the theory of stellar evolution. In the chapter by Karakas in this volumn, the evolution of low-mass stars up to the asymptotic-giant branch has been discussed more... 

The book has been organized into three parts to address the major issues in cosmochemistry. Part I of the book deals with stellar structure, nucleosynthesis and evolution of low and intermediate-mass stars. The lectures by Simon Jeffery outline stellar evolution with discussion on the basic equations, elementary solutions and numerical methods. Amanda Karakas s lectures discuss nucleosynthesis of low and intermediate-mass stars covering nucleosynthesis prior to the Asymptotic Giant Branch (AGB) phase, evolution during the AGB, nucleosynthesis during the AGB phase, evolution after the AGB and massive AGB stars. The slow neutron-capture process and yields from AGB stars are also discussed in detail by Karakas. The lectures by S Giridhar provide some necessary background on stellar classification. 

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