The Asymptotic Giant Branch

During the next 100000 years the situation is as follows in the non degenerate core, helium burns to carbon, the luminosity of the star is now 1/100 as it was at the time of the helium flash. The star shrinks, its surface temperature increases. In the H-R diagram (Fig. 9.10) the stars move to the left of the red giant branch. In this horizontal branch, stars burn helium in the core to carbon and hydrogen to He in a shell. Differences in their chemical composition affect where these stars fall on the horizontal branch. 

The strong dependence on small differences in chemical composition and other parameters like for example magnetic field strengths becomes more important toward the last stages of stellar evolution. 

Nucleosynthesis in AGB stars and its relevance for galactic enrichment and planetary system formation was reviewed by Busso, Gallino, and Wasserburg, 1999 [52]. They stressed the importance of neutron capture by a process like C(o , n) 0. This means that a carbon isotope reacts with an a particle and the result is an oxygen plus a neutron, n. 

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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. 

Fig. 4.8. Colour-magnitude (HR) diagram of the globular cluster Messier 68 with [Fe/H] —2. In order of successive evolutionary stages, MS (sd) indicates the main sequence occupied by cool subdwarfs, with the position of the Sun shown for comparison, SGB indicates the subgiant branch, RGB the red giant branch, HB the horizontal branch including a gap in the region occupied by RR Lyrae pulsating variables and AGB the asymptotic giant branch. Adapted from McClure etal. (1987).

The s process builds up an abundance distribution with peaks at mass numbers (A = Z + N) 87,138 and 208 and pronounced even-odd imbalance. The main component of the s process is associated with thermal pulsations of stars in the asymptotic giant branch (1-3 Mq) which produce neutron densities between 10 and 10 cm (Fig. 5.6). 

The s process is slow to start moving, for it is related to stars in the asymptotic giant branch. These have a maximum mass of 8 M , implying a lifetime of at least 20 million years. It is not surprising then to observe that abundances in old halo stars carry a clear r-process signature (Fig. 8.6). 

Most presolar silicon carbide and oxide grains and a significant fraction of presolar silicate grains found in meteorites come from low- to intermediate-mass stars in the asymptotic giant branch (AGB) phase (see Chapter 3). Evidence for this conclusion derives from two sources (1) spectroscopic observations of the envelopes of these stars and (2) comparison... 

Figure 3 The upper part of the HR diagram with evolutionary tracks calculated by Maeder and Meynet (1987). The branch of dots in the red part is the proposed Red Supergiant Branch. Lower to the right (at log Teff 3.4) is the uppermost part of the Asymptotic Giant Branch. The hatched area near log Teff = 3.75 is the upper part of the Cepheid branch. 

Stars belonging to the asymptotic giant branch (AGB, Gehrz et al. 1989) and supernovae (Jones et al. 1996) are considered the principal sites of cosmic dust... 

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. 

Boothroyd A. I. and Sackmann I.-J. (1988) Low-mass stars III. Low-mass stars with steady mass loss up to the asymptotic giant branch and through the final thermal pulses. Astrophys. J. 328, 653 -670. 

Busso M., GaUino R., Lambert D. L., Travagho C., and Smith V. V. (2001) Nucleosynthesis and mixing on the asymptotic giant branch III. Predicted and observed s-process abundances. J. 557, 802—821. 

The principal sources of primary N are thought to be intermediate mass stars, with masses 4 M/Mq 7, during the asymptotic giant branch (AGB) phase. A corollary of this hypothesis is that the release of N into the ISM should lag behind that of O which, as we have seen, is widely believed to be produced by massive stars which explode as Type II supernovae soon after an episode of star formation. Henry et al. (2000) calculated this time delay to be approximately 250 Myr at low metallicities the (N/O) ratio could then perhaps be used as a clock with which to measure the past rate of star formation, as proposed by Edmunds Pagel (1978). Specifically, in metal-poor galaxies which have only recently experienced a burst of star formation one may expect to find values of (N/O) below the primary plateau at (N/O) —1.5, provided the fresh Oxygen has been mixed with the ISM (Larsen, Sommer-Larsen, Pagel 2001). 

The main contributors of carbon to the interstellar medium are intermediate-mass (1-8 Mq) stars (see, for example. Wood, 1981 Yungelson et al., 1993 Timmes et al., 1995) through the asymptotic giant branch and planetary nebulae phases. A knowledge of the cosmic SFR history, together with a knowledge of the initial stellar mass function (presently still uncertain for high redshift), therefore allows for an approximate calculation of the rate of carbon production as a function of... 

Intermediate-mass stars with initial masses roughly between 3 and 8M ignite helium in a non-degenerate core, thus avoiding both the helium core flash and the core-collapse supernova fate. These stars will all evolve to the asymptotic-giant branch phase and subsequently to become CO white dwarfs. 

In low-mass stars with initial masses less than about 3 M , helium-ignition occurs in a degenerate core - usually when the helium-core mass has reached about 0.48 M . The star subsequently evolves as a horizontal-branch star (Sect. 14). With a sufficiently massive hydrogen envelope (the exact value depends strongly on metallicity), it will evolve up the asymptotic giant branch after core helium exhaustion. Otherwise it will evolve directly to the white-dwarf cooling track. 

Following core-helium exhaustion, there being no H-burning shell, the mass of the helium-rich intershell remains fixed. A few mild helium-shell ignitions may occur11, but the star is ultimately doomed not to reach the asymptotic giant branch. Instead it will contract directly to become a hybrid He/CO white dwarf. 

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... 

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