Asymptotic giant branch

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. 

Fig. la shows the abundance ratio [Ba/Fe] for this sample as a function of [C/Fe]. Thirty stars (77% of the sample) have [Ba/Fe] > +0.7, while the others have [Ba/Fe] < 0.0. There is a clear gap in the Ba abundances between the two groups, suggesting at least two different origins of the carbon excesses. Ba-enhanced stars The Ba-enhanced stars exhibit a correlation between the Ba and C abundance ratios (Fig. la). This fact suggests that carbon was enriched in the same site as Ba. The Ba excesses in these objects presumably originated from the s-process, rather than the r-process, because (1) nine stars in this group for which detailed abundance analysis is available clearly show abundance patterns associated with the s-process [2], and (2) there is no evidence of an r-process excess in the other 21 objects. Hence, the carbon enrichment in these objects most likely arises from Asymptotic Giant Branch (AGB) stars, which are also the source of the s-process elements. 

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. 

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).

Arlandini C, Kappeler F, Wisshak K, Gallino R, Lugaro M, Busso M, Staniero O (1999) Neutron capture in low-mass asymptotic giant branch statrs cross sections and abundance signatures. Astrophys J 525 886-900... 

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 main s process synthesises neufron-rich nuclei with atomic number A greater than 100 and occurs in asymptotic giant branch (AGB)-type red giant stars undergoing thermal palpitations. 

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). 

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. 

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... 

Iben, L, Jr. and Renzini, A. (1983) Asymptotic giant branch evolution and beyond. Annual Reviews of Astronomy and Astrophysics, 21, 271-342. 

Smith, V. V. and Lambert, D. L. (1990) The chemical composition of red giants. III. Further CNO isotopic and -process abundances in thermally pulsing asymptotic giant branch stars. Astrophysical Journal Supplement, 72, 387—416. 

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. 

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