Third DredgeUp and Tpagb Nucleosynthesis

Figure 26.15 shows that after the disappearance of the intershell convective zone, the external convective zone (CZ) may deepen again (this is a critical point depending on convective parameters and mixing). It reaches layers which were covered by the convective tongue and contains the products of He burning. The outer convec-tive envelope, if it reaches these layers, brings the new synthesized elements to the surface: this is the third dredge-up. The efficiency of the third dredge-up is often measured by a parameter X:

. deepening of CZ after third dredge-up 4MTDU

AM He core in interpulse phase 4MHe i.e., the ratio of the mass reduction of the H-exhausted core due to the third dredge-up to the mass increase of the H-exhausted core during the interpulse. For X = 0 there is no dredge-up and for X = 1 there is no net growth of the H-free core. It is remarkable that new elements appear immediately at the the surface. A proof is that technetium 99Tc is observed in AGB stars [587]. This element is radioactive with a half-life of 2.1 x 105 yr, which is shorter than the stellar lifetime. Thus 99Tc has been synthesized locally.

If the idea of the third dredge-up looks simple in principle, the models have difficulties to simulate this phase of mixing. First, the opacity in the intershell is electron scattering, i.e., a weak opacity which does not favor convection. In addition, there is a ^ gradient, which does not favor the inward penetration of the convective envelope. Different choices of the convective parameters help the authors to get the third dredge-up, e.g., a larger mixing length. Also, overshoot below the envelope favors the third dredge-up [248], which is more efficient at lower Z. The degree of overlap between successive convective tongues is uncertain and also depends on mixing (Sect. 26.7).

AGB stars make a rich nucleosynthesis in different sites [191, 434].

1. H-burning shell: temperature of H burning in AGB stars is higher than in MS stars (Fig. 26.18); it is between T6 = 40 and 95, being higher for higher M and lower Z. At this T, the ON loops are working; however as the very thin H shell advances fast, the ON loops do not reach equilibrium, the same for the NeNa and MgAl cycles (Sect. 25.1.5). The CN cycle (which is at equilibrium) produces 4He and 14N with some 13C, while the NeNa cycle produces mainly 23Na and 22Ne. The products of the MgAl cycle depend on T. For T6 = 55 - 75, the radionuclide 26Alg is largely produced. The half-life t1/2 = 7.1 x 105 yr of this isotope is shorter than the duration of the TP-AGB phase for stars with M > 3 M0. Thus, it accumulates without being too much destroyed before being ejected. At higher T, less 26Alg is produced, because it is partially destroyed by proton captures (Fig. 25.4).

2. He-burning in shell and in TP, s-elements: during the interpulse, the energy production by the He shell is negligible with respect to that by the H shell. T at the base of the He shell is according to the masses between T8 = 1.2 and 2.1. Some reactions nevertheless go on, like the destruction of 13C by 13C(a,n)16O in the He shell and intershell region.

He burning during the thermal pulses occurs at T8 = 2.3-3.9; thus it is orders of magnitude more powerful; in addition He is brought by convection into the most active burning layers. The main product is 12C; it is transported by the convective tongue close to the H shell, where successive third dredges-up bring it to the stellar surface. For a C/O ratio > 1, the AGB star first observed as an M giant is turned into a C star. During the pulses, the H-burning products (4He, 14N, 13C, etc.) sitting in the intershell are suddenly mixed into the He-burning tongue producing reactions such as

13C(a, n)16O 22Ne(a, y)26Mg 19C(a, p)22Ne 14N(a, y)18O 18O(a, y)22Ne 16O(a,y)20Ne 22Ne(a, n)25Mg

The injection of 13C into the He-burning region leads to the production of neutrons. In the hottest pulses, neutron production by 22Ne also occurs. Free neutrons lead to a most interesting nucleosynthesis in TP-AGB stars [81, 180]: a fraction of them is captured by elements of the Fe peak. In TP-AGB stars, the neutron flux is low enough so that the new nuclei formed by neutron capture have the time, if unstable, to ¡5- disintegrate before a new neutron is captured. This succession of n captures followed by ¡5- decays leads to heavy nuclei close to the valley of stability in the plane (N,Z). The heavy elements formed in this way (for example, Y, Sr, Zr, Ba, La, Nd, Tc and up to 208Pb) are the "s-elements", formed by slow captures of neutrons, i.e., the neutron flux is low (the average time interval between two n captures by a nucleus is ~ 10 yr). The physics of the s-element synthesis is discussed in Sect. 28.5.4. About 70% of the heavy nuclei beyond 56Fe are s-elements produced by the s process, either in AGB stars or to a lesser extent in massive stars.

There is, however, a problem: there is not enough 13C in the intershell to produce the observed amount of s-elements. Typically, the mass fractions and number ratios of 13C and 56Fe are

Thus, less than one neutron is available per 56Fe seed. The rest is captured by (n,p) and (n,y) reactions onto "poisons" for neutrons, such as 14N and other elements in the range of 12C to 30Si. There is not enough neutrons to form the s-elements. Thus, more 13C should be formed inside AGB stars.

Some of the neutrons from 13C lead to the formation of protons: 14N(n,p)14C and 26Al«(n,p)26Mg —► 18O(p,a)15N(a, y)19F. This leads to the formation of fluorine 19F. These reactions are very sensitive to T, and 19F is also destroyed by further reactions (n,y) and (a,p). The amount of 13C is also insufficient [434] to account for the observed fluorine.

3. H diffusion: some additional 13C may originate [434] from partial mixing of protons of the H-burning shell into the intershell, which is enriched in 12C by the previous convective tongue. This occurs just after the death of the convective tongue.

4. Hot bottom burning: in stars with initial masses > 4 [email protected] (high-mass AGB stars), T at the basis of the convective envelope becomes above T6 = 50 (Fig. 26.18), so that H burning and related reactions may occur. This H burning is called the hot bottom burning (HBB); the fusion layers are in contact with a huge reservoir of inactive matter and the reaction products immediately appear at the surface. Characteristic reactions are the building of 14N, the destructions of 12C (with 12C/13C « 3), 15N (with 14N/15N « 104) and 18O. Also, fluorine is destroyed by 19F(p,a)16O. HBB may also be an efficient site for the production of 26Al through the MgAl cycle.

The HBB by coupling fusion and convective mixing may also be a unique site for 7Li production by the Cameron-Fowler process [84, 192]. The reaction 3He(a, y)7Be of the ppll chain (Table 25.1) produces 7Be. Normally, this element is rapidly destroyed by proton capture (leading to 8B) and to a less extent by e- capture (leading to 7Li which would rapidly be destroyed). However, convection with a turnover time of a few 102 days has the time to evacuate 7Be toward surface layers, where e- captures which are little dependent on T turn it into 7Li. The surface abundance of 7Li may increase in a spectacular way, because of the relatively high abundance of 3He. However, this enhancement only lasts as long as lithium is effectively produced, because the convective turnover brings it back to the bottom of the convective zone where it is rapidly destroyed. As mass loss is high in these stages, the HBB synthesis may lead to a significant galactic 7Li enrichment.

X(13C) = 9 x 1(T5, X(56Fe) = 6 x 10~4 0.5 ^ 1. (26.21)

H-burning temperature Models at [Fe/H]=-1.6

- Main Sequence .

□ Red Giant - Tip _ O Red Giant - Clump . ■ AGB - Bottom of _ • Convective Envelope .

H-burning temperature Models at [Fe/H]=-1.6

Fig. 26.18 The different T at which H can burn in stars: MS shell H burning, hot bottom burning, etc. The dotted lines show the minimum T for accounting for the observed O, Na, Mg, Al abundances in globular clusters. The thick gray line is the same for H burning in convective zones. Courtesy of C. Charbonnel

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