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AGB phase nucleosynthesis

There are four sites of nucleosynthesis identifiable in AGB stars: the hydrogen burning shell; the helium burning shell; sites of extra 13C production; and hot bottom burningb.


1) The H-burning shell:
During the interpulse phase the H-burning shell synthesizes helium and some heavier elements through the p-p chains and the CN-cycle. These elements are brought to the stellar surface continuously by the deep convective envelope.

There are two nuclides, 13C and O, produced by the CNO cycles which are important for testing stellar models using observations. As discussed in section 2.2.1, all paths in the CNO cycle lead to the overproduction of 14N. Consider the production of 13C by the reaction 12C( tex2html_wrap_inline586 ) 13N( ,beta+)13C The 13C is then destroyed by the reaction 13C( tex2html_wrap_inline586 ) 14N. A similar situation occurs for 17O which is created by 16O( tex2html_wrap_inline586 ) 17F( ,beta+)17O , but destroyed by 17O(p,alpha)14N. As a result of these mechanisms, 13C and 17O are overabundant in the less proton-dense intermediate layers of LIM stars, but destroyed in the proton-dense deeper regions. The first dredge-up transports 13C and 17O to the surface. This is observed in the drastic decrease in the 12C/13C and 16O/17O ratios of red giant stars (El Eid 1994).


2) The He-burning shell:
Remnants of the H-burning shell injected into the He-burning shell also cause it to produce s-process elements/isotopes. For instance, the injection of 13C into the He-burning shell leads to the production of neutrons by the reaction 13C(alpha,n)16O. The neutrons are then used in the slow neutron addition - the s-process. As mentioned above, once the fusion of nuclei has reached 56Fe, fusion no longer takes place and the heavier elements are made by neutron addition. There are two distinct paths this process can take: rapid neutron addition (the r-process), in which the neutron additions occur before any nuclei have time to beta-decay. In this process neutrons must be added in a fraction of a second, corresponding to neutron densities of 1023cm-3. This sort of neutron addition is expected to occur in supernovae rather than AGB stars and will not be discussed further here. The alternative is slow neutron addition or s-process, in which any beta-decay that can happen, will happen before the addition of another neutron. For all beta-decays to occur, the time interval between neutron captures must be ~104years, corresponding to neutron densities of only ~105cm-3. This process is pertinent to AGB stars. In order for the s-process to operate there must be a source of neutrons. Sources of neutrons include: 22Ne nuclei which produce neutrons by the reaction: 22Ne(alpha,n)25Mg; and also 13C nuclei produced earlier by the CN-cycle which produce neutrons by the reaction 13C(alpha,n)16O.


3) Extra 13C production:
The quantity of neutrons required to account for the s-process products cannot solely come from the above 13C from the H-burning shell (Mowlavi 1998a). This can be compensated for by the injection of protons into the C-rich areas. During the interpulse phases, when the temperature increases, the following reaction can occur: 12C( tex2html_wrap_inline586 )13N(    , beta+ nu)13C, providing another source of 13C and thus another source of neutrons for the s-process. There are other sources of neutrons for the s-process, including 22Ne in the higher mass stars.


4) Hot bottom burning:
This occurs in stars with masses Mstar > ~ 4Mtex2html_wrap_inline518 , in which the bottom of the convective envelope can reach temperatures
T > ~ 5 x 107K. This initiates H-burning within the convective envelope, which is known as envelope-burning or hot bottom burning. This process destroys 12C by the reaction 12C ( tex2html_wrap_inline586 ) 13N( ,beta+)13C and 18O by the reaction
18O(p,alpha)15N(p,alpha)12C ( tex2html_wrap_inline586 ) 13N(    , beta+ nu)13C through the CNO cycle (Boothroyd et al. 1993), which adds to the apparent high abundance of 13C around some AGB stars and yields nitrogen-rich Type I planetary nebulae (Kingsburgh & Barlow 1994).


We now have the broad outline of which elements and isotopes are expected to be produced by AGB stars, and so we can discuss the dust species expected to form in their atmospheres (chapter 5).

b Hot bottom burning occurs in the H-burning shell and is, therefore, not strictly a separate site, however it requires high temperatures and core mass to operate and therefore will be regarded as separate here


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