Formation of the Neutron Donor 13C in AGB Stars Overshoot and Rotation
F. Herwig1 and N. Langer2
1Universität Potsdam, Institut für Physik, Astrophysik
2University Utrecht, Utrecht, The Netherlands
Observations of heavy elements in Red Giant stars
clearly show that low-mass AGB stars can provide a nucleosynthesis site
of the s-process. Stellar evolution models produced over the last years indicate
that radiative burning of 13C between succeeding
thermal pulses in low-mass AGB star models
may indeed provide the neutrons for the s-process.
However, although it seems clear that some mixing
between the proton-rich envelope and the carbon-rich core
may lead to the production of 13C, the physical mechanism
responsible for such mixing is not yet unambiguously identified.
We present stellar model calculations which include mixing due to
overshoot and rotation. Overshoot, with a
time-dependent and exponentially decaying efficiency, leads to a
partial mixture of protons and 12C during the third dredge-up, when
the envelope convection zone reaches deep into the core. According to
the depth-dependent ratio of protons and 12C, a small 13C pocket
forms underneath a 14N-rich layer. Overshoot
does not allow for any mixing after the envelope convection zone
retreats at the end of the third dredge-up after each pulse.
Rotation introduces mixing driven by
large angular velocity gradients which form at the envelope-core
interface in AGB stars, in particular after a thermal pulse. This
leads to partial mixing after a pulse,
as in the case of overshoot. However, both mechanisms differ
during the interpulse phase. Rotation continues to mix the
region of the 13C-pocket with a diffusion coefficient of
log D round 2 ... 3cm2 s-1. This does not only spread
the 13C-pocket, but also the more massive 14N-rich layer, and
finally leads to mixture of both layers.
By the time when the temperature there has risen to
about 9 107 K and neutron production sets in,
the 14N abundance exceeds
the 13C abundance by a factor of 5 ... 10. We analyze
the role of 14N as a neutron poison by considering
the recycling of neutrons via
and 12C(p, gamma)13N (ß+)13C qualitatively.
We find that as long as X(14N) <
X(12C), the s-process will still be possible to occur under radiative conditions.
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