Formation of the Neutron Donor

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 C 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 C, 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 C during the third dredge-up, when the envelope convection zone reaches deep into the core. According to the depth-dependent ratio of protons and C, a small C pocket forms underneath a N-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 envelopecore 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 C-pocket with a diffusion coefficient of logD ∼ 2 . . . 3cms. This does not only spread the C-pocket, but also the more massive N-rich layer, and finally leads to mixture of both layers. By the time when the temperature there has risen to about 9 10 K and neutron production sets in, the N abundance exceeds the C abundance by a factor of 5 . . . 10. We analyze the role of N as a neutron poison by considering the recycling of neutrons via N(n, p)C and C(p, γ)N(β)C qualitatively. We find that as long as X(N) ≪ X(C), the s-process will still be possible to occur under radiative conditions.