Application of the Calculating Formula for the Mean Neutron Exposure in CEPM-s and CEPM-r/s Starstwo

Abstract

Recent studies have shown that for the current s-process nucleosynthesis models of low-mass asymptotic giant branch (AGB) stars with a radiatively burning C13 pocket in the interpulse period, the neutron exposure distribution in the nucleosynthesis region can be regarded as an exponential function, and the relation between the mean neutron exposure τ0 and the model parameters is τ0=−Δτ/ln[q/(1−r+q)], in which Δτ is the exposure value of each neutron irradiation, r is the overlap factor, and q is the mass ratio of the C13 pocket to the He intershell. Using the published data resulted from fitting the observed abundances of neutron-capture elements in 20 CEMP (Carbon-Enhanced Metal-Poor)-s and CEMP-s/r stars with the parameterized s-process nucleosynthesis model of AGB stars, the reliability of the formula for calculating the mean neutron exposure is tested, and further more the application of the formula in the study of heavy-element s-process nucleosynthesis is explored preliminarily. Our results show that under the radiative s-process nucleosynthesis mechanism, the formula is suitable for the CEMP stars experienced multiple successive neutron irradiations. Combined with the parameterized AGB star nucleosynthesis model, the formula could be regarded as an effective tool to select the CEMP stars of single neutron exposure or special type. But considered the uncertainty of the C13 pocket, the role of this formula in understanding the physical conditions necessary for reproducing the observed abundances of neutron-capture elements in CEMP stars needs further studies.