Soft monopole mode in weakly bound light nuclei

A low-energy peak appears in the monopole strength functions of light nuclei composed of two weakly bound clusters. It exhausts a significant part of the soft-mode sum rules associated with the relative motion of the clusters, and a non-negligible part of the total sum rules. The peak properties mainly depend on the binding energy of the clusters. This effect is studied both in a simple potential model and in a multichannel microscopic model for nuclei of interest in astrophysics: 7Li, 7Be, 8Li, 8B and 12N. The peak value of the 8B monopoles strength function is related to the astrophysical S-factor for the 7Be(p, γ)8B reaction at zero energy. (orig.)