Empirical Abundance Scaling Laws and Implications for the Gamma-Process in Core-Collapse Supernovae

Analyzing the solar system abundances, we have found two empirical abundance scaling laws concerning the pand s-nuclei with the same atomic number. The first scaling is s/p ratios are almost constant over a wide range of the atomic number, where the p-nculei are lighter than the s-nuclei by two or four neutrons. The second scaling is p/p ratios are almost constant, where the second p-nuclei are lighter than the first p-nucleus by two neutrons. These scalings are a piece of evidence that most p-nuclei are dominantly synthesized by the γ-process in supernova explosions. The scalings lead to a novel concept of ”universality of γ-process” that the s/p and p/p ratios of nuclei produced by individual γprocesses are almost constant, respectively. We have calculated the ratios by γ-process based on core-collapse supernova explosion models under various astrophysical conditions and found that the scalings hold for materials produced by individual γ-processes independent of the astrophysical conditions assumed. The universality originates from three mechanisms: the shifts of the γ-process layers to keep their peak temperature, the weak s-process in pre-supernovae, and the independence of the s/p ratios of the nuclear reactions. The results further suggest an extended universality that the s/p ratios in the γ-process layers are not only constant but also centered on a specific value of 3. With this specific Kansai Photon Science Institute, Japan Atomic Energy Agency, Kizu, Kyoto 619-0215, Japan; [email protected] National Astronomical Observatory, Osawa, Mitaka, Tokyo 181-8588, Japan. Nuclear Data Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan. Department of Astronomy, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan. Department of Astronomy & Research Center for the Early Universe, School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan. Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa, Chiba 2778582, Japan.