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

Analyzing solar system abundances, we have found two empirical abundance scaling laws for p - and s -nuclei with the same atomic number. The first scaling law is that s / p ratios are almost constant over a wide range of atomic numbers where the p -nuclei are lighter than the s -nuclei by 2 or 4 neutrons. The second law is that p / p ratios are almost constant where the second p -nuclei are lighter than the first p -nuclei by 2 neutrons. These scalings provide evidence that most p -nuclei are dominantly synthesized by the γ-process in supernova explosions. These scalings lead to a novel concept of the "universality of the γ-process," in that the s / p and p / p ratios of nuclei produced by individual γ-processes are almost constant. We have calculated the ratios produced by the 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 shifting of the γ-process layers in order to keep their peak temperature, the weak s -process in presupernovae, and the independence of the s / p ratios from 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 value and the first scaling law, we estimate that the ratio of s -process abundance contributions from AGB stars to massive stars will be almost 6.7 for the s -nuclei of A > 90. We find that large enhancements of the s / p ratios for Ce, Er, and W are evidence that the weak s -process actually occurred before supernovae.