Nucleosynthesis inside an Accretion Disk and Disk Winds Related to Gamma‐Ray Bursts

Abstract

We investigate nucleosynthesis inside both a gamma-ray burst accretion disk and a wind launched from an inner region of the disk using one-dimensional models of the disk and wind. Far from a central black hole, the composition of accreting gas is taken to be that of an Si-rich or O-rich layer of a massive star before core collapse. We find that the inner region of the disk comprises five layers characterized by dominant elements: 16O, 28Si, 54Fe (and 56Ni), 4He, and nucleons. As the accretion rate decreases, the individual layers shift inward, retaining the overall profiles of compositions. A massive amount of 56Ni, over 0.1 M☉, is ejected through the wind from the disk where the electron fraction ≃0.5. The amount of 56Ni produced through the disk wind can be responsible for the light curves observed at the late stage in hypernovae (HNe). The yields of elements heavier than Ca produced via the disk wind are comparable to or greater than those of a normal supernova and their composition is similar to that of an aspherical HN explosion. A significant amount of Ga, Ge, and Se is ejected from the disk. A variety of neutron-rich elements, as well as p-nuclei, are also appreciably produced through the disk wind.