Nucleosynthesis in the Outflow from Gamma-Ray Burst Accretion Disks

We examine the nucleosynthesis products that are produced in the outflow from rapidly accreting disks. We find that the type of element synthesis varies dramatically with the degree of neutrino trapping in the disk and therefore the accretion rate of the disk. Disks with relatively high accretion rates such as = 10 M_☉ s^-1 can produce very neutron-rich nuclei that are found in the r-process. Disks with more moderate accretion rates can produce copious amounts of nickel, as well as the light elements such as lithium and boron. Disks with lower accretion rates such as = 1 M_☉ s^-1 produce large amounts of nickel, as well as some unusual nuclei such as ⁴⁹Ti, ⁴⁵Sc, ⁶⁴Zn, and ⁹²Mo. This wide array of potential nucleosynthesis products is due to the varying influence of electron neutrinos and antineutrinos emitted from the disk on the neutron-to-proton ratio in the outflow. We use a parameterization for the outflow and discuss our results in terms of entropy and outflow acceleration.