Neutron Capture and the Production of 60-Fe in Stellar Environments

The observation of gamma rays associated with the decay of 26Al and 60Fe can provide important information regarding ongoing nucleosynthesis in our galaxy. The half-lives of these radioisotopes (7.2 x 105 y and 1.5 x 106 y, respectively) are long compared to the interval between synthesis events such as supernovae, so they build up in a steady state in the interstellar medium (centered on the galactic plane, where massive stars reside), yet short enough that gamma radiation from their decay may be detected. Additionally, these half-lifes are short compared to the period of galactic revolution, so that observable abundances remain in the proximity of their production sites. Predicted abundances of 26Al and 60Fe vary widely between several calculations in the last decade. In 2004, the first observation of the gamma ray flux from 60Fe decay was reported, with a 60Fe/26Al flux ratio in good agreement with nucleosynthesis modeling from 1995. However, recent calculations that include well motivated updates to the stellar and nuclear physics, predict a flux ratio as much as six times higher than the observed value. It is desirable to understand the discrepancy between the latest calculation, which in principle should have been more accurate, and the observation. In the present study, the uncertainties related to two key nuclear aspects of this problem, namely the neutron capture reaction rates for 59,60Fe, are investigated. New reaction rates are modeled using local systematics as opposed to the global systematics used in previous studies. Comparisons to experimental data are made whenever possible. The sensitivity of the reaction rates to various input quantities is gauged, and estimates regarding the total uncertainty in the reaction rates are made. The resulting rates and uncertainties are used in parameterized single-zone nucleosynthesis calculations using hydrodynamic conditions typical of those found in more complex stellar models. Finally, the sensitivity of the abundance of 60Fe to the reaction rates is discussed