Possible solution to the triple alpha fine-tuning problem: Spallation reactions during planet formation

Abstract

Carbon is produced during the helium burning phase of sufficiently massive stars through the triple alpha process. The 0+ energy level of the carbon nucleus allows for resonant nuclear reactions, which act to greatly increase the carbon yields compared to the non-resonant case. Many authors have argued that small changes to the energy level of this resonance would lead to a significantly lower carbon abundance in the universe, and this sensitivity is often considered an example of fine-tuning. By considering spallation reactions occurring during the process of planet formation, this paper presents a partial solution to this triple alpha fine-tuning problem. Young stellar objects generate substantial luminosities of particle radiation (cosmic rays) that can drive nuclear reactions through spallation. If the standard triple alpha process is inoperative, stars tend to synthesize oxygen (and other alpha elements) rather than carbon. Cosmic rays can interact with oxygen nuclei to produce carbon while planets are forming. The resulting carbon abundances are significant, but much smaller than those observed in our universe. However, for a range of conditions – as delineated herein – spallation reactions can result in carbon-to-oxygen ratios roughly comparable to those found on Earth and thereby obviate the triple alpha fine-tuning problem.