The AMS-02 Cosmic-Ray Deuteron Flux is Consistent with a Secondary Origin

The recent measurements of cosmic-ray (CR) deuteron fluxes by AMS-02 show that the rigidity dependence of deuterons is similar with that of protons but flatter than 3 He, which has been attributed to the existence of primary deuterons with abundance much higher than that from the Big Bang nucleosynthesis. The requirement of highly deuteron-abundant sources imposes a serious challenge to modern astrophysics since there is no known process to produce a large amount of deuterons without violating other constraints. In this work we demonstrate that the fragmentation of heavy nuclei up to nickel plays a crucial role in shaping/enhancing the spectrum/flux of the CR deuterons. Based on the latest CR data, the predicted secondary fluxes of deuterons and 3 He are found to be reasonably consistent with the AMS-02 measurements, and a primary deuteron component is not needed. The observed differences between the spectra of D and 3 He, as well as those between the D/ 4 He (D/p) and 3 He/ 4 He ( 3 He/p) flux ratios, measured in the rigidity space, is probably due to the kinetic-energy-to-rigidity conversion and the solar modulation, given different charge-to-mass ratios of D and 3 He. More precise measurements of the fragmentation cross sections of various nuclei to produce deuterons, tritons, and 3 He in a wide energy range will be very helpful in further testing the secondary origin of cosmic-ray deuterons.