The AMS-02 cosmic ray deuteron flux is consistent with a secondary origin

The recent measurements of cosmic ray deuteron fluxes by AMS-02 show that the rigidity dependence of deuterons is similar with that of protons but flatter than ³He, 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 on the modern astrophysics since there is no known process to produce a large amount of deuterons without violating other constraints {1976Natur.263..198E}. 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 cosmic ray deuterons. Based on the latest cosmic ray data, the predicted secondary fluxes of deuterons and ³He 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 ³He, as well as those between the D/⁴He (D/p) and ³He/⁴He (³He/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 ³He. More precise measurements of the fragmentation cross sections of various nuclei to produce deuterons, tritons, and ³He in a wide energy range will be very helpful in further testing the secondary origin of cosmic ray deuterons.