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 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 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 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.