Spectra of He isotopes and the ${}^3$He/${}^4$He ratio

Abstract

Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, $e^{\pm }$, $\bar{p}$, and nuclei (H-Si, S, Fe), which have resulted in a number of breakthroughs. Besides elemental spectra, AMS-02 also measures the spectra of light isotopes albeit within a smaller rigidity range. In this paper, we use the precise measurements of He isotopes and the ${}^3$He/${}^4$He ratio by AMS-02, together with Voyager 1 data, and investigate their origin. We show that there is an excess in ${}^3$He at higher energies compared to standard calculations assuming its fully secondary origin, which may indicate a source of primary ${}^3$He, or signify a change in the propagation parameters over the Galactic volume probed by the ${}^3$He/${}^4$He ratio. We provide updated local interstellar spectra (LIS) of ${}^3$He and ${}^4$He in the rigidity range from 2-40 GV. Our calculations employ the self-consistent GalProp-HelMod framework that has proved to be a reliable tool in deriving the LIS of CR $e^{-}$, $\bar{p}$, and nuclei $Z\le 28$.

30 pages, 20 figures, 18 tables, ApJ, in press