The Superbubble Origin of²²Ne in Cosmic Rays

Abstract

We investigate a superbubble origin for the well-known anomalous 22Ne/20Ne ratio found (Binns and coworkers) for the sources of cosmic rays, which is 5 times the solar wind value. We calculate self-consistently the neon isotopes synthesized by massive stars residing in cores of superbubbles. Our model yields, based on the recent nucleosynthetic calculations, depend on two parameters: the limiting mass for Type II supernovae (SNe II) above which single massive stars lose their hydrogen envelopes and explode as Type Ib and Ic supernovae (SNe Ibc), and the cutoff mass above which stellar collapse creates black holes without supernova explosions. We then model the mean 22Ne/20Ne ratio in superbubble cores, resulting from the dispersal of this newly synthesized neon ejected in the metal-rich winds and supernova explosions together with the debris of older interstellar medium within the superbubbles. We characterize this dispersal in terms of the mean superbubble metallicity, Zsb, the elemental mass fraction heavier than helium. Finally, we determine the expected 22Ne/20Ne ratio in the local cosmic rays, based on observations of the relative fractions of supernovae occurring in superbubbles and other phases of the interstellar medium. Considering all of the uncertainties, we find that the cosmic-ray source abundance ratio of 22Ne/20Ne can be easily understood as the result of cosmic rays accelerated primarily out of superbubble cores with a mean metallicity Zsb between 2.3 and 3.1 Z☉. This metallicity, which corresponds to a mean wind and ejecta mass fraction of between 13% and 23%, is quite consistent with values of the mean superbubble core metallicity inferred from other observations and provides additional evidence for a superbubble origin of the bulk of the cosmic rays.