The unreasonable weakness of r-process cosmic rays in the neutron-star-merger nucleosynthesis scenario

TL;DR

This paper argues that neutron-star-merger ejecta are unlikely to be the main source of Galactic r-process cosmic rays due to their extremely low acceleration efficiency, challenging previous assumptions about their role in nucleosynthesis.

Contribution

It provides a robust analysis showing the inefficiency of cosmic-ray acceleration in neutron-star-merger ejecta, questioning their contribution to Galactic r-process elements.

Findings

Reverse shock in neutron-star-merger ejecta is less than 0.003% efficient in converting kinetic energy to cosmic rays.

Acceleration efficiency for supernova ejecta is less than 0.1% for cosmic rays lighter than iron.

Neutron-star-mergers may not be the primary source of Galactic r-process elements.

Abstract

We reach the robust conclusion that, by combining the observed cosmic rays of r-process elements with the fact that the velocity of the neutron-star-merger ejecta is much higher than that of the supernova ejecta, either (1) the reverse shock in the neutron-star-merger ejecta is a very inefficient accelerator that converts less than 0.003% of the ejecta kinetic energy to the cosmic-ray energy or (2) the neutron star merger is not the origin of the Galactic r-process elements. We also find that the acceleration efficiency should be less than 0.1% for the reverse shock of the supernova ejecta with the observed cosmic rays lighter than the iron.