OB Associations, Wolf-Rayet Stars, and the Origin of Galactic Cosmic Rays

TL;DR

This study analyzes isotopic abundances in galactic cosmic rays, finding strong evidence that OB associations within superbubbles are a primary source of these rays, consistent with Wolf-Rayet star models and decay times of certain isotopes.

Contribution

It provides observational support for OB associations as the origin of galactic cosmic rays, aligning isotope ratio data with Wolf-Rayet star models and decay scenarios.

Findings

Isotopic ratios match Wolf-Rayet model predictions

GCR source is ~20% WR material and ~80% solar-like material

Decay of 59Ni supports a >10^5 yr delay between nucleosynthesis and acceleration

Abstract

We have measured the isotopic abundances of neon and a number of other species in the galactic cosmic rays (GCRs) using the Cosmic Ray Isotope Spectrometer (CRIS) aboard the ACE spacecraft. Our data are compared to recent results from two-component Wolf-Rayet (WR) models. The three largest deviations of galactic cosmic ray isotope ratios from solar-system ratios predicted by these models, 12C/16O, 22Ne/20Ne, and 58Fe/56Fe, are very close to those observed. All of the isotopic ratios that we have measured are consistent with a GCR source consisting of ~20% of WR material mixed with ~80% material with solar-system composition. Since WR stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of our data with WR models suggests that OB associations within superbubbles are the likely source of at least a substantial fraction of GCRs. In previous work it has been shown that the primary 59Ni (which decays only by electron-capture) in GCRs has decayed, indicating a time interval between nucleosynthesis and acceleration of >10^5 yr.In this paper we suggest a scenario that should allow much of the 59Ni to decay in the OB association environment and conclude that the hypothesis of the OB association origin of cosmic rays appears to be viable.