The Sulfur, Argon, and Calcium Isotopic Composition of the Galactic Cosmic Ray Source

Abstract

Galactic cosmic ray measurements of the sulfur, argon, and calcium isotopes made by the Cosmic Ray Isotope Spectrometer on the NASA Advanced Composition Explorer are reported over the energy range from 100 to 400 MeV/nucleon. The propagation of cosmic rays through the Galaxy and heliosphere is modeled with observational constraints imposed by measurements. Source abundance ratios of the sulfur, argon, and calcium isotopes are deduced from this model. Cosmic rays are thought to originate in the cores of superbubbles which contain stellar ejecta mixed with the surrounding interstellar medium. The composition of the superbubble core should reflect the composition of the cosmic rays at their source. Based on the derived isotopic source ratios of sulfur, argon, and calcium, the superbubble material at the cosmic ray source is constrained to be 18%+26%-14% supernova and wind ejecta, with the remainder interstellar medium material. This mix of metal-rich ejecta and interstellar medium in the superbubble core corresponds to a cosmic ray source metallicity of 2.7+3.9-2.1 times solar metallicity.