The phosphorus/sulfur abundance ratio as a test of galactic cosmic-ray source models

Galactic cosmic-ray (GCR) elemental abundances display a fractionation compared to solar-system values that appears ordered by atomic properties such as the first ionization potential (FIP) or condensation temperature (volatility). Determining which parameter controls the observed fractionation is crucial to distinguish between GCR origin models. The Cosmic-Ray Isotope Spectrometer (CRIS) instrument on board NASA’s Advanced Composition Explorer (ACE) spacecraft can measure the abundances of several elements that break the general correlation between FIP and volatility (e.g., Na, P, K, Cu, Zn, Ga, and Ge). Phosphorus is a particularly interesting case as it is a refractory (high condensation temperature) element with a FIP value nearly identical to that of its semi-volatile neighbor, sulfur. Using a leaky-box galactic propagation model we find that the P/S and Na/Mg ratios in the GCR source favor volatility as the controlling parameter.