INVESTIGATING THE COSMIC-RAY IONIZATION RATE IN THE GALACTIC DIFFUSE INTERSTELLAR MEDIUM THROUGH OBSERVATIONS OF H+3

Abstract

Observations of H + 3 in the Galactic diffuse interstellar medium have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (ζ 2 ) is about one order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H + 3 observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7 ± 1.3) × 10 −16 s −1 < ζ 2 < (10.6 ± 8.2) × 10 −16 s −1 with a mean value of ζ 2 = (3.5 +5.3 −3.0 ) × 10 −16 s −1 . Upper limits (3σ) derived from non-detections of H + 3 are as low as ζ 2 < 0.4 × 10 −16 s −1 . These low upper limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in ζ 2 between different diffuse cloud sight lines. A study of ζ 2 versus N H (total hydrogen column density) shows that the two parameters are not correlated for diffuse molecular cloud sight lines, but that the ionization rate decreases when N H increases to values typical of dense molecular clouds. Both the difference in ionization rates between diffuse and dense clouds and the variation of ζ 2 among diffuse cloud sight lines are likely the result of particle propagation effects. The lower ionization rate in dense clouds is due to the inability of low-energy (few MeV) protons to penetrate such regions, while the ionization rate in diffuse clouds is controlled by the proximity of the observed cloud to a site of particle acceleration.