Journal Article

Reevaluation of the Cosmic-Ray Ionization Rate in Diffuse Clouds

Caselli, P, Neufeld, D. A, Obolentseva, M, Silsbee, K, Edenhofer, G, Indriolo, N, Bisbas, T. G, Ivlev, A. V, Lomeli, D

2024

Overview

All current estimates of the cosmic-ray (CR) ionization rate rely on assessments of the gas density along the probed sight lines. Until now, these have been based on observations of different tracers, with C2 being the most widely used in diffuse molecular clouds for this purpose. However, dust extinction maps have recently reached sufficient accuracy to give an independent measurement of the gas density on parsec scales. In addition, they allow us to identify the gas clumps along each sight line, thus localizing the regions where CR ionization is probed. We reevaluate H3+ observations, which are often considered as the most reliable method to measure the H2 ionization rate ζH2 in diffuse clouds. The peak density values derived from the extinction maps for 12 analyzed sight lines turn out to be, on average, an order of magnitude lower than the previous estimates and agree with the values obtained from revised analysis of C2 data. We use the extinction maps in combination with the 3d-pdr code to self-consistently compute the H3+ and H2 abundances in the identified clumps for different values of ζH2 . For each sight line, we obtain the optimum value by comparing the simulation results with observations. We show that ζH2 is systematically reduced with respect to the earlier estimates by a factor of ≈9 on average, to ≈6 × 10−17 s−1, primarily as a result of the density reduction. We emphasize that these results have profound consequences for all available measurements of the ionization rate.

• Publisher: IOP Publishing
• Subject: Chemistry / Clumps / Cosmic ray ionization / Cosmic rays / Endangered & extinct species / Estimates / Extinction / Gas density / Hydrogen / Ionization / Molecular clouds / Space telescopes / Spacecraft / Star & galaxy formation / Tracers / Visual perception