On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei

TL;DR

This study revisits the cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei, emphasizing the importance of grain physics and suggesting an enhancement factor around 40 over the standard value.

Contribution

It provides a detailed, self-consistent analysis of cosmic-ray ionization rates in diffuse clouds, highlighting the impact of grain physics on the inferred values.

Findings

Cosmic-ray ionization rate towards Zeta Persei is approximately 40 times the standard value.

Grain properties significantly influence the determination of ionization rates.

Full grain physics treatment is crucial for accurate interstellar chemistry models.

Abstract

The spatial distribution of the cosmic-ray flux is important in understanding the Interstellar Medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate in diffuse clouds compared to the standard value, zeta(average)=2.5e-17 s^-1, measured toward dense molecular clouds. In an earlier work we reported an enhancement in the cosmic ray rate of 20 towards HD185418. McCall et al. have reported enhancements of 48 towards zeta Persei based on the observed abundance of H$_{3}^+ while Le Petit et al. found a cosmic ray enhancement of ~10 to be consistent with their models for this same sight line. Here we revisit zeta Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of zeta deduced from the H$_{3}^+$ column density in the diffuse region of the sightline depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with a cosmic ray enhancement of 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.