Multiwavelength Probes of Cosmic Ray Transport in Molecular Cloud Structures

TL;DR

This paper develops a comprehensive framework to study cosmic ray transport in molecular clouds using multi-wavelength observations, revealing enhanced scattering and ionization effects in dense gas regions.

Contribution

It introduces a self-consistent model considering three CR propagation scenarios and applies it to the Taurus cloud, linking observational signatures to CR transport physics.

Findings

CR diffusion coefficients are suppressed relative to ISM values in dense clumps.

CR ionization rates are elevated at higher densities due to increased secondary production.

A detectable hard X-ray synchrotron component may be observed with future instruments.

Abstract

We investigate how cosmic ray (CR) transport in molecular clouds and their substructures can be probed using multi-wavelength observations. The detailed microphysics regulating the penetration and coupling of CRs in dense molecular structures is unsettled. Self-generated turbulence can produce scattering and diffusive transport, while ion-neutral damping in cold, dense gas promotes ballistic CR propagation. We construct a self-consistent framework for CR transport and interactions in magnetized molecular clouds, considering three limiting propagation scenarios: ballistic transport, diffusion, and a hybrid configuration with a diffusive envelope and quasi-ballistic core. By forward-modeling pion-decay $\gamma$-ray emissivities, CR-driven ionization-rate profiles, and electron synchrotron emission in the hard X-ray band, we connect GeV attenuation and propagation signatures to independent diagnostics of secondary production and CR penetration. As an illustrative example, we apply our framework to the Taurus molecular cloud complex and selected embedded clumps. We show that CR scattering may be substantially enhanced on clump scales, with inferred CR diffusion coefficients suppressed relative to canonical interstellar medium (ISM) values at GeV energies. In this interpretation, CRs are more closely coupled with dense gas in the ISM, and a diffusive envelope boosts the effective gas column density encountered by the CRs. This increases the hadronic interaction rate in the cloud. In turn, the secondary CR electron injection is also increased, and CR ionization rates are elevated at higher densities. We show that a hard X-ray synchrotron emission component is also generated, which may be detectable with near-future facilities. Finally, we discuss how future $\gamma$-ray, X-ray, and ionization constraints will provide firm tests of CR propagation theories in molecular cloud environments.