Multiwavelength probes of cosmic ray transport in molecular cloud structures

Ng HP, Owen ER, Tsuji N, Chen S (2026)

Publication Type: Journal article

Publication year: 2026

Journal

Publications of the Astronomical Society of Japan Universal Academy Press, Inc.

Book Volume: 78

Pages Range: 1390-1410

Journal Issue: 4

DOI: 10.1093/pasj/psag070

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 model framework for CR transport and interactions in magnetized molecular clouds, considering three limiting propagation scenarios: ballistic transport, diffusion, and a hybrid configuration featuring a diffusive envelope and quasi-ballistic core. By forward-modeling pion-decay (Formula presented) -ray emissivities, CR-driven ionization-rate profiles, and electron synchrotron emission in the hard X-ray band, we connect GeV-scale attenuation and propagation signatures to independent diagnostics of secondary production and low-energy 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 (Formula presented) -ray, X-ray, and ionization constraints will provide firm tests of CR propagation theories in molecular cloud environments.

Authors with CRIS profile

Sicheng Chen Lehrstuhl für Biophysik (Alexander von Humboldt-Professur)

Involved external institutions

University College London (UCL) GB United Kingdom (GB) Riken Institute of Physical and Chemical Research / 理化学研究所 JP Japan (JP) University of Tokyo JP Japan (JP)

How to cite

APA:

Ng, H.P., Owen, E.R., Tsuji, N., & Chen, S. (2026). Multiwavelength probes of cosmic ray transport in molecular cloud structures. Publications of the Astronomical Society of Japan, 78(4), 1390-1410. https://doi.org/10.1093/pasj/psag070

MLA:

Ng, Hayden P.H., et al. "Multiwavelength probes of cosmic ray transport in molecular cloud structures." Publications of the Astronomical Society of Japan 78.4 (2026): 1390-1410.

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