Effective Shielding of $\lesssim$ 10 GeV Cosmic Rays from Dense Molecular Clumps

TL;DR

This study uses gamma-ray observations to show that low-energy cosmic rays are shielded in dense molecular clumps, affecting star formation and astrochemistry by altering cosmic ray penetration models.

Contribution

It provides evidence for slow cosmic ray diffusion inside dense molecular clumps, challenging the common assumption of fast transport in such regions.

Findings

Gamma-ray deficits indicate cosmic ray shielding in dense clumps

Shielding is significant in parsec-sized dense regions, not the entire cloud

Results suggest slow diffusion of cosmic rays inside dense molecular regions

Abstract

The density of cosmic rays inside molecular clouds determines the ionization rate in the dense cores where stars form. It is also one of the drivers of astrochemistry leading to the creation of complex molecules. Through Fermi Large Area Telescope observations of nearby giant molecular clouds, we observed deficits (holes) in the gamma-ray residual map when modelling with the expected gamma-ray diffuse emission from uniform cosmic rays interacting with the molecular content. We propose that the deficit is due to the lack of penetration of the low-energy (sub-GeV to GeV) cosmic rays into denser regions or clumps. This differs from the prevailing view of fast cosmic ray transport in giant molecular clouds where the magnetic turbulence is suppressed by neutral-ion damping, as our results require a slow diffusion inside dense molecular clumps. Through modelling we find that while the shielding is negligible on the cloud scale, it becomes important in the denser, parsec-sized regions where the gravitational collapse is already at play, changing the initial condition of star formation and astrochemistry.