Cosmic-ray propagation at small scale: a support for protostellar disc formation

TL;DR

This study investigates how cosmic-ray attenuation influences magnetic decoupling in collapsing clouds, affecting protostellar disc formation, with implications for understanding magnetic braking and ionisation in star formation.

Contribution

It introduces a formalism for cosmic-ray attenuation considering magnetic fields and applies it to star formation models, highlighting its impact on magnetic decoupling zones.

Findings

Decoupling occurs only when cosmic-ray attenuation is considered.

Large dust grains increase the size of the decoupling zone.

Turbulence causes magnetic diffusion, reducing decoupling in high-mass cases.

Abstract

As long as magnetic fields remain frozen into the gas, the magnetic braking prevents the formation of protostellar discs. This condition is subordinate to the ionisation fraction characterising the inmost parts of a collapsing cloud. The ionisation level is established by the number and the energy of the cosmic rays able to reach these regions. Adopting the method developed in our previous studies, we computed how cosmic rays are attenuated as a function of column density and magnetic field strength. We applied our formalism to low- and high-mass star formation models obtained by numerical simulations of gravitational collapse that include rotation and turbulence. In general, we found that the decoupling between gas and magnetic fields, condition allowing the collapse to go ahead, occurs only when the cosmic-ray attenuation is taken into account with respect to a calculation in which the cosmic-ray ionisation rate is kept constant. We also found that the extent of the decoupling zone also depends on the dust grain size distribution and is larger if large grains (of radius about 0.1 microns) are formed by compression and coagulation during cloud collapse. The decoupling region disappears for the high-mass case due to magnetic field diffusion that is caused by turbulence and that is not included in the low-mass models. We infer that a simultaneous study of the cosmic-ray propagation during the cloud's collapse may lead to values of the gas resistivity in the innermost few hundred AU around a forming protostar that is higher than generally assumed.