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.Comment: 8 pages, CRISM 2014 conference proceeding
