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Non-ideal magnetohydrodynamic effects that rule the coupling of the magneticfield to the circumstellar gas during the low-mass star formation processdepend heavily on the local physical conditions, such as the ionizationfraction of the gas. The purpose of this work is to observationallycharacterize the level of ionization of the circumstellar gas at small enveloperadii and investigate its relation to the efficiency of the coupling betweenthe star-forming gas and the magnetic field in the Class 0 protostar B335. Wehave obtained molecular line emission maps of B335 with ALMA, which we use tomeasure the deuteration fraction of the gas, its ionization fraction, and thecosmic-ray ionization rate, at envelope radii $\lesssim$1000 au. We find largefractions of ionized gas, $\chi_{e} \simeq 1-8 \times 10^{-6}$. Ourobservations also reveal an enhanced ionization that increases at smallenvelope radii, reaching values up to $\zeta_{CR} \simeq 10^{-14}$~s$^{-1}$ ata few hundred au from the central protostellar object. We show that thisextreme ionization rate can be attributed to the presence of cosmic raysaccelerated close to the protostar. We report the first resolved map of thecosmic-ray ionization rate at scales $\lesssim 1000$~au in a solar-type Class 0protostar, finding remarkably high values. Our observations suggest that localacceleration of cosmic rays, and not the penetration of interstellar Galacticcosmic rays, may be responsible for the gas ionization in the inner envelope,potentially down to disk forming scales. If confirmed, our findings imply thatprotostellar disk properties may also be determined by local processes settingthe coupling between the gas and the magnetic field, and not only by the amountof angular momentum available at large envelope scales and the magnetic fieldstrength in protostellar cores.

Magnetically regulated collapse in the B335 protostar? II. Observational constraints on gas ionization and magnetic field coupling