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

TL;DR

This study uses ALMA observations to measure gas ionization and cosmic-ray ionization rates in the B335 protostar, revealing high local ionization likely due to cosmic-ray acceleration near the protostar, impacting magnetic coupling during star formation.

Contribution

First resolved map of cosmic-ray ionization rate at scales less than 1000 au in a Class 0 protostar, highlighting local cosmic-ray acceleration effects.

Findings

High ionization fractions of 1-8 x 10^-6 in the gas.

Enhanced ionization rates up to 10^-14 s^-1 near the protostar.

Local cosmic-ray acceleration likely influences magnetic coupling.

Abstract

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