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

Abstract

Context. It is still largely debated whether magnetic fields play a key role in dynamically shaping the products of the star formation process. For example, in magnetized protostellar formation models, magnetic braking plays a major role in the regulation of the angular momentum transported from large envelope scales to the inner envelope, and is expected to be responsible for the resulting protostellar disk sizes. However, non-ideal magnetohydrodynamic effects that rule the coupling of the magnetic field to the gas also depend heavily on the local physical conditions, such as the ionization fraction of the gas. Aims. The purpose of this work is to observationally characterize the level of ionization of the 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. Methods. We have obtained molecular line emission maps of B335 with ALMA, which we use to measure the deuteration fraction of the gas, R D , its ionization fraction, χ e , and the cosmic-ray ionization rate, ζ, at envelope radii 1000 au. Results. We find large fractions of ionized gas, χ e 1 − 8 × 10 −6 . Our observations also reveal an enhanced ionization that increases at small envelope radii, reaching values up to ζ 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. Conclusions. We report the first resolved map of the cosmic-ray ionization rate at scales 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. We stress that the gas ionization we find in B335 significantly stands out from the typical values routinely used in state-of-the-art models of protostellar formation and evolution. If the local processes of ionization uncovered in B335 are prototypical to low-mass protostars, our results are calling for revising the treatment of ionizing processes in magnetized models for star and disk formation.

Recent progress with observations and models to characterize the magnetic fields from star-forming cores to protostellar disks

“…Such high values seem inconsistent with the fiducial value if the interstellar cosmic rays flux is responsible for the gas ionization, as it should be efficiently attenuated while penetrating into the dense cores (Padovani et al, 2018). The observations of Cabedo et al (2022) also show the CRs ionization rate is increasing at small envelope radii, toward the central protostellar embryo. Several theoretical works have investigated the role of shocks at the protostellar surface and magnetic mirroring within the jets as efficient forges to accelerate locally low-energy cosmic rays, and their role in increasing the ionization rate of the shielded protostellar material (Padovani et al, 2015;Silsbee et al, 2018;Fitz Axen et al, 2021;Padovani et al, 2021).…”

“…The role of the magnetic field in setting the disk size in B335 has also been discussed in subsequent studies (Bjerkeli et al, 2019;Imai et al, 2019;Yen et al, 2019). However, new observations by Cabedo et al (2022) suggest that the gas at small envelope radii is strongly ionized in B335 and points toward conditions typical of ideal MHD. It may very well be that the star-disk building phase consists in a succession of non-ideal and ideal MHD conditions in the "life of the protostar."…”

“…Caveats: the expected velocity drift is very difficult to measure at small scales typical of inner envelopes, as the velocity offset is expected to be of the order of the best spectral resolution currently available from typical instruments, and the velocity field is complex (Yen et al, 2018;Cabedo et al, 2022). The main limitation of this method is to ensure that the selected neutral and ion species trace the same gas or are affected by optical depth effects (Pratap et al, 1997;Jørgensen et al, 2004;Girart et al, 2005;Zinchenko et al, 2009).…”

“…Such low values may suggest it is unlikely that the accretion through the disk to the central star could be driven by magneto-rotational instabilities. In Class 0 protostar B335, Cabedo et al (2022) used deuteration detected in molecular line emission maps to characterize the ionization of the gas at envelope radii ≲ 500 au (typical densities n H_{2} ~10 6 cm −3 ) and found a large cosmic ray ionization rate ζ between 10 -16 and 10 -14 s −1 . Such high values seem inconsistent with the fiducial value if the interstellar cosmic rays flux is responsible for the gas ionization, as it should be efficiently attenuated while penetrating into the dense cores (Padovani et al, 2018).…”

Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration

“…Finally, note that the models analyzed here implement nonideal MHD processes under ionization conditions that may not be prototypical at the smallest scales around protostars: for example, Cabedo et al (2022) estimated cosmic-ray ionization rates in B335 from observations probing the ∼ 100 − 500 au scales which are up to two orders of magnitude higher than the predicted ionization generated by external cosmic rays alone. If the ionization was indeed to be much stronger at small envelope radii due to locally accelerated cosmic rays, that would make the coupling of B fields to the local gas much more efficient and models that implement both ideal and nonideal MHD with realistic local ionization conditions would need to be explored in the future.…”