Protostellar disk formation and transport of angular momentum during magnetized core collapse

TL;DR

This study uses 3D MHD simulations to explore how magnetic field orientation and strength influence angular momentum transport and early disk formation during magnetized core collapse, revealing that misaligned fields reduce magnetic braking efficiency.

Contribution

It provides new insights into the effects of non-aligned magnetic fields on disk formation and angular momentum transport in collapsing cores, extending previous aligned-field studies.

Findings

Massive disks can form early even with moderate magnetic fields.

Magnetic braking dominates angular momentum transport in aligned configurations.

Disk formation is suppressed at higher magnetic field intensities.

Abstract

Theoretical studies of collapsing clouds have found that even a relatively weak magnetic field (B) may prevent the formation of disks and their fragmentation. However, most previous studies have been limited to cases where B and the rotation axis of the cloud are aligned. We study the transport of angular momentum, and its effects on disk formation, for non-aligned initial configurations and a range magnetic intensities. We perform 3D AMR MHD simulations of magnetically supercritical collapsing dense cores using the code Ramses. We compute the contributions of the processes transporting angular momentum (J), in the envelope and the region of the disk. We clearly define what could be defined as centrifugally supported disks and study their properties. At variance with earlier analyses, we show that the transport of J acts less efficiently in collapsing cores with non-aligned rotation axis and B. Analytically, this result can be understood by taking into account the bending of field lines occurring during the gravitational collapse. For the transport of J, we conclude that magnetic braking in the mean direction of B tends to dominate over both the gravitational and outflow transport of J. We find that massive disks, containing at least 10% of the initial core mass, can form during the earliest stages of star formation even for mass-to-flux ratios as small as 3 to 5 times the critical value. At higher field intensities, the early formation of massive disks is prevented. Given the ubiquity of Class I disks, and because the early formation of massive disks can take place at moderate magnetic intensities, we speculate that for stronger fields, disks will form later, when most of the envelope will have been accreted. In addition, we speculate that some observed early massive disks may actually be outflow cavities, mistaken for disks by projection effects. (Abridged version of the abstract.)