Constraints on the (re-)orientation of star-disk systems through infall

TL;DR

This study explores how late infall and accretion streamers influence the orientation and angular momentum of star-disk systems, challenging the traditional view that their rotational axes are solely inherited from core collapse.

Contribution

It demonstrates that post-collapse infall can significantly alter star-disk orientations and highlights the role of anisotropic accretion in the evolution of star systems, based on 3D MHD simulations.

Findings

Infall can substantially change star-disk system orientation.

Higher mass stars experience more prolonged orientation changes.

Post-collapse accretion is more anisotropic than initial collapse.

Abstract

It has been consensus that star-disk systems accrete most of their mass and angular momentum during the collapse of a prestellar core, such that the rotational direction of a system is equivalent to the net rotation of the core. Recent results, however, indicate that stars experience post-collapse or late infall, during which the star and its disk is refreshed with material from the protostellar environment through accretion streamers. Apart from adding mass to the star-disk system, infall potentially supplies a substantial amount of angular momentum as the infalling material is initially not bound to the collapsing prestellar core. We investigate the orientation of infall on star-disk systems by analyzing the properties of accreting tracer particles in 3D magnetohydrodynamical simulations of a molecular cloud that is (4 pc)$^3$ in volume. In contrast to the traditional picture, where the rotational axis is inherited from the collapse of a coherent pre-stellar core, the orientation of star-disk systems can change substantially during the accretion process. In agreement with previous results that show larger contributions of late infall for increasing stellar masses, infall is more likely to lead to a prolonged change in orientation for stars of higher final mass. On average, brown dwarfs and very low mass stars are more likely to form and accrete all of their mass as part of a multiple system, while stars with final masses above a few 0.1 M$_{\odot}$ are more likely to accrete part of their mass as single stars. Finally, we find an overall trend: the post-collapse accretion phase is more anisotropic than the early collapse phase. This result is consistent with a scenario, where mass accretion from infall occurs via infalling streamers along a preferred direction, while the initial collapse is less anisotropic albeit the fact that material is funneled through accretion channels.