Constraints on the primordial misalignment of star-disk systems

A consensus prevails with regard to star-disk systems accreting most of their mass and angular momentum during the collapse of a prestellar core. However, recent results have indicated that stars experience post-collapse or late infall, during which the star and its disk are 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 three-dimensional magnetohydrodynamical (3D MHD) 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 changes substantially throughout the accretion process, thereby extending the possibility of primordial misalignment as the source of large obliquities. In agreement with previous results that show larger contributions of late infall for increasing stellar masses, a misaligned 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 ⊙ are more likely to accrete part of their mass as single stars. Finally, we find an overall trend among our sample: the post-collapse accretion phase is more anisotropic than the early collapse phase. This result is consistent with a scenario of Bondi-Hoyle-Littletlon accretion during the post-collapse phase, while the initial collapse is less anisotropic – despite the fact that material is funneled through accretion channels.