Alignment of Protostars and Circumstellar Disks During the Embedded Phase

TL;DR

This paper investigates how gravitational coupling during the embedded phase of star formation aligns protostars and circumstellar disks, reducing misalignments and influencing outflow dynamics, which impacts the stellar initial mass function.

Contribution

It demonstrates that gravitational coupling during the embedded phase suppresses star-disk misalignment, affecting outflow behavior and the stellar initial mass function.

Findings

Gravitational coupling effectively suppresses star-disk misalignment.

Star-disk joint tilting causes wandering of bipolar outflows.

Outflow wandering enhances envelope clearing efficiency.

Abstract

Star formation proceeds via the collapse of a molecular cloud core over multiple dynamical timescales. Turbulence within cores results in a spatially non-uniform angular momentum of the cloud, causing a stochastic variation in orientation of the disk forming from the collapsing material. In the absence of star-disk angular momentum coupling, such disk-tilting would provide a natural mechanism for production of primordial spin-orbit misalignments in the resulting planetary systems. However, owing to high accretion rates in the embedded phase of star formation, the inner edge of the circumstellar disk extends down to the stellar surface, resulting in efficient gravitational and accretional angular momentum transfer between the star and the disk. Here, we demonstrate that the resulting gravitational coupling is sufficient to suppress any significant star-disk misalignment, with accretion playing a secondary role. The joint tilting of the star-disk system leads to a stochastic wandering of star-aligned bipolar outflows. Such wandering widens the effective opening angle of stellar outflows, allowing for more efficient clearing of the remainder of the protostar's gaseous envelope. Accordingly, the processes described in this work provide an additional mechanism responsible for sculpting the stellar Initial Mass Function (IMF).