Poster Presentation

The Rotation Dip in the Envelope-Disk Transition of HH 111: Evidence for Magnetic Braking

Author(s): Jyun-Heng Lin （NTNU/ASIAA)

Presenter: Jyun-Heng Lin (NTNU/ASIAA)

Magnetic braking can drive angular momentum loss in star formation, influencing the evolution and size of accretion disks around protostars. A previous study has shown a sign of this effect in the HH 111 VLA1 protostellar system, finding a decrease in rotation velocity in the transition region between the infalling envelope and rotating disk. Using observations from the Atacama Large Millimeter/submillimeter Array, we have analyzed the C$^{18}$O ($J = 2-1$) gas kinematics in this system within 4000 au of the central protostar. By modeling the observed kinematics, we find that the transition region (from $\sim$2000 to 160 au) can be further divided into 3 regions: (1) outer region with a significant decrease in infall velocity, dropping to approximately 30\% of the free-fall velocity; (2) middle region with a sharp drop in angular momentum and thus rotation velocity and an increase in infall velocity; and (3) inner region with rotation velocity increasing inward to connect to that of the disk and infall velocity decreasing to zero. Comparison with non-ideal MHD simulation of collapsing cores suggests that the reduced infall velocity in the outer region can be due to magnetic tension by the pinched magnetic field lines, the sharp drop of angular momentum in the middle region can be due to magnetic braking as the field lines pile up, and the rapid increase in rotation velocity in the inner region can be because of less magnetic braking due to ambipolar diffusion of the field lines. As a result, a dip appears in rotation profile between the middle and inner regions, which can be considered evidence of magnetic braking.