Poster Presentation

Unveiling Toroidal Magnetic Fields in a Protostellar Outflow

Author(s): Tao-Chung Ching (NRAO), Shih-Ping Lai (NTHU)

Presenter: Shih-Ping Lai (National Tsing Hua University)

Magnetic fields play a fundamental role in the formation of protostellar winds. In the magneto-centrifugal models, poloidal fields are required to launch winds from accretion disks, and the fast-rotating gas twists the fields into toroidal geometry. Due to the hoop stress of the toroidal fields, winds are self-collimated and accelerated to a high velocity. However, there are still no observations convincingly resolving the magnetic fields in the winds. Here we report the ALMA observations of the CO J = 2–1 linear polarization toward the NGC1333 IRAS 4A protostellar outflow. The inferred magnetic fields are perpendicular to the outflow axis and aligned with the rotational structure of the outflow, in agreement with the theoretical picture of a protostellar outflow wrapping by toroidal magnetic fields. The field strengths are about a few mG at a distance of 400 astronomical units from the star, strong enough to accelerate and collimate the outflow. Based on the Ampere law, a linear correlation is found between the curl of plane-of-sky magnetic field inferred from CO polarization and the line-of-sight electric current inferred from CO total emission. The linear correlation suggests that the drift velocity between ions and electrons must be remarkably constant a few m/s across the outflow, providing a constraint for outflow simulations. In addition, the right-hand rule provides a potential method to determine the plane-of-sky magnetic field direction which is not available from dust polarization or synchrotron emission.