Unveiling Dominant Toroidal Magnetic Fields in a Protostellar Outflow

TL;DR

This study presents polarization observations revealing toroidal magnetic fields in a protostellar outflow, supporting magneto-centrifugal models of wind launching and collimation.

Contribution

First observational evidence of toroidal magnetic fields in a protostellar outflow, constraining magnetic field strength and supporting magneto-centrifugal wind models.

Findings

Magnetic fields are perpendicular to the outflow axis and aligned with the rotational structure.

Toroidal magnetic fields have strengths of a few milligauss.

A linear correlation exists between magnetic field curl and electric current density.

Abstract

Magnetic fields play a fundamental role in the formation of protostellar winds. In the magneto-centrifugal models, poloidal magnetic fields launch winds from accretion disks, and fast-rotating gas twists the fields into toroidal geometry that collimates and accelerates winds through magnetic hoop stress. However, toroidal fields in protostellar winds remain observationally unresolved. Here we report polarization observations of carbon monoxide emission 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, indicating toroidal fields with strengths of a few milligauss, sufficient to collimate and accelerate the outflow at several hundred astronomical units from the protostar. A linear correlation is found between the curl of plane-of-the-sky magnetic field and the line-of-sight electric current density. Our analysis provides better constraints on ion-electron drift velocity in protostellar outflows and supports rotating outflows driven by the magneto-centrifugal mechanism.