Helical Magnetic Fields in the NGC1333 IRAS 4A Protostellar Outflows

Abstract

We present Submillimeter Array (SMA) polarization observations of the CO $J$ = 3--2 line toward the NGC1333 IRAS 4A. The CO Stokes $I$ maps at an angular resolution of $\sim$1\arcsec reveal two bipolar outflows from the binary sources of the NGC 1333 IRAS 4A. The kinematic features of the CO emission can be modeled by wind-driven outflows at $\sim$ 20\arcdeg inclined from the plane of the sky. Close to the protostars the CO polarization, at an angular resolution of $\sim$2\farcs3, has a position angle approximately parallel to the magnetic field direction inferred from the dust polarizations. The CO polarization direction appears to vary smoothly from an hourglass field around the core to an arc-like morphology wrapping around the outflow, suggesting a helical structure of magnetic fields that inherits the poloidal fields at the launching point and consists of toroidal fields at a farther distance of outflow. The helical magnetic field is consistent with the theoretical expectations for launching and collimating outflows from a magnetized rotating disk. Considering that the CO polarized emission is mainly contributed from the low-velocity and low-resolution data, the helical magnetic field is likely a product of the wind-envelope interaction in the wind-driven outflows. The CO data reveal a PA of $\sim$ 30\arcdeg deflection in the outflows. The variation in the CO polarization angle seems to correlate with the deflections. We speculate that the helical magnetic field contributes to $\sim$ 10\arcdeg deflection of the outflows by means of Lorenz force.Comment: 19 pages, 9 figures, ApJ Accepte