High Resolution 8 mm and 1 cm Polarization of IRAS 4A from the VLA Nascent Disk and Multiplicity (VANDAM) Survey

TL;DR

This study presents high-resolution polarization observations of IRAS 4A, revealing a circular magnetic field morphology near the protostar, indicating dust grain growth and magnetic field behavior in early star formation.

Contribution

First polarization detection of a protostar at 8-10 mm wavelengths showing a circular magnetic field morphology at high resolution.

Findings

Magnetic field is circular near the protostar, contrasting larger-scale hourglass shapes.

Dust emission is optically thin with grain growth to mm/cm sizes.

Magnetic fields are consistent with dragged-in field lines from rotating infalling material.

Abstract

Magnetic fields can regulate disk formation, accretion and jet launching. Until recently, it has been difficult to obtain high resolution observations of the magnetic fields of the youngest protostars in the critical region near the protostar. The VANDAM survey is observing all known protostars in the Perseus Molecular Cloud. Here we present the polarization data of IRAS 4A. We find that with ~ 0.2'' (50 AU) resolution at {\lambda} = 8.1 and 10.3 mm, the inferred magnetic field is consistent with a circular morphology, in marked contrast with the hourglass morphology seen on larger scales. This morphology is consistent with frozen-in field lines that were dragged in by rotating material entering the infall region. The field morphology is reminiscent of rotating circumstellar material near the protostar. This is the first polarization detection of a protostar at these wavelengths. We conclude from our observations that the dust emission is optically thin with {\beta} ~ 1.3, suggesting that mm/cm-sized grains have grown and survived in the short lifetime of the protostar.