Using magnetic fields to probe the core-fragmentation model of binary formation

2022 Fiscal Year Annual Research Report

• Project/Area Number: 18K13586
• Research Institution: National Astronomical Observatory of Japan
• Principal Investigator: HULL CHARLES 国立天文台, アルマプロジェクト, 特別客員研究員 (70814755)
• Project Period (FY): 2018-04-01 – 2023-03-31
• Keywords: Astronomy

Outline of Annual Research Achievements

This grant yielded a published study that included highest resolution and highest fidelity observations of the forming binary protostellar system BHR 71. The protostar BHR 71 IRS1 features a natal, hourglass-shaped magnetic field. In contrast, the protostar IRS2 exhibits a magnetic field that has been affected by its bipolar outflow. Toward IRS2, the polarization is confined mainly to the outflow cavity walls. The origin of the enhanced polarization in IRS2 is most likely the irradiation of the outflow cavity walls, which enables the alignment of dust grains with respect to the magnetic field - but only to a depth of ∼300 au, beyond which the dust is cold and unpolarized. However, in order to align grains deep enough in the cavity walls, and to produce the high polarization fraction seen in IRS2, the aligning photons are likely to be in the mid- to far-infrared range, which implies a degree of grain growth beyond what is typically expected in very young, Class 0 sources. Furthermore, this study revealed that even binary protostars forming during a similar epoch can have very different morphologies -- this is important given that more than half of the stars in our galaxy formed in binary/multiple systems.