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In the early phase of star formation, protostellar disks are formed to grow the central protostars into stars. The disks will later evolve into protoplanerary disks to form planets like our own ones. Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), scientists have spatially resolved the dust polarization substructure in a protostellar disk recently found to host a pair of spiral arms and discovered an anti-correlation of the polarized intensity with the spiral arms. This anti-correlation can be attributed to the self-scattering of the dust emission in the disk by the dust grains that have grown to a size of about 150 um, supporting an early grain growth in the early phase of star formation. The disk is located at the center of the HH 111 protostellar system, lying in Orion about 1300 light-years away. It has an age of about 0.5 million years and a radius of about 160 au, which is about 5 times the orbital radius of Neptune. The two spiral arms in the disk show that material is actively moving toward the center where the young star is forming. These spiral arms help scientists understand the dust polarization mechanism because different mechanisms affect the polarization in different ways. In particular, the dust polarization due to dust self-scattering is affected by asymmetry of light incoming from various directions, while the dust polarization due to the magnetically aligned grains is not. “We are so excited to have found an anti-correlation of the polarized intensity of the dust emission with the spiral arms in the HH 111 disk and been able to attribute it to the self-scattering of the dust emission by the dust grains. The material in between arms is sandwiched and illuminated by two brighter spiral arms and thus has higher polarized intensity. Our finding supports an early grain growth in the disk, opening up a window of using dust polarization to study the grain growth in the disks with substructures (e.g., spiral arms and rings),” commented Chin-Fei Lee, who is the lead author of this work. “This early grain growth can ultimately lead to an early planet formation.”

Chin-Fei Lee et al. (2024), ApJ, 971, L23