First Detection of a Linear Structure in the Midplane of the Young HH 211 Protostellar Disk: A Spiral Arm?

Spiral structures have been detected in evolved protostellar disks, driving the disk accretion toward the central protostars to facilitate star formation. However, it is still unclear if these structures can form earlier in young protostellar disks. With the Atacama Large Millimeter/submillimeter Array, we have detected and spatially resolved a very young and nearly edge-on dusty disk with a radius of only ~20 au in the HH 211 protostellar system at submillimeter wavelength. It is geometrically thick, indicating that the submillimeter-light-emitting dust grains have yet to settle to the midplane for planet formation. Intriguingly, it shows three bright linear structures parallel to the equatorial plane, resembling a three-layer pancake that has not been seen before. The top and bottom ones arise from the warm disk surfaces, unveiling the flared structure of the disk. More importantly, the middle one is in the dense midplane of the disk and can be modeled as a trailing spiral arm excited by disk gravity, as seen in evolved protostellar disks, supporting the presence of spiral structures in the very early phase for disk accretion.