2017ApJ...843...27L

HH 212 is a nearby (400 pc) Class 0 protostellar system recently found to host a "hamburger"-shaped dusty disk with a radius of ∼60 au, deeply embedded in an infalling-rotating flattened envelope. We have spatially resolved this envelope-disk system with the Atacama Large Millimeter/submillimeter Array at up to ∼16 au (0.''04) resolution. The envelope is detected in HCO⁺ J = 4-3 down to the dusty disk. Complex organic molecules (COMs) and doubly deuterated formaldehyde (D₂CO) are detected above and below the dusty disk within ∼40 au of the central protostar. The COMs are methanol (CH₃OH), deuterated methanol (CH₂DOH), methyl mercaptan (CH₃SH), and formamide (NH₂CHO, a prebiotic precursor). We have modeled the gas kinematics in HCO⁺ and COMs and found a centrifugal barrier (CB) at a radius of ∼44 au, within which a Keplerian rotating disk is formed. This indicates that HCO⁺ traces the infalling-rotating envelope down to the CB and COMs trace the atmosphere of a Keplerian rotating disk within the CB. The COMs are spatially resolved for the first time, both radially and vertically, in the atmosphere of a disk in the earliest, Class 0 phase of star formation. Our spatially resolved observations of COMs favor their formation in the disk rather than a rapidly infalling (warm) inner envelope. The abundances and spatial distributions of the COMs provide strong constraints on models of their formation and transport in low-mass star formation.