Formation and Atmosphere of Complex Organic Molecules of the HH 212
  Protostellar Disk
  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 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 centrifugal barrier and COMs trace the atmosphere of a Keplerian
rotating disk within the centrifugal barrier. 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.
