Probing the Inner 200 AU of Low-Mass Protostars with the Submillimeter Array: Dust and Organic Molecules in NGC 1333 IRAS 2A

The Submillimeter Array has opened a new window to study the innermost warm and dense regions of the envelopes and disks around deeply embedded protostars. This paper presents high angular resolution (<2'') submillimeter observations of the Class 0 young stellar object NGC 1333 IRAS 2A. Dust continuum emission and lines of complex organic molecules such as CH3OCH3 and CH3OCHO, high-excitation CH3OH transitions, and deuterated methanol, CH3OD, as well as lines of CO, HCN, H13CN, SO, and SO2, are detected on ≲200 AU scales. The observations are interpreted using detailed radiative transfer models of the physical and chemical structure, consistent with both single-dish and interferometer data. The continuum emission is explained by an extended envelope and a compact but resolved component, presumably a circumstellar disk with a diameter of 200-300 AU and a mass of ~a few times 0.01-0.1 M☉. If it is related to the rotation of the envelope, then the size of this disk suggests a centrifugal barrier of 200-300 AU, which implies that the temperature in the envelope does not increase above 100 K. Its large size also suggests that the buildup of disks proceeds rapidly throughout the early protostellar stages. The smaller (<100 AU) disks found around other deeply embedded protostars may be a result of tidal truncation. The high-resolution observations of SO can be explained with a simple constant abundance, ~10-9, constrained through single-dish observations, whereas those of H13CN and the organic species require high abundances, increased by 1-2 orders of magnitude, or an additional compact source of emission at small scales. The compact molecular emission could originate in a hot-core region of the inner envelope, but a more likely reservoir is the circumstellar disk.