2019ApJ...872..196S

We report high angular resolution observations of NGC 1333 IRAS 4A, a protostellar binary including A1 and A2, at 0.84 mm with the Atacama Large Millimeter/submillimeter Array. From the continuum observations, we suggest that the dust emission from the A1 core is optically thick, and A2 is predominantly optically thin. The A2 core, exhibiting a forest of spectral lines including complex molecules, is a well-known hot corino, as suggested by previous works. More importantly, we report, for the first time, solid evidence of complex organic molecules (COMs), including CH₃OH, ¹³CH₃OH, CH₂DOH, and CH₃CHO, associated with the A1 core seen in absorption. The absorption features mostly arise from a compact region around the continuum peak position of the A1 core. Rather than originating from a larger common envelope surrounding the protobinary, the COM features are associated with individual cores A1 and A2. Considering the signatures observed in both continuum and spectral lines, we propose two possible scenarios for IRAS 4A1: the COM absorption lines may arise from a hot-corino-like atmosphere at the surface of an optically thick circumstellar disk around A1, or the absorption may arise from different layers of a temperature-stratified, dense envelope.