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We used the Atacama Large (sub-)Millimeter Array (ALMA) and the IRAM Plateaude Bure Interferometer (PdBI) to image, with an angular resolution of 0.5$''$(120 au) and 1$''$ (235 au), respectively, the emission from 11 differentorganic molecules in the protostellar binary NGC1333 IRAS 4A. We clearlydisentangled A1 and A2, the two protostellar cores present. For the first time,we were able to derive the column densities and fractional abundancessimultaneously for the two objects, allowing us to analyse the chemicaldifferences between them. Molecular emission from organic molecules isconcentrated exclusively in A2 even though A1 is the strongest continuumemitter. The protostellar core A2 displays typical hot corino abundances andits deconvolved size is 70 au. In contrast, the upper limits we placed onmolecular abundances for A1 are extremely low, lying about one order ofmagnitude below prestellar values. The difference in the amount of organicmolecules present in A1 and A2 ranges between one and two orders of magnitude.Our results suggest that the optical depth of dust emission at thesewavelengths is unlikely to be sufficiently high to completely hide a hot corinoin A1 similar in size to that in A2. Thus, the significant contrast inmolecular richness found between the two sources is most probably real. Weestimate that the size of a hypothetical hot corino in A1 should be less than12 au. Our results favour a scenario in which the protostar in A2 is eithermore massive and/or subject to a higher accretion rate than A1, as a result ofinhomogeneous fragmentation of the parental molecular clump. This naturallyexplains the smaller current envelope mass in A2 with respect to A1 along withits molecular richness.

Complex organics in IRAS 4A revisited with ALMA and PdBI: Striking contrast between two neighbouring protostellar cores