2022ApJ...935..136O

Resolving physical and chemical structures in the vicinity of a protostar is of fundamental importance for elucidating their evolution to a planetary system. In this context, we have conducted 1.2 mm observations toward the low-mass protostellar source B335 at a resolution of 0.″03 with the Atacama Large Millimeter/submillimeter Array. More than 20 molecular species including HCOOH, NH₂CHO, HNCO, CH₃OH, CH₂DOH, CHD₂OH, and CH₃OD are detected within a few tens au around the continuum peak. We find a systematic chemical differentiation between oxygen-bearing and nitrogen-bearing organic molecules by using the principal component analysis for the image cube data. The distributions of the nitrogen-bearing molecules are more compact than those of the oxygen-bearing ones except for HCOOH. The temperature distribution of the disk/envelope system is revealed by a multiline analysis for each of HCOOH, NH₂CHO, CH₃OH, and CH₂DOH. The rotation temperatures of CH₃OH and CH₂DOH at the radius of 0.″06 along the envelope direction are derived to be 150-165 K. On the other hand, those of HCOOH and NH₂CHO, which have a smaller distribution, are 75-112 K, and are significantly lower than those for CH₃OH and CH₂DOH. This means that the outer envelope traced by CH₃OH and CH₂DOH is heated by additional mechanisms rather than protostellar heating. We here propose the accretion shock as the heating mechanism. The chemical differentiation and the temperature structure on a scale of a few au provide us with key information to further understand chemical processes in protostellar sources.