Chemical Differentiation and Temperature Distribution on a Few au Scale around the Protostellar Source B335

Abstract 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, NH 2 CHO, HNCO, CH 3 OH, CH 2 DOH, CHD 2 OH, and CH 3 OD 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, NH 2 CHO, CH 3 OH, and CH 2 DOH. The rotation temperatures of CH 3 OH and CH 2 DOH 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 NH 2 CHO, which have a smaller distribution, are 75–112 K, and are significantly lower than those for CH 3 OH and CH 2 DOH. This means that the outer envelope traced by CH 3 OH and CH 2 DOH 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.