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, 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.