2024ApJ...970...28O

Deuterium fractionation in the closest vicinity of a protostar is important in understanding its potential heritage to a planetary system. Here, we have detected the spectral line emission of CH₃OH and its three deuterated species, CH₂DOH, CHD₂OH, and CH₃OD, toward the low-mass protostellar source B335 at a resolution of 0.″03 (5 au) with Atacama Large Millimeter/submillimeter Array. They have a ring distribution within the radius of 24 au with the intensity depression at the continuum peak. We derive the column densities and abundance ratios of the above species at six positions in the disk/envelope system as well as the continuum peak. The D/H ratio of CH₃OH is ∼[0.03-0.13], which is derived by correcting the statistical weight of 3 for CH₂DOH. The [CHD₂OH]/[CH₂DOH] ratio is derived to be higher ([0.14-0.29]). On the other hand, the [CH₂DOH]/[CH₃OD] ratio ([4.9-15]) is higher than the statistical ratio of 3 and is comparable to those reported for other low-mass sources. We study the physical structure on a few astronomical unit scales in B335 by analyzing the CH₃OH (18_3,15 - 18_2,16, A) and HCOOH (12_0,12 - 11_0,11) line emission. Velocity structures of these lines are reasonably explained as the infalling-rotating motion. The protostellar mass and the upper limit to the centrifugal barrier are thus derived to be 0.03-0.07 M_⊙ and <7 au, respectively, showing that B335 harbors a young protostar with a tiny disk structure. Such youth of the protostar may be related to the relatively high [CH₂DOH]/[CH₃OH] ratio.