CH_3OH and Its Deuterated Species in the Disk/Envelope System of the Low-Mass Protostellar Source B335

TL;DR

This study detects methanol and its deuterated species in the B335 protostellar system, revealing their spatial distribution, abundance ratios, and the physical structure of the disk/envelope, providing insights into early protostellar evolution.

Contribution

First high-resolution detection of multiple deuterated methanol species in B335, analyzing their distribution, ratios, and protostellar structure at a few au scale.

Findings

Deuterium fractionation ratios suggest early protostellar stage.

Ring distribution of molecules indicates specific chemical processes.

Protostar has a very small, young disk with a mass of 0.03-0.07 solar masses.

Abstract

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 6 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 au scale in B335 by analyzing the CH3OH (183,15-182,16, A) and HCOOH (120,12-110,11) line emission. Velocity structures of these lines are reasonably explained as the infalling-rotating motion. The protostellar mass and the upper limit to centrifugal barrier are thus derived to be 0.03-0.07 M_{\odot} 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.