Stratified Distribution of Organic Molecules at the Planet-formation Scale in the HH 212 Disk Atmosphere

Abstract

Formamide (NH 2 CHO) is considered an important prebiotic molecule because of its potential to form peptide bonds. It was recently detected in the atmosphere of the HH 212 protostellar disk on the solar system scale where planets will form. Here we have mapped it and its potential parent molecules HNCO and H 2 CO, along with other molecules CH 3 OH and CH 3 CHO, in the disk atmosphere, studying its formation mechanism. Interestingly, we find a stratified distribution of these molecules, with the outer emission radius increasing from ∼24 au for NH 2 CHO and HNCO, to 36 au for CH 3 CHO, to 40 au for CH 3 OH, and then to 48 au for H 2 CO. More importantly, we find that the increasing order of the outer emission radius of NH 2 CHO, CH 3 OH, and H 2 CO is consistent with the decreasing order of their binding energies, supporting that they are thermally desorbed from the ice mantle on dust grains. We also find that HNCO, which has much lower binding energy than NH 2 CHO, has almost the same spatial distribution, kinematics, and temperature as NH 2 CHO, and is thus more likely a daughter species of desorbed NH 2 CHO. On the other hand, we find that H 2 CO has a more extended spatial distribution with different kinematics from NH 2 CHO, thus questioning whether it can be the gas-phase parent molecule of NH 2 CHO.