2018A&A...617A..10C

Context. The unprecedented combination of high-sensitivity and high angular resolution provided by the ALMA interferometer allows us to shed light on the processes leading to the formation of the jet-disc system associated with a Sun-like mass protostar.
Aims. We investigate the physical and chemical properties of the gas associated with water and interstellar complex organic molecules around a protostar on solar system scales.
Methods. The HH 212 protostellar system, in Orion B, has been mapped, thanks to ALMA-Band 7 Cycle 1 and Cycle 4 observations of dueterated water (HDO) and acetaldehyde (CH₃CHO) emission with an angular resolution down to ∼0.15" (60au).
Results. Many emission lines due to 14 CH₃CHO and 1 HDO transitions at high excitation (E_u between 163 and 335K) have been imaged in the inner ∼70au region. The local thermal equilibrium analysis of the CH₃CHO emission leads to a temperature of 78±14K and a column density of 7.6±3.2x10¹⁵cm^–2, which, when N_H2 of 10²⁴cm^–2 is assumed, leads to an abundance of X_CH3_CHO~=8x10^–9. The large velocity gradient analysis of the HDO emission also places severe constraints on the volume density, n_H2~=10⁸cm^–3. The line profiles are 5-7km/s wide, and CH₃CHO and HDO both show a ±2km/s velocity gradient over a size of ∼70au (blue-shifted emission towards the north-west and red-shifted emission towards the south-east) along the disc equatorial plane, in agreement with what was found so far using other molecular tracers.
Conclusions. The kinematics of CH₃CHO and HDO are consistent with the occurrence of a centrifugal barrier, that is, the infalling envelope-rotating disc ring, which is chemically enriched through low-velocity accretion shocks. The emission radius is ∼60au, in good agreement with what was found before for another interstellar complex organic molecule such as NH₂CHO. We support a vertical structure for the centrifugal barrier, suggesting the occurrence of two outflowing, expanding, and rotating rings above and below (of about 40-45au) the optically thick equatorial disc plane. It is tempting to speculate that these rings could probe the basis of a wind launched from this region.