Water and interstellar complex organics associated with the HH 212 protostellar disc On disc atmospheres, disc winds, and accretion shocks

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 similar to 0."15 (60 au). Results. Many emission lines due to 14 CH3CHO and 1 HDO transitions at high excitation (Eu between 163 and 335 K) have been imaged in the inner similar to 70 au region. The local thermal equilibrium analysis of the CH3CHO emission leads to a temperature of 78 +/- 14 K and a column density of 7.6 similar to 3.2 x 10(15) cm(-2), which, when N-H2 of 10(24) cm(-2) is assumed, leads to an abundance of X-CH3CHO similar or equal to 8 x 10(-9). The large velocity gradient analysis of the HDO emission also places severe constraints on the volume density, n(H2) >= 10(8) cm(-3). The line profiles are 5-7 km s(-1) wide, and CH3CHO and HDO both show a +/- 2 km s(-1) velocity gradient over a size of similar to 70 au (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 similar to 60 au, 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 40-45 au) 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.