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

Abstract

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 (CH${}_3$CHO) emission with an angular resolution down to $\sim$0.15 arcsec (60 au). Many emission lines due to 14 CH${}_3$CHO and 1 HDO transitions at high excitation ($E_{\mathrm{u}}$ between 163 K and 335 K) have been imaged in the inner $\sim$ 70 au region. The local thermal equilibrium analysis of the CH${}_3$CHO emission leads to a temperature of 78$\pm$14 K and a column density of 7.6$\pm$3.2 $\times$ 10${}^{15}$ cm${}^{-2}$, which, when $N_{{\mathrm{H}}_2}$ of 10${}^{24}$ cm${}^{-2}$ is assumed, leads to an abundance of $X_{\mathrm{C}{\mathrm{H}}_3\mathrm{CHO}}$ $\simeq$ 8 $\times$ 10${}^{-9}$. The large velocity gradient analysis of the HDO emission also places severe constraints on the volume density, n${}_{{\mathrm{H}}_2}$ $\ge$ 10${}^8$ cm${}^{-3}$. The line profiles are 5--7 km s${}^{-1}$ wide, and CH${}_3$CHO and HDO both show a $\pm$ 2 km s${}^{-1}$ velocity gradient over a size of $\sim$ 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. The kinematics of CH${}_3$CHO 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 $\sim$ 60 au, in good agreement with what was found before for another interstellar complex organic molecule such as NH${}_2$CHO. 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.

Astronomy & Astrophysics, in press