Water and interstellar complex organics associated with the HH 212
  protostellar disc - On disc atmospheres, disc winds, and accretion shocks
  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_{\rm 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 $N_{\rm H_2}$ of
10$^{24}$ cm$^{-2}$ is assumed, leads to an abundance of $X_{\rm CH_3CHO}$
$\simeq$ 8 $\times$ 10$^{-9}$. The large velocity gradient analysis of the HDO
emission also places severe constraints on the volume density, n$_{\rm H_2}$
$\geq$ 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.
