Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA
  Planets form in protoplanetary disks and inherit their chemical compositions.
It is thus crucial to map the distribution and investigate the formation of
simple organics, such as formaldehyde and methanol, in protoplanetary disks. We
analyze ALMA observations of the nearby disk-jet system around the T Tauri star
DG Tau in the o-H$_2$CO $3_{1,2}-2_{1,1}$ and CH$_3$OH $3_{-2,2}-4_{-1,4}$ E,
$5_{0,5}-4_{0,4}$ A transitions at an unprecedented resolution of $\sim0.15"$,
i.e., $\sim18$ au at a distance of 121 pc. The H$_2$CO emission originates from
a rotating ring extending from $\sim40$ au with a peak at $\sim62$ au, i.e., at
the edge of the 1.3mm dust continuum. CH$_3$OH emission is not detected down to
an r.m.s. of 3 mJy/beam in the 0.162 km/s channel. Assuming an ortho-to-para
ratio of 1.8-2.8 the ring- and disk-height-averaged H$_2$CO column density is
$\sim0.3-4\times10^{14}$ cm$^{-2}$, while that of CH$_3$OH is
$<0.04-0.7\times10^{14}$ cm$^{-2}$. In the inner $40$ au no o-H$_2$CO emission
is detected with an upper limit on its beam-averaged column density of
$\sim0.5-6\times10^{13}$ cm$^{-2}$. The H$_2$CO ring in the disk of DG Tau is
located beyond the CO iceline (R$_{\rm CO}\sim30$ au). This suggests that the
H$_2$CO abundance is enhanced in the outer disk due to formation on grain
surfaces by the hydrogenation of CO ice. The emission peak at the edge of the
mm dust continuum may be due to enhanced desorption of H$_2$CO in the gas phase
caused by increased UV penetration and/or temperature inversion. The
CH$_3$OH/H$_2$CO abundance ratio is $<1$, in agreement with disk chemistry
models. The inner edge of the H$_2$CO ring coincides with the radius where the
polarization of the dust continuum changes orientation, hinting at a tight link
between the H$_2$CO chemistry and the dust properties in the outer disk and at
the possible presence of substructures in the dust distribution.
