ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT): I.
  CO, CS, CN, and H2CO around DG Tau B
  The chemical composition of planets is inherited by the distribution of the
various molecular species in the protoplanetary disk at thetime of their
formation. As of today, only a handful of disks has been imaged in multiple
spectral lines with high spatial resolution. As part of a small campaign
devoted to the chemical characterization of disk-outflow sources in Taurus, we
report on new ALMA Band 6 observations with 20 au resolution toward the
embedded young star DG Tau B. Images of the continuum emission reveals a dust
disk with rings and, putatively, a leading spiral arm. The disk, as well as the
prominent outflow cavities, are detected in CO, H2CO, CS, and CN while they
remain undetected in SO2, HDO, and CH3OH. From the absorption of the back-side
outflow, we inferred that the disk emission is optically thick in the inner 50
au. This morphology explains why no line emission is detected from this inner
region and poses some limitations toward the calculation of the dust mass and
the characterization of the inner gaseous disk. The H2CO and CS emission from
the inner 200 au is mostly from the disk and their morphology is very similar.
The CN emission significantly differs from the other two molecules as it is
observed only beyond 150 au. This ring-like morphology is consistent with
previous observations and the predictions of thermochemical disk models.
Finally, we constrained the disk-integrated column density of all molecules. In
particular, we found that the CH3OH/H2CO ratio must be smaller than 2, making
the methanol non-detection still consistent with the only such a ratio
available from the literature (1.27 in TW Hya).
