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Context. Planets form in protoplanetary disks and inherit their chemical composition. It is therefore crucial to understand the molecular content of protoplanetary disks in their gaseous and solid components.
Aims. We aim to characterize the distribution and abundance of molecules in the protoplanetary disk of DG Tau and to compare them with its dust distribution.
Methods. In the context of the ALMA chemical survey of Disk-Outflow sources in the Taurus star forming region (ALMA-DOT) we analyze ALMA observations of the nearby disk-outflow system around the T Tauri star DG Tau in H₂CO 3_1,2-2_1,1, CS 5-4, and CN 2-1 emission at an unprecedented resolution of ∼0.15'', which means ∼18au at a distance of 121pc.
Results. Both H₂CO and CS emission originate from a disk ring located at the edge of the 1.3 mm dust continuum. CS probes a disk region that is slightly further out with respect to H₂CO; their peaks in emission are found at ∼70 and ∼60au, with an outer edge at ∼130 and ∼120au, respectively. CN originates from an outermost and more extended disk/envelope region with a peak at ∼80au and extends out to ∼500au. H₂CO is dominated by disk emission, while CS also probes two streams of material possibly accreting onto the disk with a peak in emission at the location where the stream connects to the disk. CN emission is barely detected and both the disk and the envelope could contribute to the emission. Assuming that all the lines are optically thin and emitted by the disk molecular layer in local thermodynamic equilibrium at temperatures of 20-100K, the ring- and disk-height-averaged column density of H₂CO is 2.4-8.6x10¹³cm^–2, that of CS is ∼1.7-2.5x10¹³cm^–2, while that of CN is ∼1.9-4.7x10¹³cm^–2. Unsharp masking reveals a ring of enhanced dust emission at ∼40au, which is located just outside the CO snowline (∼30au).
Conclusions. Our finding that the CS and H₂CO emission is co-spatial in the disk suggests that the two molecules are chemically linked. Both H₂CO and CS may be formed in the gas phase from simple radicals and/or desorbed from grains. The observed rings of molecular emission at the edge of the 1.3 mm continuum may be due to dust opacity effects and/or continuum over-subtraction in the inner disk, as well as to increased UV penetration and/or temperature inversion at the edge of the millimeter(mm)-dust which would cause enhanced gas-phase formation and desorption of these molecules. CN emission originates only from outside the dusty disk, and is therefore even more strongly anti-correlated with the continuum, suggesting that this molecule is a good probe of UV irradiation. The H₂CO and CS emission originate from outside the ring of enhanced dust emission, which also coincides with a change in the linear polarization orientation at 0.87 mm. This suggests that outside the CO snowline there could be a change in the dust properties that manifests itself as an increase in the intensity (and change of polarization) of the continuum and of the molecular emission.