2020A&A...644A.120C

Context. Planet formation starts around Sun-like protostars with ages ≤1 Myr, but the chemical compositions of the surrounding discs remains unknown.
Aims. We aim to trace the radial and vertical spatial distribution of a key species of S-bearing chemistry, namely H₂CS, in protoplanetary discs. We also aim to analyse the observed distributions in light of the H₂CS binding energy in order to discuss the role of thermal desorption in enriching the gas disc component.
Methods. In the context of the ALMA chemical survey of disk-outflow sources in the Taurus star forming region (ALMA-DOT), we observed five Class I or early Class II sources with the o-H₂CS(7_1,6-6_1,5) line. ALMA-Band 6 was used, reaching spatial resolutions ≃40au, that is, Solar System spatial scales. We also estimated the binding energy of H₂CS using quantum mechanical calculations, for the first time, for an extended, periodic, crystalline ice.
Results. We imaged H₂CS emission in two rotating molecular rings in the HL Tau and IRAS 04302+2247 discs, the outer radii of which are ∼140au (HL Tau) and 115au (IRAS 04302+2247). The edge-on geometry of IRAS 04302+2247 allows us to reveal that H₂CS emission peaks at radii of 60-115au, at z=±50au from the equatorial plane. Assuming LTE conditions, the column densities are ∼10¹⁴cm^–2. We estimate upper limits of a few 10¹³cm^–2 for the H₂CS column densities in DG Tau, DG Tau B, and Haro 6-13 discs. For HL Tau, we derive, for the first time, the [H₂CS]/[H] abundance in a protoplanetary disc (≃10^–14). The binding energy of H₂CS computed for extended crystalline ice and amorphous ices is 4258 and 3000-4600K, respectively, implying thermal evaporation where dust temperatures are ≥50-80K.
Conclusions. H₂CS traces the so-called warm molecular layer, a region previously sampled using CS and H₂CO. Thioformaldehyde peaks closer to the protostar than H₂CO and CS, plausibly because of the relatively high excitation level of the observed 7_1,6-6_1,5 line (60K). The H₂CS binding energy implies that thermal desorption dominates in thin, au-sized, inner and/or upper disc layers, indicating that the observed H₂CS emitting up to radii larger than 100au is likely injected in the gas phase due to non-thermal processes.