ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT): IV. Thioformaldehyde (H 2 CS) in protoplanetary discs: spatial distributions and binding energies

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 H 2 CS, in protoplanetary discs. We also aim to analyse the observed distributions in light of the H 2 CS 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-H 2 CS(7 1,6 −6 1,5 ) line. ALMA-Band 6 was used, reaching spatial resolutions ≃40 au, that is, Solar System spatial scales. We also estimated the binding energy of H 2 CS using quantum mechanical calculations, for the first time, for an extended, periodic, crystalline ice. Results. We imaged H 2 CS emission in two rotating molecular rings in the HL Tau and IRAS 04302+2247 discs, the outer radii of which are ~140 au (HL Tau) and 115 au (IRAS 04302+2247). The edge-on geometry of IRAS 04302+2247 allows us to reveal that H 2 CS emission peaks at radii of 60–115 au, at z = ±50 au from the equatorial plane. Assuming LTE conditions, the column densities are ~10 14 cm −2 . We estimate upper limits of a few 10 13 cm −2 for the H 2 CS column densities in DG Tau, DG Tau B, and Haro 6–13 discs. For HL Tau, we derive, for the first time, the [H 2 CS]/[H] abundance in a protoplanetary disc (≃10 −14 ). The binding energy of H 2 CS computed for extended crystalline ice and amorphous ices is 4258 and 3000–4600 K, respectively, implying thermal evaporation where dust temperatures are ≥50–80 K. Conclusions. H 2 CS traces the so-called warm molecular layer, a region previously sampled using CS and H 2 CO. Thioformaldehyde peaks closer to the protostar than H 2 CO and CS, plausibly because of the relatively high excitation level of the observed 7 1,6 −6 1,5 line (60 K). The H 2 CS binding energy implies that thermal desorption dominates in thin, au-sized, inner and/or upper disc layers, indicating that the observed H 2 CS emitting up to radii larger than 100 au is likely injected in the gas phase due to non-thermal processes.