ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)

Abstract

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(71,6−61,5) line. ALMA-Band 6 was used, reaching spatial resolutions ≃40 au, that is, Solar System spatial scales. We also estimated the binding energy of 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 ~140 au (HL Tau) and 115 au (IRAS 04302+2247). The edge-on geometry of IRAS 04302+2247 allows us to reveal that H2CS emission peaks at radii of 60–115 au, at z = ±50 au from the equatorial plane. Assuming LTE conditions, the column densities are ~1014 cm−2. We estimate upper limits of a few 1013 cm−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 3000–4600 K, respectively, implying thermal evaporation where dust temperatures are ≥50–80 K. 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 71,6−61,5 line (60 K). 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 100 au is likely injected in the gas phase due to non-thermal processes.