Context. Planet formation starts around Sun-like protostars with ages ≤ 1 Myr: what is the chemical compositions in disks? Aims. To trace the radial and vertical spatial distribution of H 2 CS, a key species of the S-bearing chemistry, in protoplanetary disks. 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 disk 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, i.e. 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 IRAS04302+2247 disks. The outer radii 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. Upper limits of a few 10 13 cm −2 have been estimated for the H 2 CS column densities in DG Tau, DG Tau B, and Haro 6-13 disks. For HL Tau, we derive, for the first time, the [H 2 CS]/[H] abundance in a protoplanetary disk (10 −14). The binding energy of H 2 CS computed for extended crystalline ice and amorphous ices is 4258 K and 3000-4600 K, respectively, implying a thermal evaporation where dust temperature is ≥ 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 due to the relatively high-excitation level of 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 disk layers, indicating that the observed H 2 CS emitting up to radii larger than 100 au is likely injected in the gas due to non-thermal processes.
Ethanol (CH 3 CH 2 OH) is a relatively common molecule, often found in star-forming regions. Recent studies suggest that it could be a parent molecule of several so-called interstellar complex organic molecules (iCOMs). However, the formation route of this species remains under debate. In the present work, we study the formation of ethanol through the reaction of CCH with one H 2 O molecule belonging to the ice as a test case to investigate the viability of chemical reactions based on a “radical + ice component” scheme as an alternative mechanism for the synthesis of iCOMs, beyond the usual radical–radical coupling. This has been done by means of DFT calculations adopting two clusters of 18 and 33 water molecules as ice models. Results indicate that CH 3 CH 2 OH can potentially be formed by this proposed reaction mechanism. The reaction of CCH with H 2 O on the water ice clusters can be barrierless (because of the help of boundary icy water molecules acting as proton-transfer assistants), leading to the formation of vinyl alcohol precursors (H 2 CCOH and CHCHOH). Subsequent hydrogenation of vinyl alcohol yielding ethanol is the only step presenting a low activation energy barrier. We finally discuss the astrophysical implications of these findings.
Binding energies (BEs) are one of the most important parameters for astrochemical modeling determining, because they govern whether a species stays in the gas phase or is frozen on the grain surfaces. It is currently known that, in the denser and colder regions of the interstellar medium, sulfur is severely depleted in the gas phase. It has been suggested that it may be locked into the grain icy mantles. However, which are the main sulfur carriers is still a matter of debate. This work aims to establish accurate BEs of 17 sulfur-containing species on two validated water ice structural models, the proton-ordered crystalline (010) surface and an amorphous water ice surface. We adopted density functional theory-based methods (the hybrid B3LYP-D3(BJ) and the hybrid meta-GGA M06-2X functionals) to predict structures and energetics of the adsorption complexes. London’s dispersion interactions are shown to be crucial for an accurate estimate of the BEs due to the presence of the high polarizable sulfur element. On the crystalline model, the adsorption is restricted to a very limited number of binding sites with single valued BEs, while on the amorphous model, several adsorption structures are predicted, giving a BE distribution for each species. With the exception of a few cases, both experimental and other computational data are in agreement with our calculated BE values. A final discussion on how useful the computed BEs are with respect to the snow lines of the same species in protoplanetary disks is provided.
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