First Identification and Chemical Modeling of New Thiol (−SH) Bearing Molecule in the Interstellar Medium: Dithioformic Acid

The study of complex organic molecules containing thiol (−SH) groups is essential in interstellar media because −SH plays an important role in the polymerization of amino acids (R–CH(NH₂)–COOH). Some quantum chemical studies have shown that there is a high chance of detecting the emission lines of dithioformic acid (HC(S)SH) in the highly dense and warm-inner regions of hot molecular cores and hot corinos. Therefore, we attempted to search for the emission lines of HC(S)SH toward the highly dense hot corino object NGC 1333 IRAS 4A using the Atacama Large Millimeter/Submillimeter Array (ALMA) band 7. We present the first detection of the rotational emission lines of the trans-conformer of dithioformic acid (t-HC(S)SH) toward the NGC 1333 IRAS 4A2. The column density and excitation temperature of the t-HC(S)SH toward NGC 1333 IRAS 4A2 are (2.63 ± 0.32) × 10¹⁵ cm^–2 and 255 ± 32 K, respectively. The fractional abundance of t-HC(S)SH with respect to H₂ is (2.53 ± 0.68) × 10^–9. The column density ratio of t-HC(S)SH and t-HCOOH toward NGC 1333 IRAS 4A2 is 0.36 ± 0.02. To understand the possible formation pathways of HC(S)SH, we computed a two-phase warm-up chemical model abundance of HC(S)SH using the gas-grain chemical code UCLCHEM. After chemical modeling, we claim that HC(S)SH is formed in NGC 1333 IRAS 4A2 via barrierless radical–radical reactions between CSSH and H on the grain surfaces.