The prebiotic molecular inventory of Serpens SMM1. I. An investigation of the isomers CH3NCO and HOCH2CN

Aims: Methyl isocyanate (CH₃NCO) and glycolonitrile (HOCH₂CN) are isomers and prebiotic molecules that are involved in the formation of peptide structures and the nucleobase adenine, respectively. These two species are investigated to study the interstellar chemistry of cyanides (CN) and isocyanates (NCO) and to gain insight into the reservoir of interstellar prebiotic molecules.
Methods: ALMA observations of the intermediate-mass Class 0 protostar Serpens SMM1-a and ALMA-PILS data of the low-mass Class 0 protostar IRAS 16293B are used. Spectra are analysed with the CASSIS line analysis software package in order to identify and characterise molecules.
Results: CH₃NCO, HOCH₂CN, and various other molecules are detected towards SMM1-a. HOCH₂CN is identified in the PILS data towards IRAS 16293B in a spectrum extracted at a half-beam offset position from the peak continuum. CH₃NCO and HOCH₂CN are equally abundant in SMM1-a at [X]/[CH₃OH] of 5.3 × 10⁻⁴ and 6.2 × 10⁻⁴, respectively. A comparison between SMM1-a and IRAS 16293B shows that HOCH₂CN and HNCO are more abundant in the former source, but CH₃NCO abundances do not differ significantly. Data from other sources are used to show that the [CH₃NCO]/[HNCO] ratio is similar in all these sources within ~10%.
Conclusions: The new detections of CH₃NCO and HOCH₂CN are additional evidence for a large interstellar reservoir of prebiotic molecules that can contribute to the formation of biomolecules on planets. The equal abundances of these molecules in SMM1-a indicate that their formation is driven by kinetic processes instead of thermodynamic equilibrium, which would drive the chemistry to one product. HOCH₂CN is found to be much more abundant in SMM1-a than in IRAS 16293B. From the observational data, it is difficult to indicate a formation pathway for HOCH₂CN, but the thermal Strecker-like reaction of CN⁻ with H₂CO is a possibility. The similar [CH₃NCO]/[HNCO] ratios found in the available sample of studied interstellar sources indicate that these two species are either chemically related or their formation is affected by physical conditions in the same way. Both species likely form early during star formation, presumably via ice mantle reactions taking place in the dark cloud or when ice mantles are being heated in the hot core. The relatively high abundances of HOCH₂CN and HNCO in SMM1-a may be explained by a prolonged stage of relatively warm ice mantles, where thermal and energetic processing of HCN in the ice results in the efficient formation of both species.