Seeds of Life in Space (SOLIS) II. Formamide in protostellar shocks: Evidence for gas-phase formation

CONTEXT: Modern versions of the Miller-Urey experimentclaim that formamide(NH2CHO) could be the starting point for the formation of metabolic and genetic macromolecules. Intriguingly, formamideis indeed observed in regions forming solar-type stars and in external galaxies. Aims. How NH2CHO is formed has been a puzzle for decades: our goal is to contribute to the hotly debated question of whether formamideis mostly formed via gas-phase or grain surface chemistry. METHODS: We used the NOrthern Extended Millimeter Array (NOEMA) interferometer to image NH2CHO towards the L1157-B1 blue-shifted shock, a well-known interstellar laboratory, to study how the components of dust mantles and cores released into the gas phase triggers the formation of formamide. RESULTS: We report the first spatially resolved image (size ~9″, ~2300 AU) of formamideemission in a shocked region around a Sun-like protostar: the line profiles are blueshiftedand have a FWHM ≃ 5 km s-1. A column density of NNH2CHO = 8 × 1012 cm-1 and an abundance, with respect to H-nuclei, of 4 × 10-9 are derived. We show a spatial segregation of formamidewith respect to other organic species. Our observations, coupled with a chemical modelling analysis, indicate that the formamideobserved in L1157-B1 is formed by a gas-phase chemical process and not on grain surfaces as previously suggested. CONCLUSIONS: The Seeds of Life in Space (SOLIS) interferometric observations of formamideprovide direct evidence that this potentially crucial brick of life is efficiently formed in the gas phase around Sun-like protostars.