Molecules with a peptide link in protostellar shocks: a comprehensive study of L1157

Interstellar molecules with a peptide link (-NH-C(= O)-), like formamide (NH2CHO), acetamide (NH2COCH3) and isocyanic acid (HNCO), are particularly interesting for their potential role in pre-biotic chemistry. We have studied their emission in the protostellar shock regions L1157-B1 and L1157-B2, with the IRAM 30 m telescope, as part of the ASAI Large Programme. Analysis of the line profiles shows that the emission arises from the outflow cavities associated with B1 and B2. Molecular abundances of approximate to(0.4-1.1) x10(-8) and (3.3-8.8) x10(-8) are derived for formamide and isocyanic acid, respectively, from a simple rotational diagram analysis. Conversely, NH2COCH3 was not detected down to a relative abundance of a few <= 10(-10). B1 and B2 appear to be among the richest Galactic sources of HNCO and NH2CHO molecules. A tight linear correlation between their abundances is observed, suggesting that the two species are chemically related. Comparison with astrochemical models favours molecule formation on icy grain mantles, with NH2CHO generated from hydrogenation of HNCO.