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

TL;DR

This study investigates peptide-linked molecules like formamide, acetamide, and isocyanic acid in protostellar shocks L1157-B1 and B2, revealing their abundances, origins, and potential chemical relationships in star-forming regions.

Contribution

It provides the first detailed analysis of peptide-like molecules in protostellar shocks, highlighting their abundances and chemical links, and supports formation on ice grain mantles.

Findings

Formamide and isocyanic acid are abundant in L1157-B1 and B2.

A linear correlation exists between NH2CHO and HNCO abundances.

Formamide likely forms via hydrogenation of HNCO on ice grains.

Abstract

Interstellar molecules with a peptide link -NH-C(=O)-, like formamide (NH$_2$CHO), acetamide (NH$_2$COCH$_3$) 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 30m telescope, as part of the ASAI Large Program. Analysis of the line profiles shows that the emission arises from the outflow cavities associated with B1 and B2. Molecular abundance of $\approx~(0.4-1.1)\times 10^{-8}$ and $(3.3-8.8)\times 10^{-8}$ are derived for formamide and isocyanic acid, respectively, from a simple rotational diagram analysis. Conversely, NH$_2$COCH$_3$ was not detected down to a relative abundance of a few $\leq 10^{-10}$. B1 and B2 appear to be among the richest Galactic sources of HNCO and NH$_2$CHO 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 ice grain mantles, with NH$_2$CHO generated from hydrogenation of HNCO.