Shock processing of interstellar nitrogen compounds in the solar nebula

Some organic material in chondrites (primitive meteorites) exhibits a very low 14N/ 15N, suggesting that the compounds that carry this heavy nitrogen signature formed in the interstellar medium. Other organic components of the same chondrites show a more solar isotopic signature, suggesting they derive from an isotopically solar reservoir of nitrogen such as N 2 or NH 3 in the solar nebula. In this work, we model the chemistry of the shocks that have been hypothesized as the mechanism to melt chondrules. We find that such shocks (≈ 8 km/ s) do not produce significant amounts of HCN and CN if all nitrogen is initially locked in N 2 and all carbon is locked in CO. Only when NH 3 or CH 4 (or both) were present in the initial pre-shock nebula gas do CN and HCN form. We also find that C 2H 2 (acetylene) and C 2H form in low abundances if the carbon is all locked in CO in the pre-shock gas. The presence of CH 4 facilitates the formation of acetylene and related compounds. In the absence of CH 4 or NH 3, only negligible amounts of species containing CC or CN bonds form. Acetylene and cyanide-related compounds may be precursors to the organics that condensed into meteorites about 4.5 billion years ago. We find that CN bonds largely survive these shocks; thus, the very low interstellar 14N/ 15N signature can be preserved if the 15N is carried by CN-bearing interstellar compounds.