Constraints on the abundances of various molecules in interstellar ice: laboratory studies and astrophysical implications

Ethane, acetylene, ethanol, hydrazine and hydrogen peroxyde are either predicted by theoretical models to be abundant in the icy mantles on grains in dense clouds, or were found to be produced by comets. We present measurements of the spectra of these species embedded in astrophysically relevant ice matrices. Additionally, we obtained the spectrum of the hydrozonium ion (N2H5+) which could be produced by activationless acid-base reactions. The laboratory results are compared to the ISO and ground-based spectra of NGC 7538:IRS 9. Strict upper-limits compared to the solid water could be found for ethane, ethanol and hydrogen peroxyde of < 0.4%, < 1.2%, and < 6.1%, respectively. These results give some important information on the relationship between cometary and interstellar ices and on the nature of grain surface reactions.