Origin of the 6.85 μm band near young stellar objects:: The ammonium ion (NH4+) revisited

We have investigated whether the v(4) feature of NH4+ is a viable candidate for the 6.85 mum absorption band seen towards embedded young stellar objects. To produce NH4+ astrophysical ice analogs consisting of H2O, CO2, NH3 and O-2 were UV photolysed. The IR spectra reveal peaks that are identified with the NH4+, NO2-, NO3- and HCO3- ions. It is shown that the NH4+ matches two absorption features that are observed towards embedded young stellar objects, i.e., the strong 6.85 mum feature and the 3.26 mum feature. The characteristic redshift with temperature of the interstellar 6.85 pm feature is well reproduced. The abundance of NH4+ in interstellar ices would be typically 10% relative to H2O. The experiments show that the counterions produce little distinct spectral signature but rather a pseudo-continuum if a variety of them is present in a H2O dominated environment. The anions could therefore go undetected in IR spectra of interstellar ice. In the ISM, where additional mechanisms such as surface chemistry and additional elements such as sulfur are available many acids and an even wider variety of anions could be produced. These components may be detectable once the ices sublime, e.g., in hot cores.