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

Abstract

We have investigated whether the nu₄ feature of NH₄⁺ is a viable candidate for the 6.85 mu m absorption band seen towards embedded young stellar objects. To produce NH₄⁺ astrophysical ice analogs consisting of H₂O, CO₂, NH₃ and O₂ 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 mu m feature and the 3.26 mu m feature. The characteristic redshift with temperature of the interstellar 6.85 mu m 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.

Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.