Origin of the 6.85 m band near young stellar objects: The ammonium ion (NH_4^+) revisited^*

¹ Raymond and Beverly Sackler Laboratory for Astrophysics, Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
² Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA

Corresponding author: W. A. Schutte, schutte@strw.Leidenuniv.nl

Received: 7 June 2001
Accepted: 14 November 2002

Abstract

We have investigated whether the feature of  is a viable candidate for the 6.85 μm absorption band seen towards embedded young stellar objects. To produce  astrophysical ice analogs consisting of , , and  were UV photolysed. The IR spectra reveal peaks that are identified with the NH, NO, NO and HCO ions. It is shown that the  matches two absorption features that are observed towards embedded young stellar objects, i.e., the strong 6.85 μm feature and the 3.26 μm feature. The characteristic redshift with temperature of the interstellar 6.85 μm feature is well reproduced. The abundance of  in interstellar ices would be typically 10% relative to . 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  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.