Volume 398, Number 3, February II 2003
|Page(s)||1049 - 1062|
|Section||Interstellar and circumstellar matter|
|Published online||28 January 2003|
Origin of the 6.85 m band near young stellar objects: The ammonium ion (NH4^+) revisited*
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
Accepted: 14 November 2002
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.
Key words: methods: laboratory / stars: individual: W33A / stars: individual: MonR2:IRS3 / ISM: abundances / ISM: molecules / infrared: ISM / ISM: lines and bands
© ESO, 2003
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