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Astron. Astrophys. 358, 276-286 (2000)

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A novel model for the interpretation of the unidentified infrared (UIR) bands from interstellar space: deexcitation of Rydberg Matter

L. Holmlid

Reaction Dynamics Group, Physical Chemistry, Department of Chemistry, Göteborg University, 412 96 Göteborg, Sweden (holmlid@phc.chalmers.se)

Received 27 September 1999 / Accepted 5 April 2000


The so called unidentified infrared (UIR) emission bands, observed from interstellar space for more than 25 years, are presently believed to be due to carbonaceous material in some form, for example polycyclic aromatic hydrocarbons (PAHs). However, the evidence is based on absorption data, which clearly is not adequate due to differences in the processes, for example the thermal factor. It also seems doubtful that enough carbon is available to form all the required PAHs, and that the vapor pressure is high enough to keep almost all such molecules in the gas phase. We now report on a model in which all UIR bands are due to electronic deexcitation in the condensed phase named Rydberg Matter. This type of very low-density condensed matter is formed by condensation of Rydberg states of almost any type of atom or small molecule, in space mainly hydrogen atoms and molecules. The intial formation of Rydberg states is due to desorption of alkali atoms from surfaces of small particles, especially carbon particles. This desorption can be caused by radiation or moderate heat and gives long-lived circular Rydberg states. Rydberg Matter can be produced in macroscopic quantities in the laboratory. There is no lower gas density limit for its formation. All the reported UIR peaks from different objects fall within the bands for deexcitation from Rydberg Matter states with principal quantum numbers n = 40-200. Most of the transitions are two-electron deexcitation processes, but also one-electron capture processes are identified. The UIR spectra of types A - D can be understood within this model. The agreement with observations is better for the Rydberg Matter model, with small a priori information content, than for the current PAH model.

Key words: molecular processes – ISM: lines and bands – ISM: atoms, ions – ISM: molecules

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000