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

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1. Introduction

The so called unidentified infrared bands (UIR) are observed in emission from interstellar matter, in the diffuse emission from the galactic disk, in a large number of stellar objects, in planetary and reflection nebulae, and in extragalactic sources. At present, a variable mixture of a large number of polycyclic aromatic hydrocarbon (PAH) molecules is often suggested to give rise to these bands. However, there are indications that the amount of interstellar carbon is not sufficient to form all the required PAHs (Dwek 1997). That stable molecules should give rise to this very strong type of bands seems unlikely, since the strongest peak is a factor of 103 above the continuous background and the UIR bands are observed almost everywhere in space. If this was the case, many other stable molecules, for example their precursors, would also be expected to show their vibrational emission IR signatures in the same spectral region. Here we instead propose a model where most types of atoms and small molecules take part in the emission from the condensed excited phase named Rydberg Matter (Svensson & Holmlid 1999; Wang & Holmlid 1998).

At least eight broad bands, with half-widths of the order of 0.5-1 µm, are considered to be of the UIR type, with the peak wavelengths ranging from 3.3 to 17 µm. They are of comparable intensities also in different surroundings, and this is one fact which points to a common origin of the bands. Recently, two observed spectra of UIR bands (i.e. emission spectra) were compared in detail with absorption spectra for a large number of PAHs, giving fair agreement (Allamandola et al. 1999). However, all possible gaseous origins for the UIR bands have still not been investigated. Further, the comparison should of course be made with emission data. The reasons for this are several, for example the different absorption and spontaneous emission coefficients, which differ by a factor [FORMULA], and the thermal populations of the upper vibrational states which differ a factor of 10 for transitions at 5 and 10 µm respectively at 600 K. In the present study, we show that a model of low complexity with fewer input parameters gives good agreement with the observed bands. The source of the UIR bands is proposed to be the general form of matter named Rydberg Matter (RM), which is a phase of matter of the same rank as liquid and solid. The agreement with data is better for this RM model than for other current models.

Rydberg Matter (RM) is a metallic phase of low density, at the lowest densities built up by clusters or sheaths with a thickness of one atomic layer. It is a condensed phase containing any kind of atom or small molecule with interparticle distances of the order of µm. The prediction of RM was made by Manykin et al. (1980). Quite complete quantum mechanical calculations were performed to predict a range of properties (Manykin et al. 1981, 1982, 1983, 1992a,b). The first macroscopic experimental proof was found after 10 years (Svensson et al. 1991; Svensson & Holmlid 1992), and a detailed microscopic proof was presented just recently (Wang & Holmlid 1998, 2000a). RM can be produced in various pressure regimes, and macroscopic amounts of RM can easily be produced at pressures of 1 mbar and at high temperatures, while RM at low pressures and low temperatures useful for microscopic studies is slightly more difficult to produce in the laboratory (Wang & Holmlid 1998, 2000a,b). Since RM is formed by condensation of circular high-Rydberg states, and such states which are formed by recombination of ions and electrons are little disturbed in space at very low pressures, it seems safe to predict that also RM will exist in space in large quantities. This report is part of an ongoing study to search for the signatures of RM in interstellar space, and both the diffuse interstellar bands (DIBs) and the 2.7 K microwave background can be shown to agree well with processes within RM. These results will however be reported separately.

Also events on earth seem to be due to RM. The atmospheric phenomenon named ball lightning is described in several reviews (Singer 1971; Smirnov 1993). The properties of RM agree well with the ones observed for such phenomena (Manykin et al. 1982, 1998). Applications of RM in various devices are in progress, mainly based on the extremely low electron work function of RM surfaces (Svensson & Holmlid 1992).

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000