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Astron. Astrophys. 339, 194-200 (1998)

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5. Conclusion

The main results of our analysis of the spectral shape of the mid-IR interstellar emission can be summarized as follows.

(1) The fitting of the carbon dust bands by Lorentzian profiles provides an unequivocal and hopefully universal way to quantify and separate the different components of the celestial unidentified mid-infrared dust spectra. The wide wings of the Lorentz profiles account for the broad pedestals of the bands. The true intensity is grossly underestimated if these wings are ignored. A continuum remains necessary but it has a simple linear shape.

(2) It is most unlikely that the Lorentz shape could result from a random combination of many lines. We think that, to a large extent, the width of the UIR is intrinsic to the emission of each particle.

(3) We propose to associate the width of the Lorentz profiles to the life-time of the emitting level. The very short life-time corresponds to the duration of the excitation of a given vibrational mode before the energy is internally transferred to another mode through internal vibrational redistribution (IVR). This interpretation is supported by laboratory data on infrared transitions from large molecules at high temperature. We consider that this work provides further support to one key aspect of the PAH hypothesis, namely that the carriers of the bands are heated by the absorption of single photons.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998