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Next: Diffuse bands in RN Up: PAHs in reflection nebulae Previous: NGC7023

PAHs in cool RN


  
Figure: By courtesy of K. Uchida et al. (1998), the ISOCAM spectrum of VdB133, a reflection nebula illuminated mostly by a cool star (6500K). For a direct comparison, the spectrum of NGC7023 (illuminated by a 17000K star) is shown again below, at a similar resolution.
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Recently Uchida et al. (1998) have shown for the first time the IR spectrum of a reflection nebula illuminated by a cool star, obtained by ISOCAM-CVF. It is displayed in Fig. [*] by courtesy of the authors. VdB133 is surrounding a double star; the two components are at effective temperatures 6500 and 12500K. Uchida et al. have demonstrated that only 21% of the incident flux was emitted in UV light ($\lambda <$ 400 nm), while the dominant part of the radiation is in visible light. The resulting IR spectrum is surprisingly similar to the spectrum observed in the diffuse medium (Boulanger et al. 1996), or in the reflection nebula NGC7023 (Cesarsky et al. 1996, this paper), which encounters much more UV photons. Bandwidths as well as band ratios are of comparable value, at first look. Such similarities in the IR properties would imply that the band carriers are able to absorb efficiently in a very wide spectral range, including visible light. The consequences on the excitation mechanisms and the emitter nature have to be investigated. We have to note that this constraint is not uncompatible with the PAH hypothesis, and more favourable to PAH cations or radicals, for which the first electronic absorption band lies in the visible (Léger and d'Hendecourt 1985, Allamandola et al. 1985). Another possibility is that the average size of emitting PAHs is larger around cooler stars, so that their first electronic absorption band is displaced to the longer wavelengths.


next up previous
Next: Diffuse bands in RN Up: PAHs in reflection nebulae Previous: NGC7023

10/8/1998