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The numerous conclusions we were able to extract from the analysis of PAH and fullerene signatures in
reflection nebulae are summarised here:
- The energy distribution in NGC7023 may be compared to the average ISM spectrum. The continuum
is detected all over the IR range and appears slightly weakened between 6 and
20 m if we compare to the energy distribution of the diffuse medium as modeled by Désert et al. (1990). It could be explained
by a relative depletion of very small grains, which are the intermediate population in between molecules and big grains.
- The emission band profiles are characterised by: the smoothness, the rarity of prominent substructures, the asymmetry of
some features (mostly 6.2 and 11.3 m). These properties could be explained by temperature and coupling effects within the molecules.
The 7.7m band is the most sensitive to the local conditions (Boulanger et al., this conference).
A new band at 7.45 m appears in its blue wing and
is related here for the first time. The ratio between the three features at 7.45, 7.60 and 7.80
m varies from one object to the other.
- New bands at 15.8 and 16.4 m are observed in NGC7023. They have characteristics close to those of aromatic features
and could thus be interpreted as low-frequency modes of the carbonaceous skeleton. From a comparison with a laboratory
data base of aromatic low-frequency modes, we find that the closest species could be very large and compact PAHs, perhaps containing
pentagonal rings.
- The observation of the 5797Å DIB in reflection nebulae has evidenced a broadening and a weakening of the absorption band.
The weakening may be an indication that the species get to a higher ionisation level when UV illuminated; the broadening
is the positive evidence for the rocket effect, which predicts a higher rotational temperature of plane molecules in reflection nebulae.
- A cool star illuminating a reflection nebula can produce an IR spectrum with similar properties as are found in the diffuse
medium or in nebulae surrounding hotter stars. This observation, provided by ISO, leads to the conclusion that the IR band carriers
absorb efficiently in the visible range, and not exclusively in UV. PAH cations or radicals are compatible with this property.
- An upper limit of the C60+ abundance of 0.1% of carbon is derived in NGC7023. This result is not compatible
with the attribution of 9577 and 9632 Å absorption bands by Foing and Ehrenfreund (1994). Further investigations of
the NIR oscillator strengths would be useful, especially from experiment in the gas-phase. Also it is possible that the
C60+ abundance differs from the diffuse medium (where the DIBs are observed) to the reflection nebulae (where the
vibrational modes of the cation are not detected), due to a different ionisation state.
Acknowledgements: We are grateful to ESA member states for funding the ISO mission and instruments.
Thanks to K. Uchida for providing the ISOCAM spectrum of VdB133. We gratefully acknowledge NATO Collaborative Research Grant 951347.
Next: References
Up: PAHs IN REFLECTION NEBULAE,
Previous: IR signature in NGC7023
10/8/1998