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Astron. Astrophys. 331, 742-748 (1998)

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5. Conclusions

The observations had for the first time sufficient sensitivity to analyse the mid infrared emission of a single interstellar cirrus cloud exposed to the local ISRF. The main results of the observations can be summarized as follows:
(1) We have detected the UIR bands at 7.7 and 11.3 µm.
(2) Additional emission bands seem to be present between 6 - 9 µm and 11.5 - 14 µm.
(3) A continuum emission is observed in the 10 and 16 µm filterbands. It accounts for about half of the emission between 6 and 16.5 µm.
From an analysis of the band intensities and ratios we have derived the following conclusions:
(1) The UIR band carriers in cirrus clouds are effective transformers for stellar UV radiation into IR emission throughout the galaxy. The similarity of the measured emission spectra in the faint cirrus cloud G 300.2-16.8 and in planetary and reflection nebulae with a thousandfold higher irradiation level excludes any equilibrium emission by large dust grains. It rather suggests the presence of the so called interstellar PAHs also in cirrus clouds.
(2) The 11.3/7.7 µm band ratio is [FORMULA] 0.3 - 0.4, i.e. substantially lower than predicted for neutral hydrogenated PAHs exposed to the ISRF of the solar neighbourhood. We have presented arguments that the ratio observed can be understood in terms of the PAHs being partially ionized.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998