This is a solicited proposal. The data will be made available to the community
upon request to the PI one month after the data set has become available.
We propose to investigate the nature of small dust particles in regions of the
interstellar medium which are strongly affected by the radiation filed from
nearby hot stars. Such regions have shown to lack 12 micron emission in
the IRAS data, which is currently interpreted as the destruction of small
dust particles, in particular PAH, in high UV fields. However, the details
of the physical processes leading to PAH destruction in these regions is still
not well understood and very few ISO observations have yet been
undertaken in order to tackle this specific problem.
The observations will consist of one-dimensional rasters (cuts) with
ISOCAM across regions showing high color gradients in the IRAS data (12
to 100 microns) surrounding 3 hot stars. The targets have been selected to
span the range of stellar surface temperature for which destruction is
expected to take place. The cuts will be observed in all IR features
attributed to small dust particles (PAH, HAC, QCC, ...) as well as in the
nearby continuum. We will also obtain a limited number of complete CVF
spectra with ISOCAM at selected positions along the cut, in order to help
interpret the filter data in terms of feature and continuum intensities.
Along with numerical modeling of the IR emission and radiative transfer,
the obtained data should allow to discriminate between the various physical
processes which have been invoked to explain the radiation driven
destruction of small dust particles (e.g. photo-thermo dissociation, direct
dissociation, Colomb explosion, ...). This program will complement ISO
observations which have been obtained toward galactic HII regions where
destruction of the small dust particles is also taking place. The geometry of
the proposed regions is generally much simpler than that of galactic HII
regions and the transition region where the destruction occurs is much
smoother and extends over wider scales on the sky, which will strongly
help the interpretation.