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==> In this proposal, more time is being requested for EFALGARO.COLSPEC
==> Time for this proposal should be transferred from EFALGARO.COLSPEC
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The origin of the small scale color variations of the dust emission as
revealed by the IRAS data in nearby interstellar clouds has remained a
puzzle in spite of more than four years of dedicated efforts. One well
settled result seems to be that these variations are due to local changes in
the small dust particles properties (size distribution, optical properties).
Spectral observations of the gas in a few bright mid-IR clouds (in the
millimetric, visible and UV ranges) reveal that highly energetic processes
are present in these clouds (excited transitions, broad lines, existence
of molecules formed via endothermic reactions) although these clouds belong
to the category of the "quiescent" ordinary interstellar clouds. The nature
of the energetic processes at work in this component of the interstellar
clouds is unknown (shocks, intermittent dissipation of turbulence,...).
Since we believe that this phenomenon is widespread in interstellar matter
and may be of great significance for the evolution of dust particles and of
that of the gas itself, we think that it deserves further investigation.
The spectrometers aboard ISO are ideally suited for the spectral
investigation of the properties of the highly excited gas, not only because
the small column densities of excited gas would escape detection by any less
sensitive device, but also because lines which are critical tests of the
mechanism(s) at work are accessible.