SCIENTIFIC ABSTRACT
It is now well accepted that systematic observations (of the survey type) are
of great interest for the general astrophysics outcome. ISO offers a unique
opportunity to completely explore the spectrum between 2.5 and 200 um at both
unprecedented high sensitivity and spectral resolution, especially in the still
unexplored gaps between the atmospheric windows. This opportunity has to be
fully used given that many unexpected lines are likely to be discovered in the
far infrared spectra of astronomical objects. We cannot rely on observing only
lines (atomic and molecular) and solid state features that we already know
about or expect to be seen.

In this context we propose to observe selected bright astronomical source(s)
over the spectral range covered by the ISO/LWS, from 45 to 196 um, in the high
resolution Fabry-Perot mode. The main goal of this proposal is to detect new
spectral features or lines with unexpected strength. In order to get the full
capability of the ISO/LWS, Fabry-Perot spectra should be taken towards some of
the brightest known infrared sources. Furthermore, since the proposed goal
impose to obtain a very high signal to noise ratio (at least 100 seems
mandatory) it might well be wisest to select only a limited number of objects
probing in the best possible way the different phases associated with the
interstellar medium.

The detection of unexpected spectral features will prompt subsequent studies
for the identification of the feature first, but also will need to elaborate
a strategy, depending on the strenght ot the new feature, for follow-up
observations.

OBSERVATION SUMMARY
In order to probe the different phases associated with the interstellar medium
in only a very few lines of sight, the source(s) selection procedure is made to
get both:
i. an "emission" spectrum: the line of sight should probe characteristic
regions of the interstellar medium exposed to a strong radiation field and
sampling at least regions of ionised hydrogen, of photodissociation and with
shocked gas.
ii. an "absorption" spectrum: the line of sight should also probe interstellar
material (molecules or solids) which remains protected from the radiation field
(at least from its visible and ultraviolet parts) and hence traces highly
processed material such as organic mantles on dust grains or volatile species
such as ices.
The source(s) should be chosen, naturally, among the brigthest objects in the
infrared sky. Therefore "off-source" position are not required for the proposed
programme.