Contents of: VI/111/./abstract/CGRY_ISM_IIA.abs

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SCIENTIFIC ABSTRACT
The ISO LWS gives the opportunity to perform IR spectroscopic observations of
extended, weak sources like the cirrus, evidenced by IRAS at high galactic 
latitude. It allows to study gas properties through fine structure emission 
lines and to correlate these emission with the dust spectrum observed 
simultaneously. In particular the [CII] 158 um line is believed to be the more 
effective coolant  for the considered diffuse regions so these observations 
should give information on the role of dust and small particles in the heating 
of the gas.

OBSERVATION SUMMARY
We will observe a small number of diffuse clouds, with several positions 
presenting different characteristics within the cloud (transition regions from
atomic to molecular gas or regions presenting IR colour spatial variations).

The [CII] 158 um line is expected to be the dominant emission line. Following 
the UV-absorption derived C+ cooling rate (Gry et al., 1992, A&A 266, 457) and 
the estimation of the FIR emission (Boulanger and Perault, 1988, ApJ 330, 964) 
we expect a contrast between the [CII] line and the adjacent continuum around 4.
A recent analysis of the COBE data (Benett et al., 1993, COBE preprint) finds 
this contrast to be only 1 in the Galactic plane and 0.5 at b=50. However these 
values show that grating spectra should be sufficient for accurate [CII] line 
measurements.

Other lines could be of interest in the LWS range, like [OI] 63.18 um, 
[OI] 145.53 um, [OIII] 51.82 um, [OIII] 88.36 um, [FeII] 51.30 um, 
[FeII] 87.38 um, [NII] 121.89 um, [FeIII] 51.7 um, [FeIII] 105.4 um and 
[NIII] 57.33 um. However they are expected to be much weaker than the [CII] 
line and their detection could only be marginal.

The strategy will thus be to take a full grating spectrum for each position.

The exposure times are calculated with AOT LWS01 (full grating scan) with 2 
samples per resolution element, on the basis of a S/N around 10 for the 
continuum at 100 um. They should unfortunately only give a S/N of the order of 
3 at 60 um for our objects, but a confortable S/N of 15 at 158 um.

The IRAS fluxes considered have previously been corrected for the revised
calibration of IRAS fluxes with COBE data.                                  

Off-source positions are required for extracting the background emission. For 
each object, we will observe one position close to the cloud, with the longest 
exposure time used for the object.