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==> In this proposal, more time is being requested for NEVANS.PHOTOION
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In this proposal, we continue and extend the ISO open-time project NEVANS.
PHOTOION, to study the physical and chemical conditions in the peripheries of
star-forming clouds. Specifically, using studies from the ground, we have
chosen outlying regions of the L1630, S140, and IC5146 molecular clouds, which
have modest extinction (Av = 5 - 15) and density (n = 1000/cm^3). Regions like
these include most of the mass in the interstellar medium, but their properties
and role in star formation are poorly known. We are testing these regions for
evidence of a distributed mode of star formation. Using IRAS data, we define
1-D positional cuts (see figures in the paper version), which have a range in
intensity of the interstellar radiation field (Go = 10 - 170). In NEVANS.
PHOTOION, we proposed to observe the [CII] line at 157 microns and the [OI]
lines at 63 microns toward the outskirts of S140 and IC5146 by using LWS. Though
the LWS software has yet to be released, we have used the ISO First-Look tool to
examine the preliminary data on S140 we have received.  We see good detections
of both 157 and 63 micron lines, and the line intensities are roughly consistent
with the models. Our initial analysis indicates that further observations will
allow a detailed comparison with the models and improve our understanding of
these regions. We propose to observe the rotational lines of H_2 at 28.221,
17.035, 9.665 microns, and rovibrational lines 1-0 O(3) at 2.80 microns toward
these clouds using SWS, as they directly probe the total amount of warm gas in
the PDR. These lines, together with the FIR continuum spectrum and lines from
LWS, will allow us to test PDR models in regimes where they have not previously
been tested and, together with our ground-based studies, to pin down the
physical and chemical conditions in these regions.  Knowledge of these
conditions will allow us to test tracers of hydrogen column density, chemistry
in cloud "edges", and McKee's theory that star formation is regulated by
photoionization.