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==> In this proposal, more time is being requested for PWESSELI.CO2CH4IF
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Interstellar ices are an important dust component inside dense molecular
clouds. ISO-SWS observations have revealed that CO2 is the most abundant
carbon-bearing molecule in ice mantles. The line profiles indicate the
presence of distinct ice mantle components: H2O-rich ices as well as  ices
dominated by non-polar molecules such as CO and CO2. These components may
result from outgassing of the highly volatile, non-polar molecules near
protostars or from the  predominance of different chemical routes at different
times during the  collapse or in different spatial zones around the protostar.
Understanding the influence of protostars on the composition of their natal
molecular clouds is  a key problem within astrophysics. A better understanding
of the abundances of organic molecules in interstellar ices around low mass
protostars will also  lead to a better understanding of the organic inventory
of comets and the  outer ice planets and satellites in our solar system.
Using the SWS, we propose to extend our successful observations of interstellar
solid CO2.  We propose to measure the 13CO2 stretching and 12CO2  bending
modes for sources with an optically thick 12CO2 stretching mode.  The
composition of the ice mantles in which the CO2 is trapped will be determined
from the column densities and the line profiles. Laboratory studies show that
13CO2 behaves in many ways as an independent probe of the molecular
environment, because it is always a trace species in the ice.  An important
second goal of these observations will be to determine the galactic 12C/13C
abundance ratio. This will put important constraints on models of galactic
chemical evolution.  ISO will be the only platform from which one can observe
the interstellar 12CO2 and 13CO2 molecules for the next decade(s), and thereby
determine the composition of ice mantles and the solid state 12C/13C ratio.