This is a discretionary time proposal to perform a full grating scan with the
Short Wavelength Spectrometer of the southern shock peak of the Orion IRc2
outflow. Whilst there are existing scans centred on IRc2 itself and the
northern shock peak, called Pk1, this is a proposal to form a complete data
set by observing the southern shock peak, called Pk2. Such an observation
will allow a detailed investigation of the Orion IRc2 outflow and its shock.
For instance, we can look at the differences in molecular hydrogen, H2,
excitation between Pk2, IRc2 and Pk1 which may be related to differences in
temperature and/or density on either side of IRc2, or an asymmetry in the
shock itself. Furthermore, from other molecular species such as water, CO
and CO2, which have already been detected in IRc2 and Pk1, we can study
the influence of ultraviolet radiation on the chemistry of the IRc2 outflow.
This stems from the fact that the Trapezium stars lie 1 arc minute south
east of Pk2, which in turn lies south east of IRc2 and Pk1. Geometrical
dilution of the uv field may then manifest itself as a gradient in some
chemical abundances. We have already detected bright emission from high
excitation species toward IRc2, such as SIV, which was not foreseen given
the distance of about 0.1 pc from the relatively low effective temperature
Trapezium stars (of order 35000 K). Observations of Pk2 may aid in
separating the effects of radiative ionisation from the stars and
collisional ionisation within the shock.
All studies within the literature of the IRc2 outflow consider both Pk1 and
Pk2. This has been the case since 1978 when the first molecular hydrogen map
was published by Beckwith et al. and the shock peaks defined. It would be
remiss of us not to do the same with ISO. It is the brightest and most well
studied shock region in the sky. To attain a full understanding of it and the
physics and chemistry of shocks we need to obtain a comprehensive set of
observations. This represents our last opportunity for a very long time.