We propose to observe far-infrared line emissions of CO and OI from hot,
shock-heated gas in Cepheus A (W).  Cepheus A (W) is a particularly favorable
object in which to study shock-heated interstellar gas, because it shows a very
high ratio of shock-excited line emission to infrared continuum emission.  The
proposed observations would follow-up on earlier ISO studies of shock-excited
water and molecular hydrogen in that source. While the H2 line strengths
measured in Cepheus A (W) can be successfully accounted for by our model for the
emission properties of magnetohydrodynamic shock waves (Kaufman and Neufeld
1996), our preliminary analysis of a search for shock-excited water that was
carried out during Rev. 220 (OBSID.PROPID=DNEUFELD.HOTWATER) suggests that the
water rotational line strengths are considerably smaller than expected.  At
present it remains unclear whether the absence of strong water emission argues
against a high water abundance in the warm shocked gas - contradicting an
important prediction of chemical models - or whether the shock parameters
invoked to explain the H2 emission are in error. The observations of high-J CO
line emission and OI fine structure emission proposed here will provide
valuable constraints upon the admixture of shock velocities and preshock
densities present in Cepheus A, allowing more reliable shock models to be
constructed for this source and the implications of the water observations to
be assessed more critically.  We request a total of 18767 seconds to carry out
LWS Fabry-Perot observations of six far-infrared rotational lines of CO
(J=16-15, J=22-21, J=28-27, J=31-30, J=34-33, and J=37-36) and of the two fine
structure lines of OI (at 63 and 146 microns).