The following document lists the file abstract/DNEUFELD_MHDSHOCK.abs
from catalogue VI/111.
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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).