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W.P. Blair, R. Sankrit, S.M. Tulin (JHU/CAS)
Astrophysical shock waves produce a wide range of emission lines as the gas cools down in the post-shock region. Strong lines of O~VI \lambda\lambda1032,1038, C~III \lambda977, and N~III \lambda991 are often seen, as well as many weaker lines such as S~IV \lambda\lambda1063,1073, S~VI \lambda\lambda933,944, etc. Far Ultraviolet Spectroscopic Explorer (FUSE) observations of radiative shocks in the Cygnus Loop show broad (>100 km/s) lines which are well resolved. The observed shapes of the emission lines are influenced by overlying absorption due to interstellar lines, particularly molecular hydrogen and also self-absorption in the case of strong resonance lines. In continuum sources, the presence of such absorption can be assessed directly, but the effects are much more subtle when they affect individual emission line profiles.
We are analyzing of FUSE observations of several bright, radiative filaments in the Cygnus Loop, some from the eastern limb where the filament morphology is chaotic or turbulent in appearance, and some from the western limb, where the filament morphology is much more regular in appearance.Data from both the 30\arcsec\ square LWRS aperture (~100 km/s resolution) and the 4\arcsec\times20\arcsec\ MDRS aperture (~30 km/s resolution) are available.
The observed profiles for the stronger emission lines contain a number of clues as to the effects of overlying absorption. The O VI doublet lines should mirror each other except for peak intensity, but instead their profiles look quite different from each other and vary between the different positions observed. Modeling self-absorption in these strong resonance transitions and assuming differing amounts of molecular hydrogen, we can match the observed line profiles and their variations. The S~VI lines are narrower than O~VI, and provide the best evidence that at least some of the overlying H2 must be hot, and thus possibly directly related to Cygnus Loop shocks elsewhere along the line of sight. We assess the impact of absorption on all the lines detected in our spectra in order to derive the properties of the absorbing molecular hydrogen at each position.
This work is supported by NASA Contract NAS5-32985 to the Johns Hopkins University.
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The author(s) of this abstract have provided an email address for comments about the abstract: wpb@pha.jhu.edu
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.