A detailed analysis of a Cygnus Loop shock-cloud interaction

The XA region of the Cygnus Loop is a complex zone of radiative and nonradi atve shocks interacting with interstellar clouds. We combine five far ultra violet spectral observations from the Hopkins Ultraviolet Telescope (HUT), a grid of 24 IUE spectra, and a high-resolution long-slit H alpha spectrum to study the spatial emission line variations across the region. These spec tral data are placed in context using ground-based optical emission-line im ages of the region and a far-UV image obtained by the Ultraviolet Imaging T elescope (UIT). The presence of high-ionization species (O VI, N V, and C I V) indicates a shock velocity near 170 km s(-1) while other diagnostics ind icate km s(-1). It is likely that a large range v(sh) approximate to 140 of shock velocities may exist at a spatial scale smaller than we are able to resolve. By comparing C IV lambda 1550, C III lambda 977, and C III] lambda 1909, we explore resonance scattering across the region. We find that a si gnificant column depth is present at all positions, including those not nea r bright optical or UV filaments. Analysis of the O VI doublet ratio sugges ts an average optical depth of about unity in that ion, while flux measurem ents of [Si VIII] lambda 1443 suggest a hot component in the region at just below 10(6) K. Given the brightness of the O VI emission and the age of th e interaction, we rule out the mixing-layer interpretation of the UV emissi on. Furthermore, we formulate a picture of the XA region as that of an enco unter of the blast wave with a finger of dense gas protruding inward from t he pre-supernova cavity.