A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction

Abstract

The XA region of the Cygnus Loop is a complex zone of radiative and nonradiative shocks interacting with interstellar clouds. We combine five far ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT), a grid of 24 IUE spectra and a high-resolution longslit Halpha spectrum to study the spatial emission line variations across the region. These spectral data are placed in context using ground-based, optical emission line images of the region and a far-UV image obtained by the Ultraviolet Imaging Telescope (UIT). The presence of high-ionization ions (OVI, NV, CIV) indicates a shock velocity near 170 km/s while other diagnostics indicate v_shock=140 km/s. It is likely that a large range of shock velocities may exist at a spatial scale smaller than we are able to resolve. By comparing CIV 1550, CIII 977 and CIII] 1909, we explore resonance scattering across the region. We find that a significant column depth is present at all positions, including those not near bright optical/UV filaments. Analysis of the OVI doublet ratio suggests an average optical depth of about unity in that ion while flux measurements of [SiVIII] 1443 suggest a hot component in the region at just below 10^6K. Given the brightness of the OVI emission and the age of the interaction, we rule out the mixing layer interpretation of the UV emission. Furthermore, we formulate a picture of the XA region as the encounter of the blast wave with a finger of dense gas protruding inward from the pre-SN cavity.

21 pages, 9 figures, accepted by the Astronomical Journal, July 2001 Full resolution figures available at http://fuse.pha.jhu.edu/~danforth/xa/

References in corpus (6)

• Ionization balance for optically thin plasmas: rate coefficients for all atoms and ions of the elements H to Ni
• Panoramic Views of the Cygnus Loop
• The ROSAT-HRI X-Ray Survey of the Cygnus Loop
• The Distance to the Cygnus Loop from Hubble Space Telescope Imaging of the Primary Shock Front
• The Ultraviolet Imaging Telescope: Instrument and Data Characteristics
• A Comparison of Ultraviolet, Optical, and X-Ray Imagery of Selected Fields in the Cygnus Loop

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• Far-Ultraviolet & Halpha Spectroscopy of SNR0057-7226 in the SMC HII Region N66
• High Forbidden-to-resonance Line Ratio of OVII Discovered in the Cygnus Loop
• SPEAR Far Ultraviolet Spectral Images of the Cygnus Loop
• FUSE Observations of the Cygnus Loop: OVI Emission from a Nonradiative Shock
• Spitzer IRS Observations of the XA Region in the Cygnus Loop Supernova Remnant
• Chandra Imaging and Spectroscopy of the Eastern XA Region of the Cygnus Loop Supernova Remnant
• SPEAR far UV spectral imaging of highly ionized emission from the North Galactic Pole Region
• Global Far-Ultraviolet Properties of the Cygnus Loop
• The origin of the X-ray emitting plasma in the eastern edge of the Cygnus Loop
• [Ne V] Imaging of N49 in the Large Magellanic Cloud
• Unbiased Spectroscopic Study of the Cygnus Loop with LAMOST. I. Optical Properties of Emission Lines and the Global Spectrum
• Near-Ultraviolet Radiation toward Molecular Cloud N4 in W 50/SS 433: Evidence for Direct Interaction of the Jet with Molecular Cloud