Hubble Space Telescope WF/PC Imaging of the Cygnus Loop

Details Hubble Space Telescope WF/PC Imaging of the Cygnus Loop Hubble Space Telescope WF/PC Imaging of the Cygnus Loop

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.7004h&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #70.04; Bulletin of the American Astronomical Society, Vol. 24, p.1232

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Multi-line morphological studies of the subarcsecond structure of SNR shocks offer tremendous promise for improving our understanding of the physical processes at work. Model calculations (e.g., Raymond 1979; Shull and McKee 1979) indicate that in a steady flow situation and for typical interstellar magnetic field strengths the cooling and recombination region behind a 100 km sec(-1) shock moving into a medium with density of a few cm(-3) will be of order 10(16) cm. At the distance to the Cygnus Loop, this corresponds to about 1 arcsecond, or one resolution element under moderately good ground-based conditions. HST images, on the other hand, offer of order 10 resolution elements across the cooling and recombination region, providing access to the scale lengths on which important physical processes occur. We have obtained WF/PC-I images of the region studied in detail by Raymond et al (1988), with spectacular results. In addition to accomplishing the primary objective of resolving the displacement between [O III] and [S II] emission in the ``spur'' filament (which provides an independent constraint on the degree of nonthermal support behind the shock), these data have revealed many interesting phenomena. Our findings include: a cloud that appears to have been accelerated by the blast wave and is now overtaking the shock; a complex of ``incomplete'' shocks in which the [O III] region itself is substantially extended, and which may require substantially more nonthermal support even than reported by Raymond et al (1988); a network of [O III] dominated shocks (implying low column density) that are highly reminiscent of hydrodynamic models of shock/cloud encounters; a number of subarcsecond V-shaped structures where the blast wave has overrun dense clumps a few hundred AU in size in the diffuse ISM; and a radially directed ``contrail'' structure that is perhaps best explained as a knot of SN ejecta which has recently overtaken the shock front. In short, apart from answering the specific questions that justified the observations, these data have revealed a wealth of surprises.

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