Hubble Space Telescope Imaging of the Primary Shock Front in the Cygnus Loop Supernova Remnant^*

We present Hubble Space Telescope WFPC2 narrowband Hα (F656N) and [O III] λ5007 (F502N) imaging of two fields on the northeastern limb of the Cygnus Loop supernova remnant. This region provides an outstanding example of the initial encounter between the primary blast wave and the surrounding interstellar medium. The Hα images show the primary nonradiative shock front, which, when viewed edge-on, is unresolved at WFPC2 resolution. The [O III] images show portions of filaments that are beginning to become radiative, and in these images the filaments are resolved, appearing fuzzy at WFPC2 resolution. The [O III] filament regions are not bounded directly by Hα filaments, indicating that the shock emission from the nascent radiative region is sufficient to fully ionize the local preshock gas. One field, imaged 4 yr earlier in Hα with WFPC2, is used to study the proper motion of the filament and constrain any brightness variations over this time period. In conjunction with improved models of nonradiative shocks, these data are used to place limits on the possible deceleration of the filament and refine the distance to the Cygnus Loop, arriving at a revised value of d = 540 pc (assuming no deceleration). The second field imaged contains examples of coherent Hα filaments with much more dramatic curvatures than identified previously. We discuss the possible reasons for the different morphology of these filaments and conclude that they can be accommodated with relatively modest variations in local density and shock velocity.