Presentation #133.01 in the session Planetary Nebulae, Supernova Remnants.
We present H-alpha and [O III] narrow-band images obtained with the Hubble Space Telescope (HST) of a well-studied shock along the north-eastern perimeter of the Cygnus Loop supernova remnant. The H-alpha emission traces the thin, rippled non-radiative shock front, and the [O III] emission, with a wider spatial profile, appears in places where the shock is becoming radiative, and where the leading H-alpha edge is absent. The most recent images, obtained in 2020, using the Wide Field Camera-3 (WFC3) are compared with images obtained in 2001 using the Wide Field and Planetary Camera-2 (WFPC2). The expected outward motion of the filaments is evident in the H-alpha images, and shows that the filament has retained its overall structure, and there have been no dramatic changes. The leading edge of the filament exhibits proper motions ranging from 55 to 80 mas/yr, which, for a distance of 735 pc to the Cygnus Loop (Fesen et al. 2021), correspond to shock speeds of about 195 to 280 km/s. The [O III] filaments have proper motions of 45 mas/yr, implying a shock velocity of about 160 km/s, which is consistent with slower shocks becoming radiative before faster shocks do. By measuring the proper motions relative to a still earlier WFPC2 H-alpha image obtained in 1997, we find there has been no measurable deceleration anywhere along the shock front.