The large, faint supernova remnant (SNR) G70.0─21.5 is believed to be the result of a Type Ia supernova some 90,000 yr ago at a distance of ∼1 kpc based on the Gaia proper motion and parallax of an unusual white dwarf. We have obtained narrow passband optical images and high- resolution spectroscopy to determine shock speeds of 70 to 110 km s−1. The shock itself is unusual in that the sharp Hα filaments arise from a very thin postshock zone where preshock neutral atoms are rapidly excited and ionized. Combining the shock speed with the remnant's estimated age, distance, and diameter, we investigate the SNR evolution in the post-Sedov phase. One-dimensional models that ignore such factors as magnetic fields, cosmic rays, and thermal conduction are marginally consistent with the observations.
