G107.0+9.0: A New Large Optically Bright, Radio and X-Ray Faint Galactic
  Supernova Remnant in Cepheus
  Wide-field H-alpha images of the Galactic plane have revealed a new supernova
remnant (SNR) nearly three degrees in diameter centred at l = 107.0, b = +9.0.
Deep and higher resolution H-alpha and [O III] 5007 Ang images show dozens of
H-alpha filaments along the remnant's northern, western, and southwestern
limbs, but few [O III] bright filaments. The nebula is well detected in the
H-alpha Virginia Tech Spectral-Line Survey images, with many of its brighter
filaments even visible on Digital Sky Survey images. Low-dispersion spectra of
several filaments show either Balmer dominated, non-radiative filaments or the
more common SNR radiative filaments with [S II]/H-alpha ratios above 0.5,
consistent with shock-heated line emission. Emission line ratios suggest shock
velocities ranging from <70 km/s along its western limb to ~100 km/s along its
northwestern boundary. While no associated X-ray emission is seen in ROSAT
images, faint 1420 MHz radio emission appears coincident with its western and
northern limbs. Based on an analysis of the remnant's spatially resolved
H-alpha and [O III] emissions, we estimate the remnant's distance around 1.5 -
2.0 kpc implying a physically large (dia.= 75 - 100 pc) and old (90 - 110 x
10^3 yr) SNR in its post-Sedov radiative phase of evolution expanding into a
low density interstellar medium (n = 0.05 - 0.2 cm^-3) and lying some 250 - 300
pc above the Galactic plane.
