Optical Discovery of New and Candidate Galactic Supernova Remnants Plus Optical Imaging of the Monogem Supernova Remnant
We report the discovery of new and candidate Galactic supernova remnants made through over 2000 hours of H$\alpha$ and [O III] imaging using amateur class instrumentation. These nebulae range from 0.5 to over 8 degrees in size. Five of these, namely G27.8-17.1, G115.6+9.2, G190.5+25.3, G195.9+2.2 and G239.9+7.0 exhibit low-dispersion optical spectra indicating shock emission by the presence of strong [S II] $\lambda\lambda$6716,6731 and [O I] $\lambda$6300 line emissions relative to H$\alpha$, or exhibit a nonradiative shock generated Balmer dominated spectrum. Their filamentary emission morphology also supports the presence of interstellar shock fronts. Three other optical nebulae, G191.3+11.1, G205.7-1.7, and G305.4-0.7 appear likely to be remnants and are proposed as new SNR candidates. We also present [O III] and H$\alpha$ images of optical filaments associated with the 25 degree diameter Monogem Ring SNR which reveal extensive emission-line filaments around most of the remnant's X-ray boundaries. Deep images of the X-ray suspected remnant G190.4+12.5 and the large and [O III] emission dominated SNR G206.6+6.1 seen toward or near the Monogem SNR are also included. Key findings of this work include: 1) optical surveys can sometimes detect SNRs better than radio and X-ray SNR searches, 2) Balmer nonradiative shocks appear not uncommon in high Galactic latitude SNRs, and 3) faint \O3 emission dominated SNRs may have been missed in prior H$\alpha$ emission only surveys.