Optical emission-line properties of evolved galactic supernova remnants

New optical spectrophotometric data are presented for the supernova remnants CTB 1, OA 184, VRO 42.05.01, S147, the Monoceros Loop, G206.9 + 2.3, and G65.3 + 5.7. These data are combined with published spectral data to study some of the general properties of evolved galactic supernova remnants. It is found that (1) O I and O II forbidden line strengths, when used in conjunction with the usual H-alpha S II forbidden line ratio test, provide an excellent additional diagnostic for discriminating remnants from H II regions; (2) the line ratios H-alpha forbidden line N II, H-alpha forbidden line S II, and forbidden line S II 6717/6731 A generally do not vary substantially among the filaments of an individual remnant; and (3) the observed correlation of forbidden line N II/H-alpha with S II forbidden line 6717/6731 A in remnants is the result of observational selection rather than of evolutionary effects. A galactic nitrogen abundance gradient of d log (N/H)/dR = -0.088 dex/kpc, which is in agreement with that derived from H II regions. However, no abundance gradients for oxygen or sulfur are indicated from the remnant data.