Optical emission-line properties of evolved galactic supernova remnants

Abstract

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. We find that (1) [O i] and [O n] line strengths, when used in conjunction with the usual Ha/[S n] ratio test, provide an excellent additional diagnostic for discriminating remnants from H n regions; (2) the line ratios Ha/[N n], Ha/[S n], and [S n] 26717/26731 generally do not vary substantially among the filaments of an individual remnant; and (3) the observed correlation of [N n]/Ha with [S n] 26717/26731 in remnants is the result of observational selection rather than of evolutionary effects. We derive a galactic nitrogen abundance gradient of d log (N/H)/dR =-0.088 dex kpc-1 , which is in agreement with that derived from H n regions. However, no abundance gradients for oxygen or sulfur are indicated from the remnant data.