Optical emission-line properties of M33 supernova remnants

Abstract

We have obtained spectra of the 42 supernova remnants (SNRs) and remnant candidates identified in the recent survey of the inner portion of M33. Relative fluxes for the important diagnostic emission lines in the red portion of the optical spectrum (6200-7500 A) are reported. We have confirmed that all the candidate SNRs have [S II]/Hα ratios >= 0.4, the canonical dividing line between SNR emission and emission from the photoionized gas in H II regions and planetary nebulae. Significant [O I] emission, another distinguishing characteristic of shocked gas, is also seen in the majority of the remnants. We find little evidence of emission line variation with SNR diameter, unlike that reported in previous SNR samples. Using published grids of shock models, we derive abundances and abundance gradients in the interstellar gas in M33. While the oxygen abundances and gradient derived agree (to within the rather large errors) with those derived from H II region studies, the nitrogen abundances and gradient show the same offset as previously noted in studies of other galaxies. Subject headings : galaxies : abundances galaxies : individual (M33) - H ii regions - nebulae: supernova remnants