A NEW OPTICAL-SAMPLE OF SUPERNOVA-REMNANTS IN M33

We present a new and larger sample of supernova remnants in the nearby spiral galaxy M33. The sample is based upon CCD interference filter o bservations obtained with the Kitt Peak 4 m telescope and spectroscopi c observations obtained with the Multiple Mirror Telescope. Using opti cal emission-line ratios, supplemented by a radio continuum map of M33 (Duric et al.; Gordon et al.), we have identified 98 supernova remnan t (SNR) candidates, of which 53 were previously unknown. We have obtai ned spectra of 27 SNR candidates, bringing the total number of M33 SNR s for which spectra are available to 72. All the spectra show the char acteristic signature of shock-heated gas, which leads us to believe th at the rest of the candidates are also supernova remnants. The large s ample provides a useful database to investigate the global properties of SNRs, In this paper, we present a new cumulative number-diameter [N (<D)] relation for SNRs, We find that the free expansion model is inco nsistent with the observed N(<D) relation and that the Sedov-Taylor (S T) expansion model is a much better fit. The ST expansion model is use d to derive a SN rate in M33 of one SN every 360 yr. The spectroscopic sample of SNRs is the largest of its kind in any galaxy. Our investig ations show no relation between [N II]:H alpha and [S II] lambda 6717: lambda 6731. We do, however, find weak trends between the SNR diameter and [N II]:H alpha and [S II]:H alpha. Further, we find a strong corr elation between [N II]:H alpha and the galactocentric distance (GCD) i ndicative of an abundance gradient. There is a large dispersion in the [N II]:H alpha ratio at a given GCD that cannot be explained by evolu tionary effects and that may be due to a larger abundance dispersion i n the inner part of the galaxy.