6 BALMER-DOMINATED SUPERNOVA-REMNANTS

We present optical spectra of six Balmer-dominated supernova remnants (SNRs): the Galactic remnants of SN 1572 (Tycho) and SN 1006, and four remnants in the Large Magellanic Cloud (LMC), 0505-67.9, 0509-67.5, 0519-69.0, and 0548-70.4. All except 0509-67.5 exhibit broad (580-2300 km s-1 FWHM) H-alpha emission characteristic of collisionless shocks. We have measured the line widths and compared them to models for a collisionless shock interacting with a partially neutral medium. From this comparison, we derive a shock velocity for each remnant with broad H-alpha and a possible lower limit to the shock velocity in 0509-67.5. Although previous analyses used the ratio of broad to narrow intensity as an indicator of shock velocity, our models show that this is not reliable. Further, the observed intensity ratios do not correspond to either the commonly assumed case of no equilibration (Te << T(ion)), or to the other extreme of total equilibration (T(e) = T(ion)). By combining the observed proper motions of filaments in Tycho and SN 1006 with our estimates of the shock velocities, we obtain distances to these remnants of 1.5-3.1 kpc and 1.4-2.8 kpc, respectively. The shock velocities derived for the LMC remnants allow us to estimate their ages. The ages range from approximately 10(4) yr for the two which show evidence of transition to radiative shocks, 0505-67.9 and 0548-70.4, to less than 1500 yr for the small remnants 0509-67.5 and 0519-69.0. These two young remnants bring the number of supernovae in the LMC to at least five in the past 1500 yr, making the supernova rate per unit luminosity comparable to that reported for late spiral galaxies.