  We present the first far ultraviolet (FUV) spectra of the four known
Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud,
acquired with the Far Ultraviolet Spectroscopic Explorer. The remnants DEM L 71
(0505-67.9), 0509-67.5, 0519-69.0 and 0548-70.4 are all in the non-radiative
stages of evolution and exhibit expansion speeds ranging from ~ 500 km/s to ~
5000 km/s. We have detected broad emission lines of Ly beta, Ly gamma, C III
and O VI in DEM L 71 (V(FWHM) ~ 1000 km/s) and have detected broad Ly beta and
O VI emission in 0519-69.0, (V(FWHM) ~ 3000 km/s). In addition, broad Ly beta
emission (V(FWHM) ~ 3700 km/s) has been observed in 0509-67.5, the first
detection of broad line emission from this SNR. No emission was detected in our
FUSE spectrum of 0548-70.4, allowing us to place only upper limits on the FUV
line fluxes. The spectra of these SNRs are unaffected by postshock cooling, and
provide valuable probes of collisionless heating efficiency in high Mach number
shocks. We have used the Ly beta / O VI flux ratio and relative widths of the
broad Ly beta and O VI lines to estimate the degree of electron-proton and
proton-oxygen ion equilibration in DEM L 71, 0509-67.5, and 0519-69.0. Although
our equilibration estimates are subject to considerable uncertainty due to the
faintness of the FUV lines and contributions from bulk Doppler broadening, our
results are consistent with a declining efficiency of electron- proton and
proton-oxygen ion equilibration with increasing shock speed. From our shock
velocity estimates we obtain ages of 295-585 years for 0509-67.5 and 520-900
years for 0519-69.0, respectively, in good agreement with the ages obtained
from SN light echo studies.

Astrophysics