We present spectra of optical filaments associated with the X-ray knot D in the Vela supernova remnant .
It has been suggested that Knot D is formed by a bullet of supernova ejecta , that it is a break-out of the shock front of the Vela SNR , and also that it is an outflow from the recently discovered remnant RXJ0852.0-4622 .
We find that Knot D is a bow shock propagating into an interstellar cloud with normal abundances and typical cloud densities ( n _ { H } \sim 4 – 11 cm ^ { -3 } ) .
Optical longslit spectra show that the [ S II ] \lambda \lambda 6716,6731 to H \alpha line ratio is greater than unity , proving that the optical filaments are shock excited .
The analysis of far-ultraviolet spectra obtained with the Hopkins Ultraviolet Telescope and with the Far Ultraviolet Spectroscopic Explorer ( FUSE ) LWRS aperture show that slower shocks ( \sim 100 km s ^ { -1 } ) produce most of the low ionization lines such as O III ] \lambda 1662 , while faster shocks ( \sim 180 km s ^ { -1 } ) produce the O VI \lambda \lambda 1032,1038 and other high ionization lines .
C III and O VI lines are also detected in the FUSE MDRS aperture , which was located on an X-ray bright region away from the optical filaments .
The lines have two velocity components consistent with \sim 150 km s ^ { -1 } shocks on the near and far sides of the knot .
The driving pressure in the X-ray knot , P / k _ { B } \sim 1.8 \times 10 ^ { 7 } cm ^ { -3 } K , is derived from the shock properties .
This is over an order of magnitude larger than the characteristic X-ray pressure in the Vela SNR .
The velocity distribution of the emission and the overpressure support the idea that Knot D is a bow shock around a bullet or cloud that originated near the center of the Vela remnant .