2003ApJ...589..242S

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 breakout of the shock front of the Vela supernova remnant, and also that it is an outflow from the recently discovered remnant RX J0852.0-4622. We find that knot D is a bow shock propagating into an interstellar cloud with normal abundances and typical cloud densities (n_H∼4-11 cm^–3). Optical long-slit spectra show that the [S II] λλ6716, 6731 to Hα 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 (∼100 km/s) produce most of the low-ionization lines such as O III] λ1662, while faster shocks (∼180 km/s) produce the O VI λλ1032, 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 ∼150 km/s shocks on the near and far sides of the knot. The driving pressure in the X-ray knot, P/k_B∼1.8x10⁷ 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 supernova remnant. 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.