A high resolution UV absorption spectrum of supernova ejecta in SN 1006

We report a high resolution, far-ultraviolet, STIS E140M spectrum of the strong, broad Si II, III, and IV features produced by the ejecta of SN1006 seen in absorption against the background Schweizer-Middleditch star. The spectrum confirms the extreme sharpness of the red edge of the redshifted Si II 1260 Å feature, supporting the idea that this edge represents the location of the reverse shock moving into the freely expanding ejecta. The expansion velocity of ejecta at the reverse shock is measured to be 7026± 3 (relative) ± 10 (absolute) km s. If the shock model is correct, then the expansion velocity should be decreasing at the observable rate of 2.7± 0.1 km s per year. The pre-shock velocity, post-shock velocity, and post-shock velocity dispersion are all measured from the Si II 1260 Å feature, and consistency of these velocities with the shock jump conditions implies that there is little or no electron heating in this fast (2680 km s) Si-rich shock.