  We have obtained a far-ultraviolet spectrum (1150 - 1600 Ang.) of a hot
subdwarf star behind the remnant of SN 1006 with the Faint Object Spectrograph
(FOS) on the Hubble Space Telescope. The high-quality spectrum is used to test
previous identifications of the strong absorption features discovered with the
International Ultraviolet Explorer. These features have FWHM = 4000 (+/- 300)
km/sec and are not at the rest wavelengths of known interstellar lines, as
opposed to the broader (8000 km/sec FWHM) Fe II lines from the remnant centered
at zero km/sec in near-UV FOS spectra. We confirm that the broad absorption
features are principally due to redshifted Si II, Si III, and Si IV lines,
which are centered at a radial velocity of 5100 (+/- 200) km/sec.
  The Si II 1260.4 profile is asymmetric, with a nearly flat core and sharp red
wing, unlike the Si II 1526.7 and Si IV 1393.8, 1402.8 profiles. One possible
explanation is additional absorption from another species. Previous work has
suggested that S II 1250.6, 1253.8, 1259.5 at a radial velocity of 6000 km/sec
is responsible, but this would require a sulfur to silicon abundance ratio that
is at least a factor of ten higher than expected. Another possible explanation
is that the Si II and Si IV profiles are intrinsically different, but this does
not explain the symmetric (albeit weaker) Si II 1526.7 profile.
