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The Astrophysical Journal


Department of Physics and Astronomy


We have obtained a far-ultraviolet spectrum (1150-1600 Å) of a hot subdwarf star behind the remnant of supernova 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 s-1 and are not at the rest wavelengths of known interstellar lines, as opposed to the broader (~8000 km s-1 FWHM) Fe II lines from the remnant centered at 0 km s-1 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 s-1.

The Si II λ1260.4 profile is asymmetric, with a nearly flat core and sharp red wing, unlike the Si II λ1526.7 and Si IV λλ1393.8, 1402.8 profiles. One possible explanation is additional absorption from another species. Previous work has suggested that S II λλ1250.6, 1253.8, 1259.5 at a radial velocity of ~6000 km s-1 is responsible, but this would require a sulfur-to-silicon abundance ratio that is at least a factor of 10 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 λ1526.7 profile.