Interpretation of Ultraviolet Absorption Lines in SN 1006

We present a theoretical interpretation of the broad silicon and iron ultraviolet absorption features observed with the Hubble Space Telescope (HST) in the spectrum of the Schweizer-Middleditch star behind the remnant of SN 1006. These features are caused by supernova ejecta in SN 1006. We propose that the redshifted Si II 1260 Å feature consists of both unshocked and shocked Si II. The sharp red edge of the line at 7070 km s-1 indicates the position of the reverse shock, while its Gaussian blue edge reveals shocked Si with a mean velocity of 5050 km s-1 and a dispersion of 1240 km s-1, which implies a reverse shock velocity of 2860 km s-1. The measured velocities satisfy the energy jump condition for a strong shock, provided that all the shock energy goes into ions, with little or no collisionless heating of electrons. The line profiles of the Si III and Si IV absorption features indicate that they arise mostly from shocked Si. The total mass of shocked and unshocked Si inferred from the Si II, Si III, and Si IV profiles is MSi = 0.25 ± 0.01 M☉ on the assumption of spherical symmetry. Unshocked Si extends upward from 5600 km s-1. Although there appears to be some Fe mixed with the Si at lower velocities ≲7070 km s-1, the absence of Fe II absorption with the same profile as the shocked Si II suggests little Fe mixed with Si at higher (before being shocked) velocities. The column density of shocked Si II is close to that expected for Si II undergoing steady state collisional ionization behind the reverse shock, provided that the electron to Si II ratio is low, from which we infer that most of the shocked Si is likely to be of a fairly high degree of purity, unmixed with other elements. We propose that the ambient interstellar density on the far side of SN 1006 is anomalously low compared to the density around the rest of the remnant. This would simultaneously explain the high velocity of the redshifted Si absorption, the absence of blueshifted Si absorption, and the low density of the absorbing Si compared to the high Si density required to produce the observed Si X-ray line emission. We have reanalyzed the Fe II absorption lines and have concluded that the earlier evidence for high-velocity blueshifted Fe II extending to ~-8000 km s-1 is not compelling. We interpret the blue edge on the Fe II profiles at -4200 km s-1 as the position of the reverse shock on the near side of SN 1006. The mass of Fe II inferred from the red edge of the Fe II profile is MFe II = 0.029 ± 0.004 M☉ up to 7070 km s-1, if spherical symmetry is assumed. The low ionization state of unshocked Si inferred from our analysis of the silicon features, Si II/Si = 0.92 ± 0.07, suggests a correspondingly low ionization state of unshocked iron, with Fe II/Fe = 0.66+ 0.29−0.22. If this is correct, then the total mass of Fe up to 7070 km s-1 is MFe = 0.044−0.013+0.022 M☉ with a 3 σ upper limit of MFe < 0.16 M☉. Such a low ionization state and mass of iron is consistent with the recent observation of Fe III 1123 Å with the Hopkins Ultraviolet Telescope (HUT), which indicates Fe III/Fe II = 1.1 ± 0.9 but conflicts with the expected presence of several tenths of a solar mass of iron in this suspected Type Ia supernova remnant. However, the inference from the present HST data is too indirect, and the HUT data are too noisy, to rule out a large mass of iron. Reobservation of the Fe III 1123 Å line at higher signal-to-noise ratio with Far Ultraviolet Space Explorer will be important in determining the degree of ionization and hence mass of iron in SN 1006.