We present a theoretical interpretation of the broad silicon and iron ultraviolet absorption features observed with the Hubble Space Telescope in the spectrum of the Schweizer-Middleditch star behind the remnant of Supernova 1006 .
These features are caused by supernova ejecta in SN1006 .
We propose that the redshifted \ion Si2 1260 Å feature consists of both unshocked and shocked \ion Si2 .
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 } , implying 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 \ion Si3 and \ion Si4 absorption features indicate that they arise mostly from shocked Si .
The total mass of shocked and unshocked Si inferred from the \ion Si2 , \ion Si3 and \ion Si4 profiles is M _ { Si } = 0.25 \pm 0.01 { M } _ { \sun } on the assumption of spherical symmetry .
Unshocked Si extends upwards from 5600 { km } { s } ^ { -1 } .
Although there appears to be some Fe mixed with the Si at lower velocities \la 7070 { km } { s } ^ { -1 } , the absence of \ion Fe2 absorption with the same profile as the shocked \ion Si2 suggests little Fe mixed with Si at higher ( before being shocked ) velocities .
The column density of shocked \ion Si2 is close to that expected for \ion Si2 undergoing steady state collisional ionization behind the reverse shock , provided that the electron to \ion Si2 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 SN1006 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 \ion Fe2 absorption lines , concluding that the earlier evidence for high velocity blueshifted \ion Fe2 extending to \sim - 8000 { km } { s } ^ { -1 } is not compelling .
We interpret the blue edge on the \ion Fe2 profiles at -4200 { km } { s } ^ { -1 } as the position of the reverse shock on the near side of SN1006 .
The mass of \ion Fe2 inferred from the red edge of the \ion Fe2 profile is M _ { FeII } = 0.029 \pm 0.004 { M } _ { \sun } 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 , \ion Si2/Si = 0.92 \pm 0.07 , suggests a correspondingly low ionization state of unshocked iron , with \ion Fe2/Fe = 0.66 ^ { +0.29 } _ { -0.22 } .
If this is correct , then the total mass of Fe up to 7070 { km } { s } ^ { -1 } is M _ { Fe } = 0.044 ^ { +0.022 } _ { -0.013 } { M } _ { \sun } with a 3 \sigma upper limit of M _ { Fe } < 0.16 { M } _ { \sun } .
Such a low ionization state and mass of iron is consistent with the recent observation of \ion Fe3 1123 Å with HUT , indicating \ion Fe3/ \ion Fe2 = 1.1 \pm 0.9 , but conflicts with the expected presence of several tenths of a solar mass of iron in this suspected Type Ia 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 .
Re-observation of the \ion Fe3 1123 Å line at higher signal to noise ratio with FUSE will be important in determining the degree of ionization and hence mass of iron in SN1006 .