A SEARCH FOR FE-III IN SN-1006 USING THE HOPKINS ULTRAVIOLET TELESCOPE

We have observed the 912-1840 Angstrom spectrum of the sdOB star known as the Schweizer-Middleditch star, which lies behind the remnant of S N 1006, to search for absorption lines due to the unshocked ejecta. We have also observed the sdB star PG 0839+399 for comparison. The spect ra were obtained using the Hopkins Ultraviolet Telescope on the Astro- 2 Space Shuttle mission in 1995 March. Assuming SN 1006 was a Type Ia supernova, 0.3-0.5 M. of iron should be present in the expanding remna nt. The X-ray spectrum shows no evidence for this quantity of iron in the hot gas, indicating that this material may still be interior to th e reverse shock. Broad absorption lines of Fe II have been seen in the spectrum of the Schweizer-Middleditch (SM) star previously but can on ly account for a small fraction of the expected iron. Our spectrum cov ers the region of the strongest expected Fe III resonance line at 1123 Angstrom. We use stellar model fits to the spectra to constrain the s trength of Fe III and Fe II lines in the spectral level of only 1.0 An gstrom equivalent width, below the value of similar to 2.3 Angstrom pr edicted with the current best model provided by Hamilton & Fesen. Howe ver, the Hamilton & Fesen model is not precluded by the statistics in our data, which provide a 3 sigma upper limit of 3.4 Angstrom for the equivalent width of Fe III lambda 1123. Converting into mass, our best fit corresponds to only 0.016 M. of Fe++, and the 3 sigma limit corre sponds to less than or equal to 0.054 M. of Fe++. This indicates that if the expected iron is present in the ejecta, it must be largely in i onization states higher than Fe++. No other absorption lines attributa ble to the supernova remnant are detected in the spectrum below Ly alp ha. A faint excess in the spectrum near 1170 Angstrom is unexplained b ut might be due to Ly alpha emission from the nonradiative shock on th e near side of the supernova remnant shell. This possibility is testab le with improved optical data for SN 1006.