Probing Multiple Sight Lines through the SN 1006 Remnant by Ultraviolet Absorption Spectroscopy

Absorption-line spectroscopy is an effective probe for cold ejecta within a supernova remnant (SNR), provided that suitable background UV sources can be identified. For the SN 1006 remnant we have identified four such sources, in addition to the much-studied Schweitzer-Middleditch (SM) star. We have used STIS on the Hubble Space Telescope to obtain UV spectra of all four sources, to study "core samples" of the SN 1006 interior. The line of sight closest to the center of the SNR shell, passing only 2.′0 away, is to a V = 19.5 QSO at z = 1.026. Its spectrum shows broad Fe II absorption lines, asymmetric with red wings broader than blue. The similarity of these profiles to those seen in the SM star, which is 2.′8 from the center in the opposite direction, confirms the existence of a bulge on the far side of SN 1006. The Fe II equivalent widths in the QSO spectrum are ~50% greater than in the SM star, suggesting that somewhat more iron may be present within SN 1006 than studies of the SM star alone have indicated, but this is still far short of what most Type Ia supernova models require. The absorption spectrum against a brighter z = 0.337 QSO seen at 57% of the shell radius shows broad silicon absorption lines but no iron other than narrow, probably interstellar lines. The cold iron expanding in this direction must be confined within v ≲ 5200 km s-1, also consistent with a high-velocity bulge on the far side only. The broad silicon lines indicate that the silicon layer has expanded beyond this point, and that it has probably been heated by a reverse shock—conclusions consistent with the clumpy X-ray structure and anomalous abundances observed from Chandra in this region. Finally, the spectra of two ~A0 V stars near the southern shell rim show no broad or unusually strong absorption lines, suggesting that the low-ionization ejecta are confined within 83% of the shell radius, at least at the azimuths of these background sources.