The optical spectrum of the SN 1006 supernova remnant revisited

We present the deepest optical spectrum acquired to date of Balmer-dominated shocks in the northwest rim of SN 1006. We detect the broad and narrow components of Halpha, Hbeta, and Hgamma and report the first detection of the He I lambda6678 emission line in this supernova remnant. We may have detected, at the 1.5 sigma level, faint He II lambda4686 emission. We measure a full width at half-maximum of 2290 +/- 80 km s(-1) in the broad-component Halpha line, with broad-to-narrow flux ratios of 0.84(-0.01)(+0.03) and 0.93(-0.16)(+0.18) in Halpha and Hbeta, respectively. To match these observations, our nonradiative shock models require a low degree of electron-proton equilibration at the shock front, T-e/T-p less than or equal to 0.07, and a shock speed of 2890 +/- 100 km s(-1). These results agree well with an earlier analysis of ultraviolet lines from SN 1006. The He I/Halpha and He i /He II flux ratios also indicate low equilibration. Furthermore, our models match the observations for mostly ionized (similar to90%) preshock H and mostly neutral (greater than or similar to70%) preshock He, respectively. We conclude that the high H ionization fraction cannot be explained by either photoionization from the reverse shock or relic ionization from EUV photons released in the A.D. 1006 supernova. The most plausible explanation appears to be photoionization from the Galactic Lyman continuum.