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P. Ghavamian (Rice), J. Raymond (Harvard-Smithsonian CfA), P. Hartigan (Rice), W. P. Blair (Johns Hopkins)
We report optical spectroscopy and imaging of Tycho's supernova remnant which suggest the presence of a He~II \lambda304 photoionization precursor associated with the blast wave. In longslit spectra of the eastern rim, we find diffuse H\alpha emission extending \gtrsim\,1\arcmin ahead of Knot g, a bright nonradiative shock propagating at velocity ~\,2000 km s-1. In addition to the hydrogen lines, the diffuse spectrum also exhibits [SII] \lambda\lambda 6716, 6731, [NII] \lambda 6583 and weak [OIII] \lambda 5007 emission. Narrow band imagery of Tycho's SNR shows that the diffuse emission extends ahead of most of the Balmer-dominated optical filaments, and drops sharply in the remnant interior, suggesting that the diffuse gas is overrun by the supernova shock. We compare the line ratios of the diffuse gas with predictions from a simple He~II \lambda 304 photoionization precursor model. The models predict that the radiative transfer of He~II photons in the precursor produces spatially distinct ionization layers of H0, He0-\,He+, N0-\,N++, O0-\,O++ and S+-\,S3+ ahead of the shock. The models suggest that the diffuse emission arises from collisional excitation in mostly neutral gas which has been heated to a temperature ~,12,000~K.
The author(s) of this abstract have provided an email address for comments about the abstract: parviz@sparky.rice.edu