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M. Blaylock, D. Branch, J. Deaton, E. Baron (Univ. of Okla.), P. M. Garnavich, R. P. Kirshner (CfA), SINS Team
UV spectra of SN~1998S have been obtained at four epochs with the HST by the SINS team, and extensive optical spectroscopy and photometry has been obtained from the ground. The spectra contain many narrow (~00 km s-1) absorption and emission features that formed in a circumstellar shell, especially at early epochs and short wavelengths. The spectra also contain broad (~000 - 10,000 km s-1) features that formed in the supernova ejecta. The broad features are unusually shallow, especially at early epochs. We have been unable to match the broad, shallow features with our standard parameterized supernova spectrum--synthesis code, SYNOW. We show analytically, for a simple model of supernova line formation by resonance scattering, that external illumination of the line-forming region makes a line shallower (and that for sufficient external illumination the P~Cygni line ``flips", then having absorption at the rest wavelength and blueshifted emission). Using a modified SYNOW code that allows for the effects of external illumination on the line source functions, we obtain improved fits to observation. This indicates that the broad line profiles of SN~1998S (and, presumably, those of some other core--collapse supernovae) are strongly affected by external illumination from the circumstellar interaction region. We use the modified version of SYNOW to estimate the velocity intervals within which the supernova line formation took place, at several epochs.
The author(s) of this abstract have provided an email address for comments about the abstract: blaylock@mail.nhn.ou.edu