Preliminary Spectral Analysis of the Type II Supernova 1999em

We have calculated fast direct spectral model fits to two early-time spectraof the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. Thefirst is an extremely early blue optical spectrum and the second a combined HSTand optical spectrum obtained one week later. Spectroscopically this supernovaappears to be a normal Type II and these fits are in excellent agreement withthe observed spectra. Our direct analysis suggests the presence of enhancednitrogen. We have further studied these spectra with the full NLTE generalmodel atmosphere code PHOENIX. While we do not find confirmation for enhancednitrogen (nor do we rule it out), we do require enhanced helium. An even moreintriguing possible line identification is complicated Balmer and He I lines,which we show falls naturally out of the detailed calculations with a shallowdensity gradient. We also show that very early spectra such as those presentedhere combined with sophisticated spectral modeling allows an independentestimate of the total reddening to the supernova, since when the spectrum isvery blue, dereddening leads to changes in the blue flux that cannot bereproduced by altering the ``temperature'' of the emitted radiation. Theseresults are extremely encouraging since they imply that detailed modeling ofearly spectra can shed light on both the abundances and total extinction of SNeII, the latter improving their utility and reliability as distance indicators.Comment: to appear in ApJ, 2000, 54