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We analyze the Type II Plateau supernovae (SN II-P) 2005cs and 2006bp withthe non-LTE model atmosphere code CMFGEN. We fit 13 spectra in the first monthfor SN 2005cs and 18 for SN 2006bp. {\sl Swift} ultraviolet photometry andground-based optical photometry calibrate each spectrum. Our analysis showsboth objects were discovered less than 3 days after they exploded, making thesethe earliest SN II-P spectra ever studied. They reveal broad and very weaklines from highly-ionized fast ejecta with an extremely steep density profile.We identify He{\sc ii} 4686\AA emission in the SN 2006bp ejecta. Days later,the spectra resemble the prototypical Type II-P SN 1999em, which had asupergiant-like photospheric composition. Despite the association of SN 2005cswith possible X-ray emission, the emergent UV and optical light comes from thephotosphere, not from circumstellar emission.  We surmise that the very steep density fall-off we infer at early times maybe a fossil of the combined actions of the shock wave passage and radiationdriving at shock breakout. Based on tailored CMFGEN models, the direct-fittingtechnique and the Expanding Photosphere Method both yield distances andexplosion times that agree within a few percent. We derive a distance to NGC5194, the host of SN 2005cs, of 8.9$\pm$0.5 Mpc and 17.5$\pm$0.8 Mpc for SN2006bp in NGC 3953. The luminosity of SN 2006bp is 1.5 times that of SN 1999em,and 6 times that of SN 2005cs. Reliable distances to Type II-P supernovae thatdo not depend on a small range in luminosity provide an independent route tothe Hubble Constant and improved constraints on other cosmological parameters.

Using Quantitative Spectroscopic Analysis to Determine the Properties and Distances of Type II-Plateau Supernovae: SNe 2005cs and 2006bp