Quantitative Spectroscopy of Photospheric-Phase Type II Supernovae

We present first results on the quantitative spectroscopic analysis of the photospheric-phase of Type II supernovae (SN). The analyses are based on the non-LTE model atmosphere code, CMFGEN, of Hillier & Miller (1998). A key asset of CMFGEN is its thorough treatment of line-blanketing due to metal species. From its applicability to hot star environments, the main modifications to the source code were to allow a linear velocity law, a power-law density distribution, an adaptive grid to handle the steep H recombination/ionization front occurring in cooler SN models, and a routine to compute the gray temperature structure in the presence of large velocities. In this first paper we demonstrate the ability of CMFGEN to reproduce, with a high level of accuracy, the UV and optical observations of a sample of well observed Type II SN, i.e. SN1987A and SN1999em, at representative stages of their photospheric evolution. Two principal stages of SN are modeled - that where hydrogen is fully ionized, and that in which H is only partially ionized. We provide detailed and accurate line identifications for our sample of epochs. We discuss the impact on the SED of varying the density exponent, the spatial scale of the SN, the value of Rmax and the metallicity; we illustrate the formation sites for selected lines. Both Hydrogen Balmer and Helium lines (when present) are very well reproduced, even for our adopted CNO-equilibrium abundance values adequate for a massive supergiant progenitor star. NII lines are also predicted in the blue-wing of both Hbeta and HeI 5875A, and thus not associated with peculiar absorption/emission far above the photosphere. We finally present a pedagogical discussion of P-Cygni profile formation and explain, e.g., the noticeable blue-shift of Halpha in early time spectra (much abridged abstract).