NLTE MODELS FOR SYNTHETIC SPECTRA OF TYPE IA SUPERNOVAE - THE INFLUENCE OF LINE BLOCKING

A method to compute a NLTE model of the atmosphere of a type Ia supern ova (SN Ia) near maximum light is presented. The determination of the level populations is carried out using detailed atomic models, and inc luding all important contributions to the rate equations: Thomson scat tering, bound-free (from ground and excited levels) and free-free opac ities, line absorption and emission processes. Dielectronic recombinat ion is included. The spherical radiation transfer is solved at up to 4 00 frequency points and 41 depth points. Finally, a synthetic spectrum is computed using a formal integral solution of the transfer equation based on a spatial microgrid. It is found that the SN atmosphere is e lectron scattering-dominated, and that the high velocity of the appare nt photosphere (similar to 8000 km/s) is due to the pseudo-continuum o pacity created by the thick line forest which blocks the flux in the U V and optical part of the spectrum. Increasingly more sophisticated tr eatments of the process of flux blocking in the UV (line blocking) are discussed. The necessity of treating the far-UV flux correctly is dem onstrated. Line blocking in the region 800-1300 Angstrom reduces the p hotoionization from the excited levels of several important ions (e.g. Fe II, Co II, Si II, Ca II), thus decreasing the overall degree of io nization. This effect is clearly seen in the synthetic emergent spectr a. Synthetic spectra obtained with the various methods adopted for lin e blocking are shown, and compared to one another. When line blocking is properly treated, the synthetic spectrum reproduces well the spectr um of the 'normal' SN Ia 1992A from the UV to the near-IR.