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Context. Type Ia supernovae (SN Ia) have become an invaluable cosmological tool because their exceptional brightness makes them observable even at very large distances (up to redshifts around z ≈ 1). To investigate possible systematic differences between local and distant SN Ia requires detailed models whose synthetic spectra can be compared to observations and in which the solution of the radiative transfer is a key ingredient. One commonly employed method is the Feautrier scheme, which is generally very robust but can lead to wrong results under certain conditions that frequently occur when modeling supernova ejecta or even the radiatively driven expanding atmospheres of hot stars. Aims. We attempt to improve the procedure we have developed for simulating the radiative transfer of metal-rich, intermediate- and low-density, line-dominated atmospheres to allow the method to be applied successfully even under conditions of high expansion velocities. Methods. We use a sophisticated model atmosphere code that considers the non-LTE effects and large velocity gradients that strongly affect the physics of SN Ia atmospheres at all wavelengths to simulate the formation of SN Ia spectra by the thousands of strong spectral lines that intricately interact with the "pseudo-continuum" formed entirely by these Doppler-shifted lines themselves. We focus on investigating the behavior of the Feautrier scheme under these conditions. Results. Synthetic spectra of SN Ia, a complex product of computer models replicating numerous physical processes that determine the conditions of matter and radiation in the ejecta, are affected by large spatial jumps of the line-dominated opacities and source functions for which the application of even well established methods may harbor certain pitfalls. We analyse the conditions that can lead to a breakdown of conventional procedures and we derive a modified description that yields more accurate results in the given circumstances for the Feautrier radiative transfer solver. [ABSTRACT FROM AUTHOR]

Context. In type Ia supernova (SN Ia) envelopes a huge number of lines of different elements overlap within their thermal Doppler widths, and this problem is exacerbated by the circumstance that up to 20% of these lines can have a line optical depth higher than 1. The stagnation of the lambda iteration in such optically thick cases is one of the fundamental physical problems inherent in the iterative solution of the non-LTE problem, and the failure of a lambda iteration to converge is a point of crucial importance whose physical significance must be understood completely. Aims. We discuss a general problem related to radiative transfer under the physical conditions of supernova ejecta that involves a failure of the usual non-LTE iteration scheme to converge when multiple strong opacities belonging to different physical transitions come together, similar to the well-known situation where convergence is impaired even when only a single process attains high optical depths. The convergence problem is independent of the chosen frequency and depth grid spacing, independent of whether the radiative transfer is solved in the comoving or observer's frame, and independent of whether a common complete-linearization scheme or a conventional accelerated lambda iteration (ALI) is used. The problem appears when all millions of line transitions required for a realistic description of SN Ia envelopes are treated in the frame of a comprehensive non-LTE model. The only solution to this problem is a complete-linearization approach that considers all ions of all elements simultaneously, or an adequate generalization of the established ALI technique that accounts for the mutual interaction of the strong spectral lines of different elements and which thereby unfreezes the "stuck" state of the iteration. Methods. The physics of the atmospheres of SN Ia are strongly affected by the high-velocity expansion of the ejecta, which dominates the formation of the spectra at all wavelength ranges. Thus, hydrodynamic explosion models and realistic model atmospheres that take into account the strong deviation from local thermodynamic equilibrium (LTE) are necessary for the synthesis and analysis of the spectra. In this regard one of the biggest challenges we have found in modeling the radiative transfer in SN Ia is the fact that the radiative energy in the UV has to be transferred only via spectral lines into the optical regime to be able to leave the ejecta. However, convergence of the model toward a state where this is possible is impaired when using the standard procedures. We report on improvements in our approach of computing synthetic spectra for SN Ia with respect to (i) an improved and sophisticated treatment of many thousands of strong lines that interact intricately with the "pseudo-continuum" formed entirely by Doppler-shifted spectral lines; (ii) an improved and expanded atomic database; and (iii) the inclusion of energy deposition within the ejecta arising from the radioactive decay of mostly 56Ni and 56Co. Results. We show that an ALI procedure we have developed for the mutual interaction of strong spectral lines appearing in the atmospheres of SNe Ia solves the long-standing problem of transferring the radiative energy from the UV into the optical regime. Our new method thus constitutes a foundation for more refined models, such as those including energy deposition. In this regard we furthermore show synthetic spectra obtained with various methods adopted for the released energy and compare them with observations. We discuss in detail applications of the diagnostic technique by example of a standard type Ia supernova, where the comparison of calculated and observed spectra revealed that in the early phases the consideration of the energy deposition within the spectrum-forming regions of the ejecta does not qualitatively alter the shape of the emergent spectra. Conclusions. The results of our investigation lead to an improved understanding of how the shape of the spectrum changes radically as function of depth in the ejecta, and show how different emergent spectra are formed as a result of the particular physical properties of SNe Ia ejecta and the resulting peculiarities in the radiative transfer. This knowledge provides an important insight into the process of extracting information from observed SN Ia spectra, since these spectra are a complex product of numerous unobservable SN Ia spectral features, which are thus analyzed in parallel to the observable SN Ia spectral features. [ABSTRACT FROM AUTHOR]