Applications of an improved Monte Carlo code to the synthesis of early-time Supernova spectra

A Monte Carlo code developed originally for stellar winds and subsequently modified to compute synthetic spectra for the photospheric phases of supernovae adopted the simplifying assumption of line formation by coherent scattering in the matter frame. In an improved code (Lucy 1999a), the line absorption of a packet of radiant energy is followed as before by the emission of an identical amount of energy in the matter frame but now the frequency of the packet's photons is chosen so that, statistically, the branching into the available permitted downward transitions is correctly represented. This inclusion of branching is of some importance for supernova spectra, as emphasized in the light curve studies of Pinto & Eastman (2000). An improved line list is also adopted in the new code, and the emergent spectrum is computed using the formal integral. Examples of synthetic spectra computed with this new code for supernovae of types Ia and II are presented and the improvements introduced by the incorporation of branching are discussed. Previous results obtained with a pure scattering code are shown not to be invalidated. Possible further code developments to achieve a fully NLTE Monte Carlo code are briefly described.