Radiative-transfer modeling of supernovae in the nebular-phase

Abstract

Supernova (SN) explosions play a pivotal role in the chemical evolution of the Universe and the origin of life through the metals they release. Nebular phase spectroscopy constrains such metal yields, for example through forbidden line emission associated with O I, Ca II, Fe II, or Fe III. Fluid instabilities during the explosion produce a complex 3D ejecta structure, with considerable macroscopic, but no microscopic, mixing of elements. This structure sets a formidable challenge for detailed nonlocal thermodynamic equilibrium radiative transfer modeling, which is generally limited to 1D in grid-based codes. Here, we present a novel and simple method that allows for macroscopic mixing without any microscopic mixing, thereby capturing the essence of mixing in SN explosions. With this new technique, the macroscopically mixed ejecta are built by shuffling the shells from the unmixed coasting ejecta in mass space, or equivalently in velocity space. The method requires no change to the radiative transfer, but it necessitates high spatial resolution to resolve the rapid variation in composition with depth inherent to this shuffled-shell structure. We show the results for a few radiative-transfer simulations for a Type II SN explosion from a 15 M⊙ progenitor star. Our simulations capture the strong variations in temperature or ionization between the various shells that are rich in H, He, O, or Si. Because of nonlocal energy deposition, γ rays permeate through an extended region of the ejecta, making the details of the shell arrangement unimportant. The greater physical consistency of the method delivers spectral properties at nebular times that are more reliable, in particular in terms of individual emission line strengths, which may serve to constrain the SN yields as well as the progenitor mass for core collapse SNe. The method works for all SN types.