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.
Highlights
The complicated 3D structure observed in supernova (SN) remnants from Cas A (Fesen et al 2006) to the young SN 1987A (Abellán et al 2017), or the detection of polarized radiation from Type II SNe (Shapiro & Sutherland 1982; Leonard et al 2006) provide evidence that even standard core-collapse SNe are inherently asymmetric and heterogeneous
The complex 3D ejecta structure sets a challenge for the radiative transfer modeling of SN radiation
In this Letter, we present a novel treatment of chemical mixing in spherical symmetry, which is applicable to all SN types, and requires no adjustment to the radiative transfer, which was performed with the nonlocal thermodynamic equilibrium radiative transfer code CMFGEN (Hillier & Dessart 2012) for the purposes of this study
Summary
The complicated 3D structure observed in supernova (SN) remnants from Cas A (Fesen et al 2006) to the young SN 1987A (Abellán et al 2017), or the detection of polarized radiation from Type II SNe (Shapiro & Sutherland 1982; Leonard et al 2006) provide evidence that even standard core-collapse SNe are inherently asymmetric and heterogeneous. The complex 3D distribution of elements likely arises from the neutrino-driven explosion mechanism combined with fluid instabilities triggered by the propagation of the SN shock across the progenitor envelope (Fryxell et al 1991; Kifonidis et al 2000; Wongwathanarat et al 2015; Ono et al 2020) This structure contrasts with the progenitor chemical distribution, which is thought to exhibit stacked shells of a distinct composition (this picture is being challenged by recent simulations; Couch et al 2015), and with an increasing mean atomic weight towards the denser inner layers of the star, culminating with the Fe core in the innermost layers when the massive star is ripe for core collapse (see, e.g., Arnett 1996). The common expedient assumes an overall spherical symmetry of the homologously expanding ejecta, but with an enhanced mixing of all species This mixing is both macroscopic (material from low velocity is advected out to a large velocity, and vice versa) and microscopic (the “advected” material is fully mixed with the material at its new location). Because the composition is altered in a nonphysical manner, the opacity and emissivity of the plasma is no longer accurate, which may impact the SN color, the multiband light curves, and the spectral properties
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