Nebular spectra from Type Ia supernova explosion models compared to JWST observations of SN 2021aefx

Abstract

Context. Recent JWST observations of the Type Ia supernova (SN Ia) 2021aefx in the nebular phase have paved the way for late-time studies covering the full optical to mid-infrared (MIR) wavelength range, and with it the hope to better constrain SN Ia explosion mechanisms. Aims. We investigate whether public SN Ia models covering a broad range of progenitor scenarios and explosion mechanisms (Chandrasekhar-mass, or MCh, delayed detonations, pulsationally assisted gravitationally confined detonations, sub-MCh double detonations, and violent mergers) can reproduce the full optical-MIR spectrum of SN 2021aefx at ∼270 days post explosion. Methods. We consider spherically averaged 3D models available from the Heidelberg Supernova Model Archive with a 56Ni yield in the range 0.5–0.8 M⊙. We performed 1D steady-state non-local thermodynamic equilibrium simulations with the radiative-transfer code CMFGEN, and compared the predicted spectra to SN 2021aefx. Results. The models can explain the main features of SN 2021aefx over the full wavelength range. However, no single model, or mechanism, emerges as a preferred match, and the predicted spectra are similar to each other despite the very different explosion mechanisms. We discuss possible causes for the mismatch of the models, including ejecta asymmetries and ionisation effects. Our new calculations of the collisional strengths for Ni III have a major impact on the two prominent lines at 7.35 μm and 11.00 μm, and highlight the need for more accurate collisional data for forbidden transitions. Using updated atomic data, we identify a strong feature due to [Ca IV] 3.21 μm, attributed to [Ni I] in previous studies. We also provide a tentative identification of a forbidden line due to [Ne II] 12.81 μm, whose peaked profile indicates the presence of neon all the way to the innermost region of the ejecta, as predicted for instance in violent merger models. Contrary to previous claims, we show that the [Ar III] 8.99 μm line can be broader in sub-MCh models compared to near-MCh models. Last, the total luminosity in lines of Ni is found to correlate strongly with the stable nickel yield, although ionisation effects can bias the inferred abundance. Conclusions. Our models suggest that key physical ingredients are missing from either the explosion models, or the radiative-transfer post-processing, or both. Nonetheless, they also show the potential of the near- and MIR to uncover new spectroscopic diagnostics of SN Ia explosion mechanisms.

JWST MIRI/Medium Resolution Spectrograph (MRS) Observations and Spectral Models of the Underluminous Type Ia Supernova 2022xkq

“…Detailed simulations of the simmering phase suggest a single-spot ignition close to the center at ≈35-50 km s −1 Zingale et al 2011), but possibly out to 100 km s −1 (Zingale et al 2011). For the normal-bright SN 2021aefx, Blondin et al (2023) found that the delayed detonation model based on a strong off-center multispot ignition (Seitenzahl et al 2013) fails to reproduce the strength of Ni lines by factors of 2 to 3, whereas delayed-detonation models starting from a singlespot ignition can reproduce the Ni lines within the model uncertainties (DerKacy et al 2023). For SN 2022xkq, we find good agreement in the Ni lines with single-spot ignition.…”

“…Over timescales of ∼5 billion years, gravitationally driven diffusion may allow 22 Ne settling in the core of a crystallized WD (Deloye & Bildsten 2002). For an initial composition of solar metallicity, the amount may or may not be sufficient to account for some [Ni II] emission observed at about 1.9 μm in some other SNe Ia (e.g., Friesen et al 2014;Diamond et al 2015;Hoeflich et al 2021;Blondin et al 2023). However, in SN 2022xkq, we see Ni in all ionization stages.…”

Ground-based and JWST Observations of SN 2022pul. II. Evidence from Nebular Spectroscopy for a Violent Merger in a Peculiar Type Ia Supernova

“…Only the MERGER model displayed a strong, centrally peaked line due to [Ne II] 12.81 μm, resulting from the presence of Ne in the innermost ejecta layers of this model. None of the other models considered byBlondin et al (2023) presented such a strong [Ne II] feature, and only the GCD model displayed a weak flat-topped line, whose intensity at the line center was a factor ∼ 3 smaller than observed in SN 2021aefx. Only by artificially setting a minimum Ne mass fraction of 10 −2 in the inner ejecta of the GCD model were they able to match the peak intensity and profile shape of the potential weak[Ne II] line in SN 2021aefx.…”

“…Moreover, the higher density in layers 10,000 km s −1 enhances the recombination rate there, which in turn results in a lower mean ionization state compared to the other explosion models. For comparing to the nebular spectrum of SN 2022pul, we recomputed the four models studied by Blondin et al (2023) at a later time of 338 days postexplosion, using the same numerical setup. The full optical through MIR comparison of these models to the observed spectrum is displayed in Figure 7 and highlights of the results are shown in Figure 8.…”

Origin of High-velocity Ejecta, Excess Emission, and Redward Color Evolution in the Infant Type Ia Supernova 2021aefx

“…Radio observations from 2 to 8 days and a late-phase (+118 days since peak) spectrum rule out wind from a symbiotic binary progenitor and disfavors the presence of swept-up H, He, and O from the companion (Hosseinzadeh et al 2022). In addition, nebular-phase (255 and 323 days post-peak) spectra from the James Webb Space Telescope (JWST) reveal offset (∼1000 km s −1 ) core velocities and stable 58 Ni, indicating that SN 2021aefx may originate from an off-center ignited explosion (Kwok et al 2023) of a ∼1.38 M e WD (DerKacy et al 2023); though, this is argued to be inconclusive (Blondin et al 2023).…”