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1. JWST Low-resolution MIRI Spectral Observations of SN 2021aefx: High-density Burning in a Type Ia Supernova

   https://doi.org/10.3847/2041-8213/acb8a8  

   DerKacy, J. M. ; Ashall, C. ; Hoeflich, P. ; Baron, E. ; Shappee, B. J. ; Baade, D. ; Andrews, J. ; Bostroem, K. A. ; Brown, P. J. ; Burns, C. R. ; et al ( February 2023 , The Astrophysical Journal Letters)

   We present a JWST/MIRI low-resolution mid-infrared (MIR) spectroscopic observation of the normal Type Ia supernova (SN Ia) SN 2021aefx at +323 days past rest-frameB-band maximum light. The spectrum ranges from 4 to 14μm and shows many unique qualities, including a flat-topped [Ariii] 8.991μm profile, a strongly tilted [Coiii] 11.888μm feature, and multiple stable Ni lines. These features provide critical information about the physics of the explosion. The observations are compared to synthetic spectra from detailed non–local thermodynamic equilibrium multidimensional models. The results of the best-fitting model are used to identify the components of the spectral blends and provide a quantitative comparison to the explosion physics. Emission line profiles and the presence of electron capture elements are used to constrain the mass of the exploding white dwarf (WD) and the chemical asymmetries in the ejecta. We show that the observations of SN 2021aefx are consistent with an off-center delayed detonation explosion of a near–Chandrasekhar mass (M_Ch) WD at a viewing angle of −30° relative to the point of the deflagration to detonation transition. From the strengths of the stable Ni lines, we determine that there is little to no mixing in the central regions of the ejecta. Based on both the presence of stable Ni and the Ar velocity distributions, we obtain a strict lower limit of 1.2M_⊙for the initial WD, implying that most sub-M_Chexplosions models are not viable models for SN 2021aefx. The analysis here shows the crucial importance of MIR spectra in distinguishing between explosion scenarios for SNe Ia.

    

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2. Stable nickel production in type Ia supernovae: A smoking gun for the progenitor mass?

   https://doi.org/10.1051/0004-6361/202142323  

   Blondin, S. ; Bravo, E. ; Timmes, F. X. ; Dessart, L. ; Hillier, D. J. ( April 2022 , Astronomy & Astrophysics)

   Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit ( M Ch ≈ 1.4 M ⊙ ) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable iron-group elements synthesized in the explosion has been suggested as a possible discriminant between M Ch and sub- M Ch events. In particular, it is thought that the higher-density ejecta of M Ch WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni  II ] lines in late-time spectra (≳150 d past explosion). Aims. We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from M Ch and sub- M Ch progenitors. We explore the potential for lines of [Ni  II ] in the optical an near-infrared (at 7378 Å and 1.94 μm) in late-time spectra to serve as a diagnostic of the exploding WD mass. Methods. We reviewed stable Ni yields across a large variety of published SN Ia models. Using 1D M Ch delayed-detonation and sub- M Ch detonation models, we studied the synthesis of stable Ni isotopes (in particular, 58 Ni) and investigated the formation of [Ni  II ] lines using non-local thermodynamic equilibrium radiative-transfer simulations with the CMFGEN code. Results. We confirm that stable Ni production is generally more efficient in M Ch explosions at solar metallicity (typically 0.02–0.08 M ⊙ for the 58 Ni isotope), but we note that the 58 Ni yield in sub- M Ch events systematically exceeds 0.01 M ⊙ for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction 57 Co( p ,  γ ) 58 Ni is the dominant production mode for 58 Ni in both M Ch and sub- M Ch models, while the α -capture reaction on 54 Fe has a negligible impact on the final 58 Ni yield. More importantly, we demonstrate that the lack of [Ni  II ] lines in late-time spectra of sub- M Ch events is not always due to an under-abundance of stable Ni; rather, it results from the higher ionization of Ni in the inner ejecta. Conversely, the strong [Ni  II ] lines predicted in our 1D M Ch models are completely suppressed when 56 Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements. Conclusions. [Ni  II ] lines in late-time SN Ia spectra have a complex dependency on the abundance of stable Ni, which limits their use in distinguishing among M Ch and sub- M Ch progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni  II ] lines would most likely result from a Chandrasekhar-mass progenitor. 

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

   https://doi.org/10.3847/1538-4357/ad0b7b  

   DerKacy, J. M. ; Ashall, C. ; Hoeflich, P. ; Baron, E. ; Shahbandeh, M. ; Shappee, B. J. ; Andrews, J. ; Baade, D. ; Balangan, E. F. ; Bostroem, K. A. ; et al ( January 2024 , The Astrophysical Journal)

   We present a JWST mid-infrared (MIR) spectrum of the underluminous Type Ia Supernova (SN Ia) 2022xkq, obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) ∼130 days post-explosion. We identify the first MIR lines beyond 14μm in SN Ia observations. We find features unique to underluminous SNe Ia, including the following: isolated emission of stable Ni, strong blends of [Tiii], and large ratios of singly ionized to doubly ionized species in both [Ar] and [Co]. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the [Coiii] 11.888μm feature and the SN light-curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements, we constrain the mass of the exploding WD. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD ($M_{\mathrm{WD}}$≈1.37M_⊙) of high central density (ρ_c≥ 2.0 × 10⁹g cm⁻³) seen equator-on, which producedM(⁵⁶Ni) =0.324M_⊙andM(⁵⁸Ni) ≥0.06M_⊙. The observed line widths are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of subsonic carbon burning followed by an off-center deflagration-to-detonation transition beginning at a single point. Additional observations may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.

    

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4. The extraplanar type II supernova ASASSN-14jb in the nearby edge-on galaxy ESO 467-G051

   https://doi.org/10.1051/0004-6361/201834972  

   Meza, Nicolás ; Prieto, J. L. ; Clocchiatti, A. ; Galbany, L. ; Anderson, J. P. ; Falco, E. ; Kochanek, C. S. ; Kuncarayakti, H. ; Sánchez, S. F. ; Brimacombe, J. ; et al ( September 2019 , Astronomy & Astrophysics)

   We present optical photometry and spectroscopy of the Type II supernova ASASSN-14jb, together with Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) integral field observations of its host galaxy and a nebular-phase spectrum. This supernova, in the nearby galaxy ESO 467-G051 ( z  = 0.006), was discovered and followed-up by the all-sky automated survey for supernovae (ASAS-SN). We obtained well-sampled las cumbres network (LCOGTN) B V g r i and Swift w 2 m 1 w 1 u b v optical, near-UV/optical light curves, and several optical spectra in the early photospheric phases. The transient ASASSN-14jb exploded ∼2 kpc above the star-forming disk of ESO 467-G051, an edge-on disk galaxy. The large projected distance from the disk of the supernova position and the non-detection of any H II region in a 1.4 kpc radius in projection are in conflict with the standard environment of core-collapse supernova progenitors and suggests the possible scenario that the progenitor received a kick in a binary interaction. We present analysis of the optical light curves and spectra, from which we derived a distance of 25 ± 2 Mpc using state-of-the-art empirical methods for Type II SNe, physical properties of the SN explosion ( 56 Ni mass, explosion energy, and ejected mass), and properties of the progenitor; namely the progenitor radius, mass, and metallicity. Our analysis yields a 56 Ni mass of 0.0210  ±  0.0025  M ⊙ , an explosion energy of ≈0.25 × 10 51 ergs, and an ejected mass of ≈6  M ⊙ . We also constrained the progenitor radius to be R *  = 580  ±  28  R ⊙ which seems to be consistent with the sub-Solar metallicity of 0.3  ±  0.1  Z ⊙ derived from the supernova Fe II λ 5018 line. The nebular spectrum constrains strongly the progenitor mass to be in the range 10–12 M ⊙ . From the Spitzer data archive we detect ASASSN-14jb ≈330 days past explosion and we derived a total dust mass of 10 −4   M ⊙ from the 3.6 μ m and 4.5 μ m photometry. Using the F U V , N U V , B V g r i , K s , 3.6 μ m, and 4.5 μ m total magnitudes for the host galaxy, we fit stellar population synthesis models, which give an estimate of M *  ≈ 1 × 10 9   M ⊙ , an age of 3.2 Gyr, and a SFR ≈0.07  M ⊙ yr −1 . We also discuss the low oxygen abundance of the host galaxy derived from the MUSE data, having an average of 12 + log(O/H) = 8.27 +0.16 −0.20 using the O 3 N 2 diagnostic with strong line methods. We compared it with the supernova spectra, which is also consistent with a sub-Solar metallicity progenitor. Following recent observations of extraplanar H II regions in nearby edge-on galaxies, we derived the metallicity offset from the disk, being positive, but consistent with zero at 2 σ , suggesting enrichment from disk outflows. We finally discuss the possible scenarios for the unusual environment for ASASSN-14jb and conclude that either the in-situ star formation or runaway scenario would imply a low-mass progenitor, agreeing with our estimate from the supernova nebular spectrum. Regardless of the true origin of ASASSN-14jb, we show that the detailed study of the environment roughly agree with the stronger constraints from the observation of the transient. 

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5. SN 2020jgb: A Peculiar Type Ia Supernova Triggered by a Helium-shell Detonation in a Star-forming Galaxy

   https://doi.org/10.3847/1538-4357/acbb5e  

   Liu, Chang ; Miller, Adam A. ; Polin, Abigail ; Nugent, Anya E. ; De, Kishalay ; Nugent, Peter E. ; Schulze, Steve ; Gal-Yam, Avishay ; Fremling, Christoffer ; Anand, Shreya ; et al ( April 2023 , The Astrophysical Journal)

   Abstract The detonation of a thin (≲0.03 M ⊙ ) helium shell (He-shell) atop a ∼1 M ⊙ white dwarf (WD) is a promising mechanism to explain normal Type Ia supernovae (SNe Ia), while thicker He-shells and less massive WDs may explain some recently observed peculiar SNe Ia. We present observations of SN 2020jgb, a peculiar SN Ia discovered by the Zwicky Transient Facility (ZTF). Near maximum brightness, SN 2020jgb is slightly subluminous (ZTF g -band absolute magnitude −18.7 mag ≲ M g ≲ −18.2 mag depending on the amount of host-galaxy extinction) and shows an unusually red color (0.2 mag ≲ g ZTF − r ZTF ≲ 0.4 mag) due to strong line-blanketing blueward of ∼5000 Å. These properties resemble those of SN 2018byg, a peculiar SN Ia consistent with an He-shell double detonation (DDet) SN. Using detailed radiative transfer models, we show that the optical spectroscopic and photometric evolution of SN 2020jgb is broadly consistent with a ∼0.95–1.00 M ⊙ (C/O core + He-shell) progenitor ignited by a ≳0.1 M ⊙ He-shell. However, one-dimensional radiative transfer models without non-local-thermodynamic-equilibrium treatment cannot accurately characterize the line-blanketing features, making the actual shell mass uncertain. We detect a prominent absorption feature at ∼1 μ m in the near-infrared (NIR) spectrum of SN 2020jgb, which might originate from unburnt helium in the outermost ejecta. While the sample size is limited, we find similar 1 μ m features in all the peculiar He-shell DDet candidates with NIR spectra obtained to date. SN 2020jgb is also the first peculiar He-shell DDet SN discovered in a star-forming dwarf galaxy, indisputably showing that He-shell DDet SNe occur in both star-forming and passive galaxies, consistent with the normal SN Ia population. 

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