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1. SN 2022joj: A Potential Double Detonation with a Thin Helium Shell

   https://doi.org/10.3847/1538-4357/ad19c9  

   Padilla Gonzalez, E. ; Howell, D. A. ; Terreran, G. ; McCully, C. ; Newsome, M. ; Burke, J. ; Farah, J. ; Pellegrino, C. ; Bostroem, K. A. ; Hosseinzadeh, G. ; et al ( March 2024 , The Astrophysical Journal)

   We present photometric and spectroscopic data for SN 2022joj, a nearby peculiar Type Ia supernova (SN Ia) with a fast decline rate (Δm_15,B= 1.4 mag). SN 2022joj shows exceedingly red colors, with a value of approximatelyB−V≈ 1.1 mag during its initial stages, beginning from 11 days before maximum brightness. As it evolves, the flux shifts toward the blue end of the spectrum, approachingB−V≈ 0 mag around maximum light. Furthermore, at maximum light and beyond, the photometry is consistent with that of typical SNe Ia. This unusual behavior extends to its spectral characteristics, which initially displayed a red spectrum and later evolved to exhibit greater consistency with typical SNe Ia. Spectroscopically, we find strong agreement between SN 2022joj and double detonation models with white dwarf masses of around 1M_⊙and a thin He shell between 0.01 and 0.05M_⊙. Moreover, the early red colors are explained by line-blanketing absorption from iron peak elements created by the double detonation scenario in similar mass ranges. The nebular spectra in SN 2022joj deviate from expectations for double detonation, as we observe strong [Feiii] emission instead of [Caii] lines as anticipated, though this is not as robust a prediction as early red colors and spectra. The fact that as He shells get thinner these SNe start to look more like normal SNe Ia raises the possibility that this is the triggering mechanism for the majority of SNe Ia, though evidence would be missed if the SNe are not observed early enough.

    

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2. Double detonations: variations in Type Ia supernovae due to different core and He shell masses – II. Synthetic observables

   https://doi.org/10.1093/mnras/stac2665  

   Collins, Christine E. ; Gronow, Sabrina ; Sim, Stuart A. ; Röpke, Friedrich K. ( October 2022 , Monthly Notices of the Royal Astronomical Society)

   Double detonations of sub-Chandrasekhar mass white dwarfs are a promising explosion scenario for Type Ia supernovae, whereby a detonation in a surface helium shell triggers a secondary detonation in a carbon-oxygen core. Recent work has shown that low-mass helium shell models reproduce observations of normal SNe Ia. We present 3D radiative transfer simulations for a suite of 3D simulations of the double detonation explosion scenario for a range of shell and core masses. We find light curves broadly able to reproduce the faint end of the width–luminosity relation shown by SNe Ia, however, we find that all of our models show extremely red colours, not observed in normal SNe Ia. This includes our lowest mass helium shell model. We find clear Ti ii absorption features in the model spectra, which would lead to classification as peculiar SNe Ia, as well as line blanketing in some lines of sight by singly ionized Cr and Fe-peak elements. Our radiative transfer simulations show that these explosion models remain promising to explain peculiar SNe Ia. Future full non-LTE simulations may improve the agreement of these explosion models with observations of normal SNe Ia.

    

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3. Local 2D Simulations of the Ignition of a Helium Shell Detonation on a White Dwarf by an Impacting Stream

   https://doi.org/10.3847/1538-4357/ada034  

   Rajavel, Nethra ; Townsley, Dean_M ; Shen, Ken_J ( January 2025 , The Astrophysical Journal)

   The double detonation model is one of the prevalent explosion mechanisms of Type Ia supernovae (SNe Ia) wherein an outer helium shell detonation triggers a core detonation in the white dwarf (WD). The dynamically driven double degenerate double detonation (D⁶) is the double detonation of the more massive WD in a binary WD system where the localized impact of the mass transfer stream from the companion sets off the initial helium shell detonation. To have high numerical resolution and control over the stream parameters, we have implemented a study of the local interaction of the stream with the WD surface in 2D. In cases with lower base density of the shell, the stream's impact can cause surface detonation soon after first impact. With higher base densities, after the stream hits the surface, hot material flows around the star and interacts with the incoming stream to produce a denser and narrower impact. Our results therefore show that (1) a directly impacting stream for both a relatively high resolution and for a range of stream parameters can produce a surface detonation, (2) thinner helium shells ignite more promptly via impact, doing so sooner, and (3) there are lower limits on ignition in both shell density and incoming stream speed with lower limits on density being well below those shown by other work to be required for normal appearing SN Ia. This supports stream ignition and therefore the D⁶scenario, as a viable mechanism for normal SNe Ia.

    

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4. SN 2016dsg: A Thermonuclear Explosion Involving a Thick Helium Shell

   https://doi.org/10.3847/1538-4357/ac75eb  

   Dong, Yize ; Valenti, Stefano ; Polin, Abigail ; Boyle, Aoife ; Flörs, Andreas ; Vogl, Christian ; Kerzendorf, Wolfgang E. ; Sand, David J. ; Jha, Saurabh W. ; Wyrzykowski, Łukasz ; et al ( July 2022 , The Astrophysical Journal)

   Abstract A thermonuclear explosion triggered by a He-shell detonation on a carbon–oxygen white-dwarf core has been predicted to have strong UV line blanketing at early times due to the iron-group elements produced during He-shell burning. We present the photometric and spectroscopic observations of SN 2016dsg, a subluminous peculiar Type I supernova consistent with a thermonuclear explosion involving a thick He shell. With a redshift of 0.04, the i -band peak absolute magnitude is derived to be around −17.5. The object is located far away from its host, an early-type galaxy, suggesting it originated from an old stellar population. The spectra collected after the peak are unusually red, show strong UV line blanketing and weak O i λ 7773 absorption lines, and do not evolve significantly over 30 days. An absorption line around 9700–10500 Å is detected in the near-infrared spectrum and is likely from the unburnt He in the ejecta. The spectroscopic evolution is consistent with the thermonuclear explosion models for a sub-Chandrasekhar-mass white dwarf with a thick He shell, while the photometric evolution is not well described by existing models. 

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5. A JWST Near- and Mid-infrared Nebular Spectrum of the Type Ia Supernova 2021aefx

   https://doi.org/10.3847/2041-8213/acb4ec  

   Kwok, Lindsey A. ; Jha, Saurabh W. ; Temim, Tea ; Fox, Ori D. ; Larison, Conor ; Camacho-Neves, Yssavo ; Brenner Newman, Max J. ; Pierel, Justin D. R. ; Foley, Ryan J. ; Andrews, Jennifer E. ; et al ( February 2023 , The Astrophysical Journal Letters)

   We present JWST near-infrared (NIR) and mid-infrared (MIR) spectroscopic observations of the nearby normal Type Ia supernova (SN) SN 2021aefx in the nebular phase at +255 days past maximum light. Our Near Infrared Spectrograph (NIRSpec) and Mid Infrared Instrument observations, combined with ground-based optical data from the South African Large Telescope, constitute the first complete optical+NIR+MIR nebular SN Ia spectrum covering 0.3–14μm. This spectrum unveils the previously unobserved 2.5−5μm region, revealing strong nebular iron and stable nickel emission, indicative of high-density burning that can constrain the progenitor mass. The data show a significant improvement in sensitivity and resolution compared to previous Spitzer MIR data. We identify numerous NIR and MIR nebular emission lines from iron-group elements as well as lines from the intermediate-mass element argon. The argon lines extend to higher velocities than the iron-group elements, suggesting stratified ejecta that are a hallmark of delayed-detonation or double-detonation SN Ia models. We present fits to simple geometric line profiles to features beyond 1.2μm and find that most lines are consistent with Gaussian or spherical emission distributions, while the [Ariii] 8.99μm line has a distinctively flat-topped profile indicating a thick spherical shell of emission. Using our line profile fits, we investigate the emissivity structure of SN 2021aefx and measure kinematic properties. Continued observations of SN 2021aefx and other SNe Ia with JWST will be transformative to the study of SN Ia composition, ionization structure, density, and temperature, and will provide important constraints on SN Ia progenitor and explosion models.

    

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