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1. Almost All Carbon/Oxygen White Dwarfs Can Host Double Detonations

   https://doi.org/10.3847/1538-4357/ad7379  

   Shen, Ken J; Boos, Samuel J; Townsley, Dean M (October 2024, The Astrophysical Journal)

   Abstract Double detonations of sub-Chandrasekhar-mass white dwarfs (WDs) in unstably mass-transferring double WD binaries have become one of the leading contenders to explain most Type Ia supernovae. However, past theoretical studies of the explosion process have assumed relatively ad hoc initial conditions for the helium shells in which the double detonations begin. In this work, we construct realistic C/O WDs to use as the starting points for multidimensional double detonation simulations. We supplement these with simplified one-dimensional detonation calculations to gain a physical understanding of the conditions under which shell detonations can propagate successfully. We find that C/O WDs ≲1.0M_⊙, which make up the majority of C/O WDs, are born with structures that can support double detonations. More massive C/O WDs require ∼10⁻³M_⊙of accretion before detonations can successfully propagate in their shells, but such accretion may be common in the double WD binaries that host massive WDs. Our findings strongly suggest that if the direct impact accretion stream reaches high enough temperatures and densities during mass transfer from one WD to another, the accreting WD will undergo a double detonation. Furthermore, if the companion is also a C/O WD ≲1.0M_⊙, it will undergo its own double detonation when impacted by the ejecta from the first explosion. Exceptions to this outcome may explain the newly discovered class of hypervelocity supernova survivors. 

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2. Carbon Burning Cannot Explain Puffy Hypervelocity White Dwarfs

   https://doi.org/10.1088/1538-3873/ae05cc  

   Yamaguchi, Natsuko; El-Badry, Kareem; Wong, Tin_Long Sunny; Shen, Ken J (September 2025, Publications of the Astronomical Society of the Pacific)

   Abstract Several hypervelocity white dwarfs (HVWDs) with space velocities of ≳1000 km s⁻¹have recently been discovered. One possible origin of these stars is the dynamically driven double-degenerate double-detonation (D6) scenario, in which an accreting sub-Chandrasekhar mass carbon-oxygen (CO) WD detonates as a SN Ia. In this scenario, the less massive WD may survive its companion’s detonation and be ejected as a HVWD. Most of the observed HVWDs are hotter and puffier than normal WDs, perhaps due to their recent proximity to a SN. In this work, we test whether these properties can be explained by long-lived stable carbon (C) burning in the interiors of CO WD donors triggered by a SN shock. We model the long-term evolution of CO WDs following rapid energy injection using 1D models. We find that stable C burning can be ignited in CO WDs with masses of 0.95–1.10M_⊙if SN energy penetrates sufficiently deeply. The resulting born-again stars settle on the C-burning main sequence while they convert their interiors from C and O to Ne and Mg, where they have temperatures and radii similar to some of the observed HVWDs. However, the timescale over which C-burning WDs remain inflated is ≲10⁵yr, which is at least an order of magnitude shorter than the kinematic ages of observed hot HVWDs. We conclude that observed HVWDs are unlikely to be inflated by C burning. The stellar evolution of observed HVWDs remains an open problem. 

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3. Seismic Signatures of the ¹² C(α, γ) ¹⁶ O Reaction Rate in White Dwarf Models with Overshooting

   https://doi.org/10.3847/1538-4357/ace620  

   Chidester, Morgan T; Timmes, F X; Farag, Ebraheem (August 2023, The Astrophysical Journal)

   Abstract We consider the combined effects that overshooting and the¹²C(α,γ)¹⁶O reaction rate have on variable white dwarf (WD) stellar models. We find that carbon–oxygen (CO) WD models continue to yield pulsation signatures of the current experimental¹²C(α,γ)¹⁶O reaction rate probability distribution function when overshooting is included in the evolution. These signatures hold because the resonating mantle region, encompassing ≃0.2M_⊙in a typical ≃0.6M_⊙WD model, still undergoes radiative helium burning during the evolution to a WD. Our specific models show two potential low-order adiabatic g-modes,g₂andg₆, that signalize the¹²C(α,γ)¹⁶O reaction rate probability distribution function. Both g-mode signatures induce average relative period shifts of ΔP/P= 0.44% and ΔP/P= 1.33% forg₂andg₆, respectively. We find thatg₆is a trapped mode, and theg₂period signature is inversely proportional to the¹²C(α,γ)¹⁶O reaction rate. Theg₆period signature generally separates the slower and faster reaction rates, and has a maximum relative period shift of ΔP/P= 3.45%. We conclude that low-order g-mode periods from CO WDs may still serve as viable probes for the¹²C(α,γ)¹⁶O reaction rate probability distribution function when overshooting is included in the evolution. 

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4. Nucleosynthesis imprints from different Type Ia supernova explosion scenarios and implications for galactic chemical evolution

   https://doi.org/10.1051/0004-6361/202038721  

   Lach, F.; Röpke, F. K.; Seitenzahl, I. R.; Coté, B.; Gronow, S.; Ruiter, A. J. (December 2020, Astronomy & Astrophysics)

   null (Ed.)

   We analyze the nucleosynthesis yields of various Type Ia supernova explosion simulations including pure detonations in sub-Chandrasekhar mass white dwarfs; double detonations and pure helium detonations of sub-Chandrasekhar mass white dwarfs with an accreted helium envelope; a violent merger model of two white dwarfs; and deflagrations and delayed detonations in Chandrasekhar mass white dwarfs. We focus on the iron peak elements Mn, Zn, and Cu. To this end, we also briefly review the different burning regimes and production sites of these elements, as well as the results of abundance measurements and several galactic chemical evolution studies. We find that super-solar values of [Mn/Fe] are not restricted to Chandrasekhar mass explosion models. Scenarios including a helium detonation can significantly contribute to the production of Mn, in particular the models proposed for calcium-rich transients. Although Type Ia supernovae are often not accounted for as production sites of Zn and Cu, our models involving helium shell detonations can produce these elements in super-solar ratios relative to Fe. Our results suggest a re-consideration of Type Ia supernova yields in galactic chemical evolution models. A detailed comparison with observations can provide new insight into the progenitor and explosion channels of these events. 

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5. The occurrence and impact of carbon-oxygen shell mergers in massive stars

   https://doi.org/10.1051/0004-6361/202554461  

   Roberti, L; Pignatari, M; Brinkman, H E; Jeena, S K; Sieverding, A; Falla, A; Limongi, M; Chieffi, A; Lugaro, M (June 2025, Astronomy & Astrophysics)

   Context.In their final stages before undergoing a core-collapse supernova, massive stars may experience mergers between internal shells where carbon (C) and oxygen (O) are consumed as fuels for nuclear burning. This interaction, known as a C-O shell merger, can dramatically alter the internal structure of the star, leading to peculiar nucleosynthesis and potentially influencing the supernova explosion and the propagation of the subsequent supernova shock. Aims.Our understanding of the frequency and consequences of C-O shell mergers remains limited. This study aims to identify, for the first time, early diagnostics in the stellar structure that lead to C-O shell mergers in more advanced stages. We also assess their role in shaping the chemical abundances in the most metal poor stars of the Galaxy. Methods.We analyzed a set of 209 stellar evolution models available in the literature, with different initial progenitor masses and metallicities. We then compared the nucleosynthetic yields from a subset of these models with the abundances of odd-Zelements in metal-poor stars. Results.We find that the occurrence of C-O shell mergers in stellar models can be predicted with a good approximation based on the outcomes of the central He burning phase, specifically, from the CO core mass (M_CO) and the¹²C central mass fraction (X_C12): 90% of models with a C-O merger have X_C12<0.277 and M_CO<4.90 M_⊙, with average values of M_CO= 4.02 M_⊙and X_C12= 0.176. The quantities X_C12and M_COare indirectly affected from several stellar properties, including the initial stellar mass and metallicity. Additionally, we confirm that the Sc-rich and K-rich yields from models with C-O mergers would solve the long-standing underproduction of these elements in massive stars. Conclusions.Our results emphasize the crucial role of C-O shell mergers in enriching the interstellar medium, particularly in the production of odd-Zelements. This highlights the necessity of further investigations to refine their influence on presupernova stellar properties and their broader impact on Galactic chemical evolution. 

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