Abstract
Type Ia supernovae (SNe Ia) are manifestations of stars that are deficient in hydrogen and helium, and disrupt in a thermonuclear runaway. While explosions of carbon-oxygen white dwarfs are thought to account for the majority of events, part of the observed diversity may be due to varied progenitor channels. We demonstrate that helium stars with masses between ∼1.8 and 2.5 M⊙ may evolve into highly degenerate cores with near-Chandrasekhar mass and helium-free envelopes that subsequently ignite carbon and oxygen explosively at densities of ∼(1.8−5.9) × 109 g cm−3. This occurs either due to core growth from shell burning (when the core has a hybrid CO/NeO composition), or following ignition of residual carbon triggered by exothermic electron captures on 24Mg (for a NeOMg-dominated composition). We argue that the resulting thermonuclear runaway is likely to prevent core collapse, leading to the complete disruption of the star. The available nuclear energy at the onset of explosive oxygen burning suffices to create ejecta with a kinetic energy of ∼1051 erg, as in typical SNe Ia. Conversely, if these runaways result in partial disruptions, the corresponding transients would resemble SN Iax events similar to SN 2002cx. If helium stars in this mass range indeed explode as SNe Ia, then the frequency of events would be comparable to the observed SN Ib/c rates, thereby sufficing to account for the majority of SNe Ia in star-forming galaxies.
Highlights
Despite their central role in Astrophysics and Cosmology, the origin and physics of Type Ia supernovae (SNe Ia) remain uncertain (Maoz et al 2014)
We show that near-MCh (C)NeO cores originating from intermediate-mass helium stars (∼1.8−2.5 M ), a common product of binary interactions, can ignite their residual carbon and oxygen explosively at densities 6 × 109 g cm−3, before the onset of 20Ne(e−,νe)20F electron-capture reactions at ∼1010 g cm−3
We have shown that at least some stars capable of developing near-MCh (C)NeO cores after losing their hydrogen envelopes may explode as SNe Ia instead of undergoing a core-collapse ECSN
Summary
Despite their central role in Astrophysics and Cosmology, the origin and physics of Type Ia supernovae (SNe Ia) remain uncertain (Maoz et al 2014). Recent three-dimensional hydrodynamical simulations of oxygen deflagrations in ONe cores at central densities ≥1010 g cm−3, that is, after the onset of electron-captures on 20Ne, suggest that a large fraction of the star (≥1 M ) may be ejected in a so-called thermonuclear ECSN, leaving behind only a small bound remnant (Jones et al 2016, 2019) In light of these results, we revisit the work of Waldman & Barkat (2006) and Waldman et al (2008) using modern tools and updated input physics. We demonstrate that a thermonuclear runaway leading to a SN Ia can be initiated during the late evolution of a degenerate core of neonoxygen (NeO) or carbon-neon-oxygen (CNeO) composition as it approaches MCh. We show that near-MCh (C)NeO cores originating from intermediate-mass helium stars (∼1.8−2.5 M ), a common product of binary interactions, can ignite their residual carbon and oxygen explosively at densities 6 × 109 g cm−3, before the onset of 20Ne(e−,νe)20F electron-capture reactions at ∼1010 g cm−3 This mechanism does not require accretion from the binary companion and may contribute significantly to the SN Ia rate in young stellar populations (Sect. 3)
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