The gravitational collapse of ONe electron-degenerate cores and white dwarfs: The role of $^\mathsf{24}$Mg and $^\mathsf{12}$C revisited

Abstract

Abstract. The final stages of the evolution of electron-degenerate ONe cores, resulting from carbon burning in "heavy weight" intermediate-mass stars (8 M < ∼ M < ∼ 11 M ) and growing in mass, either from carbon burning in a shell or from accretion of matter in a close binary system, are examined in the light of their detailed chemical composition. In particular, we have modelled the evolution taking into account the abundances of the following minor nuclear species, which result from the previous evolutionary history: 12 C, 23 Na, 24 Mg, and 25 Mg. Both 23 Na and 25 Mg give rise to Urca processes, which are found to be unimportant for the final outcome of the evolution. 24 Mg was formerly considered a major component of ONe cores (hence called ONeMg cores), but updated evolutionary calculations in this mass range have severely reduced its abundance. Nevertheless, we have parameterized it and we have found that the minimum amount of 24 Mg required to produce NeO burning at moderate densities is ∼23%, a value exceedingly high in the light of recent evolutionary models. Finally, we have determined that models with relatively small abundances of unburnt carbon (X( 12 C) ∼ 0.015) could be a channel to explosion at low to moderate density (∼1 × 10 9 g cm −3 ). This is clearly below the current estimate for the explosion/collapse threshold and would have interesting consequences.

Thermohaline mixing in super-AGB stars

“…However, most importantly for our purpose, the details of the explosion depend sensitively on the core composition. Gutiérrez et al (2005) showed that traces of unburnt carbon as low as 1.5% in mass can trigger a low-to moderate-density explosion that would completely disrupt the star instead of a high density explosion where the star collapses to form a neutron star. Our results including thermohaline mixing indicate that the amount of carbon left in the core is large enough to produce a complete disruption of the ONe core.…”

The Supernova Channel of Super‐AGB Stars

“…Recent studies by Ritossa et al (1996Ritossa et al ( , 1999, Garcia-Berro et al (1997), Iben et al (1997), and Siess (2006) have shown that the mass fraction of 24 Mg in the ONe core is smaller than previously thought, which diminishes the role of electron captures on 24 Mg. While Gutiérrez et al (2005) found that unburnt carbon in the degenerate ONe core could trigger an explosion at densities of $10 9 g cm À3 , we disregard this possibility further on since its observational implications are not worked out, and therefore this scenario cannot yet be confronted with supernova observations.…”

The Formation and Evolution of ONe White Dwarfs: Prospects for Accretion Induced Collapse

“…In this case the white dwarf can accrete mass from the secondary and the central density can eventually become larger than the threshold for electron captures on the ashes of carbon burning, as occurs for single stars with masses larger than ∼10.5 M . As previously mentioned, in both cases the final evolution depends sensitively on the final abundances of 12 C, 16 O, 20 Ne, and 24 Mg, and the final outcome (explosion or collapse) is crucially dependent on the adopted prescription for convective mixing (Gutiérrez et al 1996(Gutiérrez et al , 2005. In any case, it is clear that at least some of these stars will produce produce massive ONe white dwarfs.…”

“…This has implications for accretion induced collapse, in the event that a white dwarf or a degenerate core of a SAGB star of initially ∼1.1 M can grow in mass to reach the effective Chandrasekhar limit of ∼1.37 M (Miyaji & Nomoto 1987). In such a case carbon will ignite before oxygen does and the energy released by the burning of even small amounts of carbon (∼2%) is sufficent to completely disrupt the star (Gutiérrez et al 1996;Gutiérrez et al 2005). Note also the absence of 22 Ne in the very outer layers of the carbon-rich degenerate core, where this element has been converted to 25 Mg. On top of these regions a helium-rich buffer exists (not shown).…”