The Intrinsic Stochasticity of the ⁵⁶Ni Distribution of Single-degenerate Near-Chandrasekhar-mass SN Ia

Abstract

Binary Chandrasekhar-mass white dwarfs accreting mass from non-degenerate stellar companions through the single-degenerate channel have reigned for decades as the leading explanation of Type Ia supernovae. Yet, a comprehensive theoretical explanation has not yet emerged to explain the expected properties of the canonical near-Chandrasekhar-mass white dwarf model. A simmering phase within the convective core of the white dwarf leads to the ignition of one or more flame bubbles scattered across the core. Consequently, near-Chandrasekhar-mass single-degenerate SNe Ia are inherently stochastic, and are expected to lead to a range of outcomes, from subluminous SN 2002cx-like events, to overluminous SN 1991T-like events. However, all prior simulations of the single-degenerate channel carried through the detonation phase have set the ignition points as free parameters. In this work, for the first time, we place ignition points as predicted by ab initio models of the convective phase leading up to ignition, and follow through the detonation phase in fully three-dimensional simulations. Single-degenerates in this framework are characteristically overluminous. Using a statistical approach, we determine the 56 Ni mass distribution arising from stochastic ignition. While there is a total spread of 0.2M for detonating models, the distribution is strongly left-skewed, and with a narrow standard deviation of 0.03M . Conversely, if single-degenerates are not overluminous but primarily yield normal or failed events, then the models require fine-tuning of the ignition parameters, or otherwise require revised physics or progenitor models. We discuss implications of our findings for the modeling of single-degenerate SNe Ia.

Models of pulsationally assisted gravitationally confined detonations with different ignition conditions

“…Our explosions proceed similar to the models presented in earlier works (Plewa et al 2004;Plewa 2007;Townsley et al 2007;Jordan et al 2008;Meakin et al 2009;Seitenzahl et al 2016), andByrohl et al (2019): The deflagration front burns toward the surface, burned ashes spread around the WD and collide at the opposite side of the star. This collision produces an inwardly directed jet which compresses and heats material ahead and leads to the initiation of a detonation.…”

“…This is a reasonable explanation for their similar spectra near maximum light and their very different luminosity and post-maximum evolution. It should be noted that the ignition of a deflagration in a M Ch WD is a stochastic process, and that strong deflagrations resulting from an ignition near the center can be considered a rare event (Nonaka et al 2012;Byrohl et al 2019). However, more simmering phase simulations with different WD models are still needed to validate the findings of Nonaka et al (2012).…”

“…For the single-spot off-center ignited M Ch WD models considered by L21, triggering of a detonation in a late phase of the explosion process is also a possibility. Such scenarios have been discussed by Plewa et al (2004) as gravitationally confined detonations and were later investigated in more detail by Plewa (2007); Townsley et al (2007); ; Jordan et al (2008Jordan et al ( , 2009; Meakin et al (2009); Seitenzahl et al (2009aSeitenzahl et al ( , 2016; García-Senz et al (2016), andByrohl et al (2019). The idea is that the deflagration flame rises toward the surface of the WD and sweeps around the gravitationally bound core.…”

“…Indeed, the faintest model in our study, Model r10_d2.6, exhausts the viability of the PGCD mechanism since critical conditions for a detonation initiation are only reached at the very end of the contraction phase. However, subluminous SNe Ia resulting from the GCD scenario should still be considered rare events as pointed out by Byrohl et al (2019). They extract a probability density function (PDF) for the ignition radius using data from Nonaka et al (2012) to interpret their results.…”

Supernovae Ia in 2019 (review): a rising demand for spherical explosions

“…On the observational side, Kawabata et al (2019) study the high-velocity SNe Ia SN 2019ein, and find its properties to be compatible with deflagration to detonation M Ch explosion. However, there are still open issues and observations that the M Ch model cannot explain (yet), e.g., Byrohl et al (2019).…”