The Last Minutes of Oxygen Shell Burning in a Massive Star

Abstract

We present the first 4π-3D simulation of the last minutes of oxygen shell burning in an 18M supernova progenitor up to the onset of core collapse. A moving inner boundary is used to accurately model the contraction of the silicon and iron core according to a 1D stellar evolution model with a self-consistent treatment of core deleptonization and nuclear quasi-equilibrium. The simulation covers the full solid angle to allow the emergence of large-scale convective modes. Due to core contraction and the concomitant acceleration of nuclear burning, the convective Mach number increases to ∼0.1 at collapse, and an = 2 mode emerges shortly before the end of the simulation. Aside from a growth of the oxygen shell from 0.51M to 0.56M due to entrainment from the carbon shell, the convective flow is reasonably well described by mixing length theory, and the dominant scales are compatible with estimates from linear stability analysis. We deduce that artificial changes in the physics, such as accelerated core contraction, can have precarious consequences for the state of convection at collapse. We argue that scaling laws for the convective velocities and eddy sizes furnish good estimates for the state of shell convection at collapse and develop a simple analytic theory for the impact of convective seed perturbations on shock revival in the ensuing supernova. We predict a reduction of the critical luminosity for explosion by 12 . . . 24% due to seed asphericities for our 3D progenitor model relative to the case without large seed perturbations.

Angular Momentum Fluctuations in the Convective Helium Shell of Massive Stars

“…In the present study we found large-scale fluctuations of angular momentum in the convective helium zone of an evolved massive star, while the energy transport is similar to that expected from MLT, strengthening the validity of our simulation. Future studies will explore similar phenomena in silicon and oxygen shells (as done by, e.g., Mueller et al 2016)and effects of rotation (as done by, e.g., Chatzopoulos et al 2016).…”

Formation of Double Neutron Star Systems

“…Finally, it has been suggested that multi-dimensional effects in the convective burning shells at the onset of core collapse can have an effect on the NS kick amplitude (e.g. Burrows & Hayes 1996;Arnett & Meakin 2011;Müller et al 2016), i.e. precollapse asymmetries in the progenitor star could influence the asymmetry parameter of Eq.…”

Neutrino-Driven Explosions

“…The initial explosion asymmetries that are intrinsically connected to the neutrino-driven explosion mechanism must therefore be included in self-consistent hydrodynamic 3D simulations of the SN blast wave from its initiation to the late stages of homologous expansion, if SN models shall be compared to the observational properties of SN explosions and SN remnants. These initial asymmetries can either grow stochastically from (small-scale) random fluctuations, or they might be triggered by large-scale asymmetries in the convective silicon and oxygen burning shells of the progenitor at the onset of core collapse (Burrows and Hayes, 1996;Arnett and Meakin, 2011;Couch and Ott, 2013;Müller and Janka, 2015;Müller et al, 2016b;Müller, 2016).…”

“…For example, they still lack sufficient (some more, some less) numerical resolution for convergence on the description of the turbulent flow in the postshock layer, employ a variety of (different) approximations for the 3D neutrino transport, omit important microphysics, obtain explosions only with special and not generally accepted assumptions such as rapid rotation or nonstandard modifications of the neutrino opacities (Melson et al, 2015a), and start from spherically symmetric initial conditions instead of the multidimensional flow that perturbs the convective burning shells around the iron core already prior to collapse Müller et al, 2016b;Müller, 2016). It is well possible that the neutrino mechanism will work well and can explain the observational properties (e.g., explosion energies and radioactive yields) for a wide range of massive progenitors once all these remaining deficiencies of current models have been removed.…”