Constraints on explosive silicon burning in core-collapse supernovae from measured Ni/Fe ratios

Measurements of explosive nucleosynthesis yields in core-collapse supernovae provide tests for explosion models. We investigate constraints on explosive conditions derivable from measured amounts of nickel and iron after radioactive decays using nucleosynthesis networks with parameterized thermodynamic trajectories. The Ni/Fe ratio is for most regimes dominated by the production ratio of Ni-58/(Fe-54 + Ni-56), which tends to grow with higher neutron excess and with higher entropy. For SN 2012ec, a supernova (SN) that produced a Ni/Fe ratio of 3.4 +/- 1.2 times solar, we find that burning of a fuel with neutron excess eta approximate to 6 x 10(-3) is required. Unless the progenitor metallicity is over five times solar, the only layer in the progenitor with such a neutron excess is the silicon shell. SNe producing large amounts of stable nickel thus suggest that this deep-lying layer can be, at least partially, ejected in the explosion. We find that common spherically symmetric models of M-ZAMS less than or similar to 13 M-circle dot stars exploding with a delay time of less than one second (M-cut < 1.5 M-circle dot) are able to achieve such silicon-shell ejection. SNe that produce solar or subsolar Ni/Fe ratios, such as SN 1987A, must instead have burnt and ejected only oxygen-shell material, which allows a lower limit to the mass cut to be set. Finally, we find that the extreme Ni/Fe value of 60-75 times solar derived for the Crab cannot be reproduced by any realistic entropy burning outside the iron core, and neutrino-neutronization obtained in electron capture models remains the only viable explanation.

Burning Your Own

Novel

The Last Minutes of Oxygen Shell Burning in a Massive Star

We present the first 3D simulation of the last minutes of oxygen shell burning in an 18 solar mass 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 l=2 mode emerges shortly before the end of the simulation. Aside from a growth of the oxygen shell from 0.51 to 0.56 solar masses 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.