Comparison of the Core-Collapse Evolution of Two Nearly Equal Mass Progenitors

Abstract

We compare the core-collapse evolution of a pair of 15.8 $M_{\odot }$ stars with significantly different internal structures, a consequence of bimodal variability exhibited by massive stars during their late evolutionary stages. The 15.78 and 15.79 $M_{\odot }$ progenitors have core masses of 1.47 and 1.78 $M_{\odot }$ and compactness parameters ${\xi }_{1.75}$ of 0.302 and 0.604. The core collapse simulations are carried out in 2D to nearly 3 s post-bounce and show substantial differences in the times of shock revival and explosion energies. The 15.78 $M_{\odot }$ model explodes promptly at 120 ms post-bounce when a strong density decrement at the Si--Si/O shell interface encounters the stalled shock. The 15.79 $M_{\odot }$ model, which lacks the density decrement, takes 100 ms longer to explode but ultimately produces a more powerful explosion. Larger mass accretion rate of the 15.79 $M_{\odot }$ model during the first 0.8 s post-bounce results in larger ${\nu }_e$/${\bar{\nu }}_e$ luminosities and rms energies. The ${\nu }_e$/${\bar{\nu }}_e$ luminosities and rms energies arising from the inner core are also larger in the 15.79 $M_{\odot }$ model throughout due to the larger negative temperature gradient of this core due to greater adiabatic compression. Larger luminosities and rms energies in the 15.79 $M_{\odot }$ model and a flatter and higher density heating region, result in more energy deposition behind the shock and more ejected matter with higher enthalpy. We find the ejected ${}^{56}$Ni mass of the 15.79 $M_{\odot }$ model is more than double that of the 15.78 $M_{\odot }$ model. Most of the ejecta in both models is moderately proton-rich, though counterintuitively the highest electron fraction ($Y_e=0.61$) ejecta in either model is in the less energetic 15.78 $M_{\odot }$ model while the lowest electron fraction ($Y_e=0.45$) ejecta in either model is in the 15.79 $M_{\odot }$ model.

24 pages; Accepted for publication in ApJ

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