Production and Distribution of $^{44}$Ti and $^{56}$Ni in a Three-dimensional Supernova Model Resembling Cassiopeia A

TL;DR

This study uses 3D supernova simulations to show that observed asymmetries in Cas A's distribution of radioactive isotopes can be explained by neutrino-driven explosion mechanisms, without requiring rapid rotation or jets.

Contribution

It demonstrates that neutrino-driven supernova explosions can produce large-scale asymmetries consistent with observations of Cas A, challenging previous jet-driven explosion models.

Findings

Simulated $^{44}$Ti distribution resembles Cas A's observed morphology.

Neutrino-driven explosions can account for high $^{44}$Ti abundance without rapid rotation.

Recoil of neutron star aligns with mass ejection asymmetries, supporting the gravitational tugboat mechanism.

Abstract

The spatial and velocity distributions of nuclear species synthesized in the innermost regions of core-collapse supernovae can yield important clues about explosion asymmetries and the operation of the still disputed explosion mechanism. Recent observations of radioactive $^{44}$Ti with high-energy satellite telescopes (Nuclear Spectroscopic Telescope Array [NuSTAR], INTEGRAL) have measured gamma-ray line details, which provide direct evidence of large-scale explosion asymmetries in SN 1987A and in Cassiopeia A (Cas A) even by mapping of the spatial brightness distribution (NuSTAR). Here we discuss a 3D simulation of a neutrino-driven explosion, using a parameterized neutrino engine, whose $^{44}$Ti distribution is mostly concentrated in one hemisphere pointing opposite to the neutron star (NS) kick velocity. Both exhibit intriguing resemblance to the observed morphology of the Cas A remnant, although neither the progenitor nor the explosion was fine-tuned for a perfect match. Our results demonstrate that the asymmetries observed in this remnant can, in principle, be accounted for by a neutrino-driven explosion, and that the high $^{44}$Ti abundance in Cas A may be explained without invoking rapid rotation or a jet-driven explosion, because neutrino-driven explosions generically eject large amounts of high-entropy matter. The recoil acceleration of the NS is connected to mass ejection asymmetries and is opposite to the direction of the stronger explosion, fully compatible with the gravitational tugboat mechanism. Our results also imply that Cas A and SN 1987A could possess similarly "one-sided" Ti and Fe asymmetries, with the difference that Cas A is viewed from a direction with large inclination angle to the NS motion, whereas the NS in SN 1987A should have a dominant velocity component pointing toward us.