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

Abstract

The spatial and velocity distributions of nuclear species synthesized in the innermost regions of core-collapse supernovae (SNe) 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 (NuSTAR, INTEGRAL) have measured gamma-ray line details, which provide direct evidence of large-scale explosion asymmetries in Supernova 1987A, and in Cassiopeia A (Cas A) even by mapping of the spatial brightness distribution (NuSTAR). Here, we discuss a three-dimensional (3D) simulation of a neutrino-driven explosion, using a parametrized 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 progenitor nor explosion were 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 genericly eject large amounts of highentropy 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 tug-boat 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.

About cosmic gamma ray lines

“…Also, comparing to the Chandra X-ray image showing Fe as seen in recombination lines, differences show up which are unexpected, if Ti and Fe both would be co-produced in the inner supernova regions only. One explanation saving the beliefs about Fe and Ti origins is that the X-ray recombination emission includes a bias from the ionisation of Fe, which thus does not reflect all of the Fe in the expanding remnant: Fe that is co-located with the 44 Ti emission clumps may be missed, and rather the Fe already behind the forward and reverse shocks dominates the brightness in the Fe image. But as both these images show clearly a dominance of several separate emission regions, rather than a spherically-symmetric centrally-dominated emission morphology, deviations from spherical symmetry apparently go hand in hand with 44 Ti enhancements as observed here.…”

“…Diffuse nuclear-line and annihilation-line emission will be discussed thereafter, with respect to the underlying source populations. 44 Ti ccSN interior nucleosynthesis 0.078 radioactive decay: 44 Ti ccSN interior nucleosynthesis 0.122 radioactive decay: 57 Ni supernova nucleosynthesis 0.158 radioactive decay: 56 Ni supernova nucleosynthesis 0.478 radioactive decay: 7 Be nova nucleosynthesis 0.511 positron annihilation nucleosynthesis, compact stars, binaries 0.812 radioactive decay: 56 Ni supernova nucleosynthesis 0.847 radioactive decay: 56 Co supernova nucleosynthesis 1.157 radioactive decay: 44 Ti ccSN interior nucleosynthesis 1.173 radioactive decay: 60 Fe,Co ccSN ejected nucleosynthesis 1.238 radioactive decay: 56 Co supernova nucleosynthesis 1.275 radioactive decay: 22 Na nova nucleosynthesis 1.332 radioactive decay: 60 Fe,Co ccSN ejected nucleosynthesis 1.634 nuclear excitation: 20 Ne cosmic ray / ISM interactions 1.809 radioactive decay: 26 Al massive-star and ccSN nucleosynthesis 2.230 neutron capture by H energetic nucleon interactions 2.313 nuclear excitation: 14 N cosmic ray / ISM interactions 2.754 nuclear excitation: 24 Mg cosmic ray / ISM interactions 4.438 nuclear excitation: 12 C cosmic ray / ISM interactions 6.129 nuclear excitation: 16 O cosmic ray / ISM interactions…”

“…More measurements on the prominent core collapse events SN1987A and Cas A were needed and made, thereafter [38,39,40,41], essentially confirming this issue. The NuSTAR hard X-ray mirror telescope finally has been able to image the low-energy lines at 68 and 78 keV from 44 Ti decay [32] (Fig. 5).…”

“…The latter aspect has been strengthened from other observations of core collapses, in particular from the link of gamma ray burst events to collapsing massive stars [34], angular momentum conservation of material falling onto a centrally forming black hole being connected to jet-like relativistic outflows causing the gamma-ray burst. Also, ejecta which were believed to originate from the innermost regions close to the mass cut between ejecta and the newly forming neutron star [35], were interpreted and related to deviations from spherical symmetry: Only with such models, it was possible to get larger amounts of 44 Ti ejected, as they had been inferred from the detection of the 44 Ti gamma-ray line at 1156 keV with COMPTEL [36] and constraints on SN1987A and the 44 Ca content of the solar system [37]. More measurements on the prominent core collapse events SN1987A and Cas A were needed and made, thereafter [38,39,40,41], essentially confirming this issue.…”

“…The nuclear-line part of astronomical instrumentation (see Fig.1) ranges from the nuclear transitions of 44 Ti at 68 and 78 keV, and 60 Fe at 58 keV at the lowest energies to nuclear lines at highest energies of ∼15 MeV, with important lines at the upper end from the 16 O transitions at 6.1 and 7 MeV and the 12 C transition at 15.1 MeV. The line and continuum signatures from the annihilation of positrons, with its prominent line at 511 keV, are of different origin, but part of the same astronomical window, and special in their astrophysical importance.…”