Properties of Type Iax Supernova 2019muj in the Late Phase: Existence, Nature and Origin of the Iron-rich Dense Core

Abstract: Type Iax Supernovae (SNe Iax) form a class of peculiar SNe Ia, whose early-phase spectra share main spectral line identifications with canonical SNe Ia but with higher ionization and much lower line velocities. Their late-time behaviors deviate from usual SNe Ia in many respects; SNe Iax keep showing photospheric spectra over several 100 days and the luminosity decline is very slow. In the present work, we study the late-time spectra of SN Iax 2019muj including a newly-presented spectrum at ~500 days. The spectrum is still dominated by allowed transitions but with lower ionization state, with possible detection of [O I]6300, 6363. By comprehensively examining the spectral formation processes of allowed transitions (Fe II, Fe I, and the Ca II NIR triplet) and forbidden transitions ([Ca II]7292, 7324 and the [O I]), we quantitatively constrain the nature of the innermost region and find that it is distinct from the outer ejecta; the mass of the innermost component is ~0.03 Msun dominated by Fe (which can be initially 56Ni), expanding with the velocity of ~760 km/s. We argue that the nature of the inner component is explained by the failed/weak white-dwarf thermonuclear explosion scenario. We suggest that a fraction of the 56Ni-rich materials initially confined in (the envelope of) the bound remnant can later be ejected by the energy input through the 56Ni/Co/Fe decay, forming the `second' unbound ejecta component which manifests itself as the inner dense component seen in the late phase.