We present our spectropolarimetric observations of SN 2017egm, a Type I superluminous supernova (SLSN-I) in a nearby galaxy NGC 3191, with the Subaru telescope at +185.0 days after the g-band maximum light. This is the first spectropolarimetric observation for SLSNe at late phases. We find that the degree of the polarization in the late phase significantly changes from that measured at the earlier phase. The spectrum at the late phase shows a strong Ca emission line and therefore we reliably estimate the interstellar polarization (ISP) component assuming that the emission line is intrinsically unpolarized. By subtracting the estimated ISP, we find that the intrinsic polarization at the early phase is only ∼0.2%, which indicates an almost spherical photosphere, with an axial ratio ∼1.05. The intrinsic polarization at the late phase increases to ∼0.8%, which corresponds to the photosphere with an axial ratio ∼1.2. A nearly constant position angle of the polarization suggests the inner ejecta are almost axisymmetric. By these observations, we conclude that the inner ejecta are more aspherical than the outer ejecta. This may suggest the presence of a central energy source producing aspherical inner ejecta.

The explosion mechanism of core-collapse supernovae is not fully understood yet. In this work, we give constraints on the explosion timescale based on 56Ni synthesized by supernova explosions. First, we systematically analyze multiband light curves of 82 stripped-envelope supernovae (SESNe) to obtain bolometric light curves, which is among the largest samples of the bolometric light curves of SESNe derived from the multiband spectral energy distribution. We measure the decline timescale and the peak luminosity of the light curves and estimate the ejecta mass (M ej) and 56Ni mass (M Ni) to connect the observed properties with the explosion physics. We then carry out one-dimensional hydrodynamics and nucleosynthesis calculations, varying the progenitor mass and the explosion timescale. From the calculations, we show that the maximum 56Ni mass that 56Ni-powered SNe can reach is expressed as M Ni ≲ 0.2 M ej. Comparing the results from the observations and the calculations, we show that the explosion timescale shorter than 0.3 s explains the synthesized 56Ni mass of the majority of the SESNe.

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Loss of Purkinje cells (PCs) in the cerebellum causes severe motor deficits. The peculiar lamellar structures known as “giant lamellar bodies” (GLBs) have been reported in PCs of patients with Werdnig-Hoffman disease, 13q deletion syndrome, and Krabbe’s disease. GLBs are localized to PC dendrites and are associated with neurodegeneration. They have been noted, however, only in case reports following autopsy, and reports of their existence have been very limited. Here we show that GLBs were reproducibly formed in PC dendrites of a mouse model, in which the CCCTC-binding factor (CTCF) was deleted. CTCF orchestrates gene expression by organizing the three-dimensional chromatin structure. The mouse model showed progressive motor dysfunction and abnormal dendritic morphology in PCs, including dendritic self-avoidance defects and proximal shift in the climbing fibre innervation territory on PC dendrites. GLBs were not clearly found in PC dendrites at infancy but instead developed with age. In conjunction with GLB development, the endoplasmic reticulum was almost absent around the nuclei, the mitochondria were markedly swollen and their cristae had decreased drastically, and almost all PCs eventually disappeared as severe motor deficits manifested. Thus, our results are the first experimental demonstration that GLBs represent a pathological alteration of PCs and suggest different genetic backgrounds involved in the induction of GLBs.

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