Strongly Bipolar Inner Ejecta of the Normal Type IIP Supernova ASASSN-16at

Subhash Bose; Subo Dong; N. Elias-Rosa; B. J. Shappee; David Bersier; Stefano Benetti; M. D. Stritzinger; D. Grupe; C. S. Kochanek; J. L. Prieto; Ping Chen; H. Kuncarayakti; Seppo Mattila; Antonia Morales-Garoffolo; Nidia Morrell; F. Onori; Thomas M Reynolds; A. Siviero; Auni Somero; K. Z. Stanek; Giacomo Terreran; Todd A. Thompson; L. Tomasella; C. Ashall; Christa Gall; M. Gromadzki; T. W. -S. Holoien

doi:10.3847/2041-8213/ab0558

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arXiv · 1810.12304

Strongly Bipolar Inner Ejecta of the Normal Type IIP Supernova ASASSN-16at

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Abstract

We report distinctly double-peakedH-alpha and H-beta emission lines in the late-time, nebular-phase spectra (>~200 d) of the otherwise normal at early phases (<~ 100 d) Type IIP supernova ASASSN-16at (SN 2016X). Such distinctly double-peaked nebular Balmer lines have never been observed for a Type II SN. The nebular-phase Balmer emission is driven by the radioactive Co56 decay, so the observed line-profile bifurcation suggests a strong bipolarity in the Ni56 distribution or in the line-forming region of the inner ejecta. The strongly bifurcated blue- and red-shifted peaks are separated by ~3x10^3 km/s and are roughly symmetrically positioned with respect to the host-galaxy rest frame, implying that the inner ejecta are composed of two almost detached blobs. The red peak progressively weakens relative to the blue peak, and disappears in the 740 d spectrum. One possible reason for the line-ratio evolution is increasing differential extinction from continuous formation of dust within the envelope, which is also supported by the near-infrared flux excess that develops after ~100 d.

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Subhash Bose, Subo Dong, N. Elias-Rosa, B. J. Shappee, David Bersier, Stefano Benetti, M. D. Stritzinger, D. Grupe, C. S. Kochanek, J. L. Prieto, Ping Chen, H. Kuncarayakti, Seppo Mattila, Antonia Morales-Garoffolo, Nidia Morrell, F. Onori, Thomas M Reynolds, A. Siviero, Auni Somero, K. Z. Stanek, Giacomo Terreran, Todd A. Thompson, L. Tomasella, C. Ashall, Christa Gall, M. Gromadzki, T. W. -S. Holoien. 2018-10-29. Strongly Bipolar Inner Ejecta of the Normal Type IIP Supernova ASASSN-16at. https://doi.org/10.3847/2041-8213%2Fab0558

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