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
Publisher: IOP PUBLISHING LTD
: 2019
: Astrophysical Journal Letters
: ASTROPHYSICAL JOURNAL LETTERS
: ASTROPHYS J LETT
: L3
: 873
: 1
: 14
: 2041-8205
: 2041-8213
DOI: https://doi.org/10.3847/2041-8213/ab0558
: https://research.utu.fi/converis/portal/detail/Publication/39944271
We report distinctly double-peaked H alpha and H beta emission lines in the late-time, nebular-phase spectra (greater than or similar to 200 days) of the otherwise normal at early phases (less than or similar to 100 days) 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 Co-56 decay, so the observed line profile bifurcation suggests a strong bipolarity in the Ni-56 distribution or in the line-forming region of the inner ejecta. The strongly bifurcated blueshifted and redshifted peaks are separated by similar to 3 x 10(3) km s(-1) 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 days 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 similar to 100 days.
