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

TL;DR

This paper reports the first observation of distinctly double-peaked nebular emission lines in a Type IIP supernova, indicating a strongly bipolar inner ejecta structure likely caused by asymmetric Ni56 distribution.

Contribution

It presents the discovery of bipolar inner ejecta in a normal Type IIP supernova through late-time spectral analysis, a phenomenon not previously observed in such supernovae.

Findings

Double-peaked nebular Balmer lines observed after 200 days.

Inner ejecta consist of two almost detached blobs.

Evolution of line ratio suggests dust formation within the envelope.

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.