Direct evidence of two-component ejecta in supernova 2016gkg from
  nebular spectroscopy
  Spectral observations of the type-IIb supernova (SN) 2016gkg at 300-800 days
are reported. The spectra show nebular characteristics, revealing emission from
the progenitor star's metal-rich core and providing clues to the kinematics and
physical conditions of the explosion. The nebular spectra are dominated by
emission lines of [O I] $\lambda\lambda6300, 6364$ and [Ca II]
$\lambda\lambda7292, 7324$. Other notable, albeit weaker, emission lines
include Mg I] $\lambda4571$, [Fe II] $\lambda7155$, O I $\lambda7774$, Ca II
triplet, and a broad, boxy feature at the location of H$\alpha$. Unlike in
other stripped-envelope SNe, the [O I] doublet is clearly resolved due to the
presence of strong narrow components. The doublet shows an unprecedented
emission line profile consisting of at least three components for each [O
I]$\lambda6300, 6364$ line: a broad component (width $\sim2000$ km s$^{-1}$),
and a pair of narrow blue and red components (width $\sim300$ km s$^{-1}$)
mirrored against the rest velocity. The narrow component appears also in other
lines, and is conspicuous in [O I]. This indicates the presence of multiple
distinct kinematic components of material at low and high velocities. The
low-velocity components are likely to be produced by a dense, slow-moving
emitting region near the center, while the broad components are emitted over a
larger volume. These observations suggest an asymmetric explosion, supporting
the idea of two-component ejecta that influence the resulting late-time spectra
and light curves. SN 2016gkg thus presents striking evidence for significant
asymmetry in a standard-energy SN explosion. The presence of material at low
velocity, which is not predicted in 1D simulations, emphasizes the importance
of multi-dimensional explosion modeling of SNe.
