Low-luminosity Type IIP Supernova 2016bkv with early-phase circumstellar
  interaction
  We present optical and near-infrared observations of a low-luminosity Type
IIP supernova (SN) 2016bkv from the initial rising phase to the plateau phase.
Our observations show that the end of the plateau is extended to $\gtrsim 140$
days since the explosion, indicating that this SN takes one of the longest time
to finish the plateau phase. among Type IIP SNe (SNe IIP), including
low-luminosity (LL) SNe IIP. The line velocities of various ions at the middle
of the plateau phase are as low as 1,000--1,500 km s$^{-1}$, which is the
lowest even among LL SNe IIP. These measurements imply that the ejecta mass in
SN 2016bkv is larger than that of the well-studied LL IIP SN 2003Z. In the
early phase, SN 2016bkv shows a strong bump in the light curve. In addition,
the optical spectra in this bump phase exhibit a blue continuum accompanied
with a narrow H$\alpha$ emission line. These features indicate an interaction
between the SN ejecta and the circumstellar matter (CSM) as in SNe IIn.
Assuming the ejecta-CSM interaction scenario, the mass loss rate is estimated
to be $\sim 1.7 \times 10^{-2} M_{\odot}$ yr$^{-1}$ within a few years before
the SN explosion. This is comparable to or even larger than the largest mass
loss rate observed for the Galactic red supergiants ($\sim 10^{-3} M_{\odot}$
yr$^{-1}$ for VY CMa). We suggest that the progenitor star of SN 2016bkv
experienced a violent mass loss just before the SN explosion.
