Nebular Spectroscopy of the `Blue Bump' Type Ia Supernova 2017cbv
  We present nebular phase optical and near-infrared spectroscopy of the Type
Ia supernova (SN) 2017cbv. The early light curves of SN~2017cbv showed a
prominent blue bump in the $U$, $B$ and $g$ bands lasting for $\sim$5 d. One
interpretation of the early light curve was that the excess blue light was due
to shocking of the SN ejecta against a nondegenerate companion star -- a
signature of the single degenerate scenario. If this is the correct
interpretation, the interaction between the SN ejecta and the companion star
could result in significant H$\alpha$ (or helium) emission at late times,
possibly along with other species, depending on the companion star and its
orbital separation. A search for H$\alpha$ emission in our +302 d spectrum
yields a nondetection, with a $L_{H\alpha}$$<$8.0$\times$10$^{35}$ erg/s (given
an assumed distance of $D$=12.3 Mpc), which we have verified by implanting
simulated H$\alpha$ emission into our data. We make a quantitative comparison
to models of swept-up material stripped from a nondegenerate companion star,
and limit the mass of hydrogen that might remain undetected to $M_{\rm H} < 1
\times 10^{-4}$ $\rm M_{\odot}$. A similar analysis of helium star related
lines yields a $M_{\rm He} < 5 \times 10^{-4}$ $\rm M_{\odot}$. Taken at face
value, these results argue against a nondegenerate H or He-rich companion in
Roche lobe overflow as the progenitor of SN 2017cbv. Alternatively, there could
be weaknesses in the envelope-stripping and radiative transfer models necessary
to interpret the strong H and He flux limits.
