Nebular Spectroscopy of the `Blue Bump' Type Ia Supernova 2017cbv

Abstract

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$5 d. One interpretation of the early light curve was that the excess blue light was due to shocking of the SN ejecta against a nondegenerate companion star -- a signature of the single degenerate scenario. If this is the correct interpretation, the interaction between the SN ejecta and the companion star could result in significant H$\hat{I}\pm$ (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for H$\hat{I}\pm$ emission in our +302 d spectrum yields a nondetection, with a $L_{H\hat{I}\pm }$$<$8.0$\times$10${}^{35}$ erg/s (given an assumed distance of $D$=12.3 Mpc), which we have verified by implanting simulated H$\hat{I}\pm$ 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 $M_{\mathrm{H}}<1\times 10^{-4}$ ${\mathrm{M}}_{\odot }$. A similar analysis of helium star related lines yields a $M_{\mathrm{He}}<5\times 10^{-4}$ ${\mathrm{M}}_{\odot }$. 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.

11 pages, 3 figures, 2 tables. ApJ accepted