Keywords

On 2017 March 11, the DLT40 Transient Discovery Survey discovered SN 2017cbv in NGC 5643, a Type 2 Seyfert Galaxy in the Lupus Constellation. SN 2017cbv went on to become a bright Type Ia supernova, with a V_max of 11.51 ± 0.05 mag. We present early time optical and infrared photometry of SN 2017cbv covering the rise and fall of over 68 days. We find that SN 2017cbv has a broad light curve Δm₁₅(B) = 0.88 ± 0.07, a B-band maximum at 2457,840.97 ± 0.43, a negligible host galaxy reddening where E(B − V)_host ≈ 0, and a distance modulus of 30.49 ± 0.32 to the SN, corresponding to a distance of ${12.58}_{-1.71}^{+1.98}$ Mpc. We also present the results of two different numerical models we used for analysis in this paper: SALT2, an empirical model for Type Ia supernova optical light curves that accounts for variability components; and SNooPy, the CSP-II light-curve model that covers both optical and near-infrared wavelengths and is used for distance estimates.

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 ∼5 days. One interpretation of the early light curve is that the excess blue light is 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 Hα (or helium) emission at late times, possibly along with other species, depending on the companion star and its orbital separation. A search for Hα emission in our +302 d spectrum yields a nondetection, with a L_Hα < 8.0 × 10³⁵ erg s⁻¹ (given an assumed distance of D = 12.3 Mpc), which we verified by implanting simulated Hα 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_H < 1 × 10⁻⁴ M_⊙. A similar analysis of helium star related lines yields a M_He < 5 × 10⁻⁴ M_⊙. 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.

Nearby type Ia supernovae (SNe Ia), such as SN 2017cbv, are useful events to address the question of what the elusive progenitor systems of the explosions are. Hosseinzadeh et al. suggested that the early blue excess of the light curve of SN 2017cbv could be due to the supernova ejecta interacting with a non-degenerate companion star. Some SN Ia progenitor models suggest the existence of circumstellar (CS) environments in which strong outflows create low-density cavities of different radii. Matter deposited at the edges of the cavities should be at distances at which photoionization due to early ultraviolet (UV) radiation of SNe Ia causes detectable changes to the observable Na i D and Ca ii H&K absorption lines. To study possible narrow absorption lines from such material, we obtained a time series of high-resolution spectra of SN 2017cbv at phases between −14.8 and +83 days with respect to B-band maximum, covering the time at which photoionization is predicted to occur. Both narrow Na i D and Ca ii H&K are detected in all spectra, with no measurable changes between the epochs. We use photoionization models to rule out the presence of Na i and Ca ii gas clouds along the line of sight of SN 2017cbv between ∼8 × 10¹⁶–2 × 10¹⁹ cm and ∼10¹⁵–10¹⁷ cm, respectively. Assuming typical abundances, the mass of a homogeneous spherical CS gas shell with radius R must be limited to ${M}_{{\rm{H}}\,{\rm{I}}}^{\mathrm{CSM}}\lt 3\times {10}^{-4}\times {(R/{10}^{17}[\mathrm{cm}])}^{2}$ ${M}_{\odot }$. The bounds point to progenitor models that deposit little gas in their CS environment.

We present very early, high-cadence photometric observations of the nearby Type Ia SN 2017cbv. The light curve is unique in that it has a blue bump during the first five days of observations in the U, B, and g bands, which is clearly resolved given our photometric cadence of 5.7 hr during that time span. We model the light curve as the combination of early shocking of the supernova ejecta against a nondegenerate companion star plus a standard SN Ia component. Our best-fit model suggests the presence of a subgiant star 56 R_☉ from the exploding white dwarf, although this number is highly model-dependent. While this model matches the optical light curve well, it overpredicts the observed flux in the ultraviolet bands. This may indicate that the shock is not a blackbody, perhaps because of line blanketing in the UV. Alternatively, it could point to another physical explanation for the optical blue bump, such as interaction with circumstellar material or an unusual nickel distribution. Early optical spectra of SN 2017cbv show strong carbon (C ii λ6580) absorption up through day −13 with respect to maximum light, suggesting that the progenitor system contains a significant amount of unburned material. These early results on SN 2017cbv illustrate the power of early discovery and intense follow-up of nearby supernovae to resolve standing questions about the progenitor systems and explosion mechanisms of SNe Ia.