Nebular Spectroscopy of Kepler's Brightest Supernova

Abstract

We present late-time ($\sim$240-260 days after peak brightness) optical photometry and nebular (+236 and +264 days) spectroscopy of SN 2018oh, the brightest Type Ia supernova (SN Ia) observed by the Kepler telescope. The Kepler/K2 30-minute cadence observations started days before explosion and continued past peak brightness. For several days after explosion, SN 2018oh had blue "excess" flux in addition to a normal SN rise. The flux excess can be explained by the interaction between the SN and a Roche-lobe filling non-degenerate companion star. Such a scenario should also strip material from the companion star, that would emit once the SN ejecta become optically thin, imprinting relatively narrow emission features in its nebular spectrum. We search our nebular spectra for signs of this interaction, including close examination of wavelengths of hydrogen and helium transitions, finding no significant narrow emission. We place upper limits on the luminosity of these features of $2.6,2.9\mathrm{and}2.1\times 10^{37}\mathrm{erg}{\mathrm{s}}^{-1}$ for H$\alpha$, He I $\lambda$5875, and He I $\lambda$6678, respectively. Assuming a simple model for the amount of swept-up material, we estimate upper mass limits for hydrogen of $5.4\times 10^{-4}{\mathrm{M}}_{\odot }$ and helium of $4.7\times 10^{-4}{\mathrm{M}}_{\odot }$. Such stringent limits are unexpected for the companion-interaction scenario consistent with the early data. No known model can explain the excess flux, its blue color, and the lack of late-time narrow emission features.

10 pages, 5 figures, accepted for publication in APJ Letters

References in corpus (7)

• Constraining the Type Ia Supernova Progenitor: The Search for Hydrogen in Nebular Spectra
• Variable Sodium Absorption in a Low-Extinction Type Ia Supernova
• Early Blue Excess from the Type Ia Supernova 2017cbv and Implications for Its Progenitor
• A hybrid type Ia supernova with an early flash triggered by helium-shell detonation
• The Rising Light Curves of Type Ia Supernovae
• The Rise and Fall of Type Ia Supernova Light Curves in the SDSS-II Supernova Survey
• Nebular-Phase Spectra of Nearby Type Ia Supernovae

Cited by in corpus (5)

• H-$\alpha$ emission in the nebular spectrum of the Type Ia supernova ASASSN-18tb
• The Deepest Radio Observations of Nearby Type IA Supernovae: Constraining Progenitor Types and Optimizing Future Surveys
• Explaining the early excess emission of the Type Ia supernova 2018oh by the interaction of the ejecta with disk-originated matter (DOM)
• Spectral signatures of H-rich material stripped from a non-degenerate companion by a Type Ia supernova
• Nebular H$\alpha$ Limits for Fast Declining Type Ia Supernovae