Abstract
We present late-time (∼240-260 days after peak brightness) optical photometry and nebular (+236 and +264 days) spectroscopy of SN 2018oh, the brightest 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 and 2.1 ×1037 erg s-1 for Hα, He i λ5875, and He i λ6678, respectively. Assuming a simple model for the amount of swept-up material, we estimate upper mass limits for hydrogen of 5.4 ×10-4 M o and helium of 4.7 ×10-4 M o. 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.
| Original language | English (US) |
|---|---|
| Article number | L14 |
| Journal | Astrophysical Journal Letters |
| Volume | 870 |
| Issue number | 2 |
| DOIs | |
| State | Published - Jan 10 2019 |
Funding
We thank the anonymous referee for the helpful comments that improved the clarity and presentation of this paper. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The UCSC team is supported in part by NASA grants 14-WPS14-0048, NNG16PJ34G, and NNG17PX03C; NSF grants AST-1518052 and AST-1815935; the Gordon & Betty Moore Foundation; the Heising-Simons Foundation; and by a fellowship from the David and Lucile Packard Foundation to R.J.F. Facilities: Swope, Keck:I (LRIS), Keck:II (DEIMOS).
Keywords
- supernovae: general
- supernovae: individual (SN 2018oh)
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science