No Stripped Companion Material in the Nebular Spectrum of the “Two-Component” Type Ia Supernova ASASSN-18bt

We analyze a Keck I/Low Resolution Imager and Spectrograph nebular spectrum taken 268 days after B-band maximum of ASASSN-18bt (SN 2018oh), a Type Ia supernova observed by K2 at the time of explosion. ASASSN-18bt exhibited a two-component rise to peak brightness, possibly the signature of an interaction between the supernova ejecta and a large (≳20 R⊙) nearby, non-degenerate companion. We search for emission signatures of stripped material from a non-degenerate companion in the nebular spectrum and find no evidence for any unbound material. We place an upper limit of <0.006 M⊙ on the amount of stripped/ablated H-rich material that could go undetected in our spectrum, effectively ruling out all hydrogen-rich donor stars. Additionally, we place a more tentative upper limit on He i emission in the observed spectrum of ≲0.02 M⊙ which also rules out helium star companions. Our deep limits rule out a non-degenerate companion as the explanation for the early-time feature in ASASSN-18bt.