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

Abstract

We analyze a KeckI/LRIS nebular spectrum taken 268 days after $B$-band maximum of ASASSN-18bt (SN~2018oh), a Type Ia supernova (SN Ia) observed by {\it K2} at the time of explosion. ASASSN-18bt exhibited a two-component rise to peak brightness, possibly the signature of an interaction between the SN ejecta and a large ($\gtrsim 20R_{\odot }$) 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 $<0.006M_{\odot }$ 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 HeI emission in the observed spectrum of $\lesssim 0.02M_{\odot }$ 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.

7 pages, 4 figures, 1 table, accepted by ApJL