Using TESS and SALT to Understand the Unusual Type Ia ASASSN-18tb

ASASSN-18tb is notable for being the first relatively normal Type Ia supernova to exhibit clear broad (∼1000 km s^-1) Hα emission in its nebular-phase spectra. This is a potential indicator of a single-degenerate progenitor system. Despite the late-time Hα signature, TESS observations of the early rise show no evidence for deviations from a single-component power-law, arguing against the presence of a non-degenerate companion. Leveraging extensive spectroscopic observations with SALT, we find that the Hα luminosity remains approximately constant after its initial detection at phase +37 d, and that the Hα velocity evolution does not trace that of the FeIII λ4660 emission. These suggest that the Hα emission arises from a circumstellar medium rather than swept-up material from a non-degenerate companion. However, ASASSN-18tb is strikingly different from other known CSM-interacting Type Ia supernovae in a number of significant ways.