ASASSN-18tb: A Most Unusual Type Ia Supernova Observed by TESS and SALT

Understand

We present photometric and spectroscopic observations of the unusual Type Ia supernova ASASSN-18tb, including a series of SALT spectra obtained over the course of nearly six months and the first observations of a supernova by the Transiting Exoplanet Survey Satellite (TESS).

• We confirm a previous observation by Kollmeier et al.
• (2019) showing that ASASSN-18tb is the first relatively normal Type Ia supernova to exhibit clear broad ($\sim1000$ km s$^{-1}$) H$\alpha$ emission in its nebular phase spectra.
• We find that this event is best explained as a sub-Chandrasekhar mass explosion with $M_{Ni} \approx 0.3\; \rm{M}_\odot$.