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

Abstract

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 ($\sim 1000$ km s${}^{-1}$) H$\hat{I}\pm$ emission in its nebular phase spectra. We find that this event is best explained as a sub-Chandrasekhar mass explosion with $M_{Ni}\approx 0.3{\mathrm{M}}_{\odot }$. Despite the strong H$\hat{I}\pm$ signature at late times, we find that the early rise of the supernova shows no evidence for deviations from a single-component power-law and is best fit with a moderately shallow power-law of index $1.69\pm 0.04$. We find that the H$\hat{I}\pm$ luminosity remains approximately constant after its initial detection at phase +37 d, and that the H$\hat{I}\pm$ velocity evolution does not trace that of the Fe~III$\lambda 4660$ emission. These suggest that the H$\hat{I}\pm$ emission arises from circumstellar medium (CSM) 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. Those objects typically show an H$\hat{I}\pm$ luminosity two orders of magnitude higher than what is seen in ASASSN-18tb, pushing them away from the empirical light-curve relations that define "normal" Type Ia supernovae. Conversely, ASASSN-18tb exhibits a fairly typical light curve and luminosity for an underluminous or transitional SN Ia, with $M_R\approx -18.1$ mag. Moreover, ASASSN-18tb is the only SN Ia showing H$\hat{I}\pm$ from CSM interaction to be discovered in an early-type galaxy.

13 pages, 8 figures