SN2019yvq Does Not Conform to SN Ia Explosion Models

M. A. Tucker, C. Ashall, B. J. Shappee, P. J. Vallely, C. S. Kochanek, M. E. Huber, G. S. Anand, J. V. Keane, E. Y. Hsiao, T. W. -S. Holoien

Published 2020-09-16Version 1

We present new photometric and spectroscopic observations of SN 2019yvq, a Type Ia supernova (SN Ia) exhibiting several peculiar properties including an excess of UV/optical flux within days of explosion, a high SiII velocity, and a low peak luminosity. Photometry near the time of first light places new constraints on the rapid rise of the UV/optical flux excess and shows that it peaks $\sim 2$ days earlier than previously thought. A near-infrared spectrum at $+173$ days after maximum light places strict limits on the presence of H or He emission, effectively excluding the presence of a nearby non-degenerate star at the time of explosion. New optical spectra, acquired at +128 and +150 days after maximum light, confirm the presence of CaII$\lambda 7300~$\r{A} and persistent CaII NIR triplet emission as SN 2019yvq transitions into the nebular phase. The lack of [OI]$\lambda 6300~$\r{A} emission disfavors the violent merger of two C/O white dwarfs (WDs) but the merger of a C/O WD with a He WD cannot be excluded. We compare our findings with several models in the literature postulated to explain the early flux excess including double-detonation explosions, $^{56}$Ni mixing into the outer ejecta during ignition, and interaction with H- and He-deficient circumstellar material. Each model may be able to explain both the early flux excess and the nebular [CaII] emission, but none of the models can reconcile the high photospheric velocities with the low peak luminosity without introducing new discrepancies.