Abstract: | Early observations of Type Ia supernovae (SNe$,$Ia) provide essential clues
for understanding the progenitor system that gave rise to the terminal
thermonuclear explosion. We present exquisite observations of SN$,$2019yvq,
the second observed SN$,$Ia, after iPTF$,$14atg, to display an early flash of
emission in the ultraviolet (UV) and optical. Our analysis finds that
SN$,$2019yvq was unusual, even when ignoring the initial flash, in that it was
moderately underluminous for a SN$,$Ia ($M_g approx -18.5$$,$mag at peak)
yet featured very high absorption velocities ($v approx
15,000$$,mathrm{km,s}^{-1}$ for Si II $lambda$6355 at peak). We find that
many of the observational features of SN$,$2019yvq, aside from the flash, can
be explained if the explosive yield of radioactive $^{56}$Ni is relatively low
(we measure $M_{^{56}mathrm{Ni}} = 0.31 pm 0.05,M_odot$) and it and other
iron-group elements are concentrated in the innermost layers of the ejecta. To
explain both the UV/optical flash and peak properties of SN$,$2019yvq we
consider four different models: interaction between the SN ejecta and a
nondegenerate companion, extended clumps of $^{56}$Ni in the outer ejecta, a
double-detonation explosion, and the violent merger of two white dwarfs. Each
of these models has shortcomings when compared to the observations; it is clear
additional tuning is required to better match SN$,$2019yvq. In closing, we
predict that the nebular spectra of SN$,$2019yvq will feature either H or He
emission, if the ejecta collided with a companion, strong [Ca II] emission, if
it was a double detonation, or narrow [O I] emission, if it was due to a
violent merger. |