A Bright Ultraviolet Excess in the Transitional 02es-like Type Ia Supernova 2019yvq

We present photometric and spectroscopic observations of the nearby Type Ia SN 2019yvq, from its discovery ∼1 day after explosion to ∼100 days after its peak brightness. This SN exhibits several unusual features, most notably an extremely bright UV excess seen within ∼5 days of its explosion. As seen in Swift UV data, this early excess outshines its "peak"brightness, making this object more extreme than other supernovae (SNe) with early UV/blue excesses (e.g., iPTF14atg and SN 2017cbv). In addition, it was underluminous M B = -18.4, relatively quickly declining (Δm 15(B) = 1.37), and shows red colors past its early blue bump. Unusual (although not unprecedented) spectral features include extremely broad-lined and high-velocity Si absorption. Despite obvious differences in peak spectra, we classify SN 2019yvq as a transitional member of the 02es-like subclass due to its similarities in several respects (e.g., color, peak luminosity, peak Ti, and nebular [Ca ii]). We model this data set with a variety of published models, including SN ejecta-companion shock interaction and sub-Chandrasekhar-mass white dwarf (WD) double-detonation models. Radio constraints from the VLA place an upper limit of (4.5-20) 10-8 M o˙ yr-1 on the mass-loss rate from a symbiotic progenitor, which does not exclude a red giant or main-sequence companion. Ultimately, we find that no one model can accurately replicate all aspects of the data set, and further we find that the ubiquity of early excesses in 02es-like SNe Ia requires a progenitor system that is capable of producing isotropic UV flux, ruling out some models for this class of objects.