Opt. phot. and GMOS & NIRES spectra of SN2019yvq

Photometry near the time of first light places new constraints on the rapid rise of the UV/optical flux excess. 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 nondegenerate 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 and persistent CaII NIR triplet emission as SN 2019yvq transitions into the nebular phase. The lack of [OI]{lambda}6300 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.