Origin of High-velocity Ejecta, Excess Emission, and Redward Color Evolution in the Infant Type Ia Supernova 2021aefx

SN 2021aefx is a normal Type Ia supernova (SN) showing excess emission and redward color evolution over the first similar to 2 days. We present analyses of this SN using our high-cadence KMTNet multiband photometry, spectroscopy, and publicly available data, including first measurements of its explosion epoch (MJD 59529.32 +/- 0.16) and onset of power-law rise (t(PL) = MJD 59529.85 +/- 0.55; often called first light) associated with the main ejecta Ni-56 distribution. The first KMTNet detection of SN 2021aefx precedes t(PL) by similar to 0.5 hr, indicating presence of additional power sources. Our peak-spectrum confirms its intermediate Type Ia subclassification between core-normal and broad-Line, and we estimate an ejecta mass of similar to 1.34 M-circle dot. The spectral evolution identifies material reaching >40,000 km s(-1) (fastest ever observed in Type Ia SNe) and at least two split-velocity ejecta components expanding homologously: (1) a normal-velocity (similar to 12,400 km s(-1)) component consistent with typical photospheric evolution of near-Chandrasekhar-mass ejecta; and (2) a high-velocity (similar to 23,500 km s(-1)) secondary component visible during the first similar to 3.6 days post-explosion, which locates the component within the outer <16% of the ejecta mass. Asymmetric subsonic explosion processes producing a nonspherical secondary photosphere provide an explanation for the simultaneous appearance of the two components, and may also explain the excess emission via a slight Ni-56 enrichment in the outer similar to 0.5% of the ejecta mass. Our 300 days post-peak nebular-phase spectrum advances constraints against nondegenerate companions and further supports a near-Chandrasekhar-mass explosion origin. Off-center ignited delayed-detonations are likely responsible for the observed features of SN 2021aefx in some normal Type Ia SNe.