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

• Published in: The Astrophysical journal Vol. 959; no. 2; pp. 132 - 156
• Main Authors: Ni, Yuan Qi, Moon, Dae-Sik, Drout, Maria R., Matzner, Christopher D., Leong, Kelvin C. C., Kim, Sang Chul, Park, Hong Soo, Lee, Youngdae
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-12-2023; IOP Publishing
• Subjects: Astrophysics; Binary stars; Ejecta; Emissions; Evolution; Explosions; Photosphere; Power sources; Spectroscopy; Spectrum analysis; Supernova; Supernovae; Time domain astronomy; Transient sources; Type Ia supernovae; Velocity; White dwarf stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad0640

Abstract
SN 2021aefx
is a normal Type Ia supernova (SN) showing excess emission and redward color evolution over the first ∼ 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
56
Ni distribution. The first KMTNet detection of SN 2021aefx precedes
t
PL
by ∼ 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 ∼ 1.34
M
⊙
. 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 (∼ 12,400 km s
−1
) component consistent with typical photospheric evolution of near-Chandrasekhar-mass ejecta; and (2) a high-velocity (∼ 23,500 km s
−1
) secondary component visible during the first ∼ 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
56
Ni enrichment in the outer ∼ 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.