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

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 56Ni 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 56Ni 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.

Helium-deficient ER UMa-type Dwarf Nova below the Period Minimum with a Hot Secondary

“…As part of the Korea Microlensing Telescope Network (KMTNet) Supernova Program (KSP; Moon et al 2016) aimed at detecting infant/early supernovae (Afsariardchi et al 2019;Moon et al 2021;Ni et al 2022Ni et al , 2023aNi et al , 2023b, we monitored the 4 square degree field around the lenticular galaxy NGC 2380 in the BVI bands with an average cadence of 8 hr for each band between 2017 December and 2019 March using the three 1.6 m telescopes of the KMTNet located at Chile, South Africa, and Australia (Kim et al 2016). All our images are 60 sec exposures reaching an average detection limit of 21.5 mag.…”

Extrapolation of Type Ia Supernova Spectra into the Near-infrared Using Principal Component Analysis

“…In Figure 15, the template approximation of SNe 2011fe, 2014J, 2021fxy, and 2021aefx is overlayed on top of the extrapolations shown here in earlier sections. The templates were given the same phase as well as values of s BV for each SN Ia: s BV = 0.95 for SN 2011fe (Ashall et al 2019), s BV = 0.99 for SN 2021fxy (DerKacy et al 2023), and s BV = 1.05 for SN 2021aefx (Ni et al 2023). We also use s BV = 1.00 for SN 2014J by converting its measured value of Δm 15 (B) = 0.98 (Li et al 2019) to s BV using the relation determined by Burns et al (2018).…”