Optical observations on the young Type Ia SN 2021fxy with detached high velocity features

We present optical observations on the young type Ia supernova (SN Ia) SN 2021fxy obtained within a few days after the explosion, with a focus on its prominent high-velocity features (HVFs). It reached a B-band maximum of M max(B) = -19.36 plus or minus0.31 mag, corresponding to a bolometric luminosity of about 1.3 times10 43 with a synthesized 56 Ni mass of 0.58 plus or minus0.14 M. The early spectra exhibit strong HVFs of intermediate-mass elements that are significantly detached from the photospheric components. In particular, the velocity of the Si II lambda6355 HVFs follows a power-law evolution (beta about 0.1), shallower than the expected photospheric velocity evolution expected for an assumed n=10 density profile (beta about 0.22) under homologous expansion. This behavior is consistent with the HVFs forming in intrinsic ejecta structures at least partially decoupled from the bulk outer ejecta, providing a possible constraint on the explosion physics of SNe Ia.