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 $M_{\mathrm{m}\mathrm{a}\mathrm{x}}(B)=-19.36\pm 0.31$ mag, corresponding to a bolometric luminosity of $\sim 1.3\times {10}^{43}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$ with a synthesized ${}^{56}$Ni mass of $0.58\pm 0.14$ M${}_{\odot }$. 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 $\lambda 6355$ HVFs follows a power-law evolution ($\beta \approx 0.1$), shallower than the expected photospheric velocity evolution expected for an assumed $n=10$ density profile ($\beta \approx 0.22$) 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.