The Blue Excess of High-velocity Type Ia Supernovae: Dust Scattering of Circumstellar Material

Type Ia supernovae (SNe Ia) originate from thermonuclear explosions of carbon-oxygen white dwarfs at masses approaching the Chandrasekhar limit, and are widely used as standard candles for cosmological distances. However, the progenitor system and explosion mechanism of SNe Ia are still unclear. The circumstellar environment of SNe Ia has received increasing attention in recent decades. The distance and other geometric properties of the circumstellar material provide essential clues for exploring the physical origin of SNe Ia. At the same time, the scattering of circumstellar dust can produce observable effects on the light curve, spectrum, and polarization during the late phase of SNe Ia. The spectroscopically normal SNe Ia can be classified into two categories: normal-velocity and high-velocity ones. The high-velocity SNe Ia have an apparent blue excess within a few months after the maximum brightness. This blue excess can be fitted by circumstellar dust scattering. Meanwhile, fitting the late-phase spectrum or imaging polarization of SNe Ia can constrain the grain size or the geometric distribution of circumstellar dust, respectively. The result indicates that multi-epoch image polarimetry during the late phase of SNe Ia is a crucial probe to reveal the characteristics of circumstellar dust.