High-velocity Feature as the Indicator of the Stellar Population of Type Ia Supernovae

Although Type Ia supernovae (SNe Ia) are very useful in many astrophysical fields, their exact nature is still unclear, e.g., the progenitor and explosion models. The high-velocity features (HVFs) in optical spectra of SNe Ia could provide some meaningful information to constrain the nature of SNe Ia. Here, I show strong evidence that the SNe Ia with a strong Ca ii infrared triple (Ca ii IR3) HVF around maximum brightness are associated with a relatively younger population than those with a weak Ca ii IR3 HVF, e.g., the SNe Ia with a strong Ca ii IR3 HVF tend to occur in a late-type galaxy or in an early-type galaxy with significant star formation. In addition, using pixel statistics, I find that the SNe Ia with a strong maximum-light Ca ii IR3 HVF show a higher degree of association with the star formation index, e.g., H? or near-UV emission, than those with a weak Ca ii IR3 HVF. Moreover, I find that the strength of the Ca ii IR3 HVF is linearly dependent on the difference of the absorption-weighted velocities between the Ca ii IR3 and Si ii 635.5 nm absorption lines, which then is a good index for diagnosing whether there is a high-velocity component in the Ca ii IR3 absorption feature in the spectra of SNe Ia. I finally discuss the origin of the HVFs and the constraints from our discoveries on the progenitor model of SNe Ia.